The Periodic Table as a Predictive Model

The periodic table does much more than organize the chemical elements.

It allows us to predict properties.

If we know where an element is located, we can often predict:

• whether it is a metal or non-metal
• its number of valence electrons
• its likely ion charge
• whether it tends to gain or lose electrons
• its likely bonding behaviour
• its relative atomic size
• its relative reactivity
• some of its physical properties
• the types of compounds it may form

This means that we can make useful predictions even about an element we have never studied before.

A useful reasoning chain is:

position → electron arrangement → periodic trends → properties → chemical behaviour → applications

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Groups and Periods

Every element has a position described by its group and period.

A group is a vertical column.

A period is a horizontal row.

Both provide important information.

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What Does the Group Tell Us?

For many main-group elements, the group tells us about the number of valence electrons.

For example:

Group 1 → 1 valence electron

Group 2 → 2 valence electrons

Group 17 → 7 valence electrons

Group 18 → complete outer shell

Elements in the same group therefore tend to have similar chemical behaviour.

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What Does the Period Tell Us?

The period tells us the number of occupied principal electron shells in the simple shell model.

For example:

Sodium is in Period 3.

Electron arrangement:

2,8,1

It has:

3 occupied shells

Potassium is in Period 4.

Electron arrangement:

2,8,8,1

It has:

4 occupied shells

This helps us predict properties such as atomic radius and shielding.

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Electron Arrangement Connects Position to Behaviour

The periodic table's patterns exist because atomic structure changes systematically.

Consider:

Li → 2,1

Na → 2,8,1

K → 2,8,8,1

All three are in Group 1.

They all have:

one valence electron

Therefore, they show similar chemical behaviour.

However, they are in different periods, so their atoms have different numbers of occupied shells.

That produces trends within the group.

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Predicting Metal or Non-Metal

An unfamiliar element's location provides a quick indication of whether it is likely to behave as a metal or non-metal.

Broadly:

left and centre → metals

upper-right → non-metals

staircase boundary → metalloids

This is one of the first observations to make when predicting an unfamiliar element.

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Predicting Metallic Properties

If an unfamiliar element lies well within the metallic region, we might predict that it:

• conducts electricity
• conducts thermal energy
• has metallic lustre
• tends to lose electrons
• forms positive ions
• may form ionic compounds with non-metals

The exact strength, density, melting point, and reactivity depend on the element's particular family and position.

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Predicting Non-Metallic Properties

If an unfamiliar element lies in the upper-right non-metal region, we might predict that it:

• is a relatively poor electrical conductor in its elemental form
• tends to gain or share electrons
• forms covalent bonds with other non-metals
• may form negative ions when reacting with metals

The exact properties still depend strongly on its group.

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The Major Element Families

Several groups have especially recognizable properties.

Important families include:

• Group 1 – alkali metals
• Group 2 – alkaline earth metals
• Group 17 – halogens
• Group 18 – noble gases
• transition metals

Knowing these families allows us to make much more detailed predictions.

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Group 1: Alkali Metals

Examples include:

• lithium
• sodium
• potassium
• rubidium
• caesium

They have:

one valence electron

They typically lose it:

M → M⁺ + e⁻

Therefore, Group 1 metals form:

+1 ions

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Predicting Group 1 Physical Properties

An unfamiliar Group 1 element would likely be:

• metallic
• relatively soft
• a good conductor
• relatively low in density compared with many other metals
• relatively low-melting compared with many transition metals

These properties distinguish Group 1 metals from many transition metals.

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Predicting Group 1 Chemical Behaviour

Group 1 metals are highly reactive.

They react by losing their outer electron.

Reactivity generally:

increases down the group.

Therefore:

Li < Na < K < Rb < Cs

Down the group, atoms become larger and shielding increases.

The outer electron becomes easier to remove.

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Predicting an Unfamiliar Alkali Metal

Suppose element X is located below potassium in Group 1.

We can predict:

• X is metallic
• X has one valence electron
• X forms X⁺
• X should be very reactive
• X should be more reactive than potassium
• X should react with suitable non-metals to form ionic compounds

We can make all of these predictions without knowing X's name.

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Group 2: Alkaline Earth Metals

Group 2 includes elements such as:

• magnesium
• calcium
• strontium
• barium

These elements have:

two valence electrons

They usually lose both:

M → M²⁺ + 2e⁻

Therefore:

Group 2 → +2 ions

An unfamiliar Group 2 element would therefore be expected to show metallic behaviour and form ionic compounds with many non-metals.

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Group 17: Halogens

Important halogens include:

• fluorine
• chlorine
• bromine
• iodine

They have:

seven valence electrons

They tend to gain one:

X + e⁻ → X⁻

Therefore:

Group 17 → −1 ions

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Predicting Halogen Properties

An unfamiliar Group 17 element would likely:

• be a non-metal
• exist as a diatomic substance, X₂
• have seven valence electrons
• form X⁻ ions
• react with metals
• form halide compounds
• participate in displacement reactions

Reactivity generally:

decreases down Group 17.

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Physical Trends in the Halogens

The physical properties also change down Group 17.

At room temperature:

Fluorine → gas

Chlorine → gas

Bromine → liquid

Iodine → solid

Moving down the group:

• atoms and molecules become larger
• electron clouds become more polarizable
• intermolecular attractions generally become stronger
• melting and boiling points generally increase

Therefore, position can help us estimate physical properties as well as chemical behaviour.

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Group 18: Noble Gases

Examples include:

• helium
• neon
• argon
• krypton
• xenon

They have complete outer electron shells.

Therefore, an unfamiliar noble gas would be expected to be:

• generally very unreactive
• monatomic
• a gas under ordinary conditions for the familiar members of the group
• unlikely to form common simple ions

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Transition Metals

Transition metals occupy the central d-block.

Examples include:

• iron
• copper
• nickel
• chromium
• titanium

Compared with Group 1 metals, transition metals are generally:

• harder
• stronger
• denser
• higher-melting
• less reactive

Many also:

• have variable oxidation states
• form coloured compounds
• act as catalysts

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Predicting an Unfamiliar Transition Metal

If element X is located in the central d-block, we might predict:

• X is metallic
• X conducts heat and electricity
• X is relatively strong and dense
• X forms positive ions
• X may form more than one oxidation state
• X may form coloured compounds
• X may have catalytic properties

These predictions are useful, although transition-metal behaviour is less uniform than Group 1 or Group 17 behaviour.

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Ion Charges from Position

For many main-group elements, common simple ion charges can be predicted directly.

Group 1 → +1

Group 2 → +2

Group 13 → +3

Group 15 → −3

Group 16 → −2

Group 17 → −1

Group 18 → no common simple ion

This pattern lets us predict compounds involving unfamiliar elements.

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Predicting Bonding

Position also helps predict the likely type of bonding.

A useful introductory rule is:

metal + non-metal → usually ionic

non-metal + non-metal → usually covalent

metal + metal → metallic bonding or alloys

This is a powerful first prediction.

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Predicting an Unfamiliar Ionic Compound

Suppose X is in Group 2.

Suppose Y is in Group 17.

Predict the ions:

X²⁺

Y⁻

One X²⁺ requires two Y⁻ ions.

Therefore:

XY₂

Because X is metallic and Y is non-metallic, the compound is expected to be predominantly:

ionic

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Another Unfamiliar Compound

Suppose X is in Group 1.

Y is in Group 16.

Predict:

X → X⁺

Y → Y²⁻

Charge balance requires:

2 X⁺ → +2

1 Y²⁻ → −2

Therefore:

X₂Y

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A More Difficult Formula Prediction

Suppose X is a Group 13 metal.

Y is a Group 16 non-metal.

Predict:

X³⁺

Y²⁻

The lowest common total charge is 6.

Two X³⁺:

+6

Three Y²⁻:

−6

Therefore:

X₂Y₃

This is the same charge pattern found in:

Al₂O₃

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Predicting Covalent Behaviour

Suppose X and Y are both non-metals.

Instead of complete electron transfer, they are more likely to share electrons.

This suggests:

covalent bonding

Their valence-electron arrangements can then help us predict how many bonds they may form.

For example:

Group 17 non-metal → commonly 1 bond

Group 16 non-metal → commonly 2 bonds

Group 15 non-metal → commonly 3 bonds

Carbon → commonly 4 bonds

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Periodic Trends Help Refine Predictions

Group membership tells us about similarities.

Position within the group tells us how properties change.

Important periodic trends include:

• atomic radius
• ionization energy
• electronegativity
• metallic character
• shielding

These allow more sophisticated predictions.

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Atomic Radius

Atomic radius generally:

increases down a group

and:

decreases across a period from left to right

Why?

Down a group, additional occupied electron shells are added.

Across a period, proton number increases while electrons are added to the same principal shell, so effective nuclear attraction generally increases.

Atomic size affects how strongly outer electrons are held.

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Shielding

Electron shielding generally increases when additional occupied shells are added.

More inner electrons reduce the effective attraction between the nucleus and outer electrons.

Therefore, moving down a group generally means:

greater shielding

This helps explain important reactivity trends.

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Ionization Energy

Ionization energy tells us how difficult it is to remove an electron.

Broadly:

low ionization energy → electron loss easier

high ionization energy → electron loss harder

This is particularly useful when predicting metallic behaviour.

Group 1 metals have relatively low first ionization energies, helping explain why they readily form +1 ions.

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Electronegativity

Electronegativity describes how strongly an atom attracts bonding electrons.

Broadly:

electronegativity increases toward the upper-right of the periodic table.

Fluorine is particularly electronegative.

Electronegativity helps us predict:

• bond polarity
• electron attraction
• metallic vs non-metallic behaviour
• whether bonding may have strong ionic or covalent character

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Metallic Character

Metallic character generally:

increases toward the lower-left of the periodic table.

Non-metallic character generally becomes stronger toward the upper-right.

Therefore, an unfamiliar element near the lower-left would likely:

• lose electrons relatively readily
• form positive ions
• show metallic physical properties

An unfamiliar element near the upper-right would more likely:

• attract electrons strongly
• gain or share electrons
• show non-metallic behaviour

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Predicting Physical Properties Down a Group

Physical properties can also show trends.

For example, in Group 17:

melting and boiling points generally increase down the group.

In Group 1:

melting points generally decrease down the group.

In Group 18:

boiling points generally increase down the group.

These trends allow us to estimate the physical properties of unfamiliar members.

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Predicting an Unknown Halogen's State

Suppose an unfamiliar halogen X is below iodine.

We know that down Group 17:

• atomic and molecular size increase
• intermolecular attractions become stronger
• melting and boiling points generally increase

Therefore, we would predict X to be:

less volatile than iodine

and likely to have:

higher melting and boiling points

This is a trend-based prediction.

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Predicting an Unknown Group 1 Metal

Suppose X is located below potassium.

We can predict:

• larger atomic radius than potassium
• greater shielding
• lower first ionization energy in the general trend
• easier electron loss
• +1 ion formation
• greater reactivity
• likely lower melting point than potassium following the general Group 1 trend

Several predictions can therefore be made from a single periodic-table position.

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Predicting an Unknown Noble Gas

Suppose element X is below krypton in Group 18.

We would predict:

• complete outer electron shell
• monatomic behaviour
• very low chemical reactivity
• greater atomic size than krypton
• stronger dispersion forces
• higher boiling point than krypton

The element's group predicts its chemical behaviour.

Its position down the group helps predict physical trends.

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Predicting an Unknown Transition Metal

Suppose element X lies between familiar transition metals in the d-block.

We might expect:

• metallic conductivity
• high density relative to many main-group metals
• useful mechanical strength
• positive ion formation
• possible variable oxidation states
• possible coloured compounds
• possible catalytic activity

However, we should be more cautious because transition-metal trends can be less regular than simple Group 1 or Group 17 trends.

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Predicting Applications from Properties

Properties determine how elements can be used.

Therefore, if we can predict properties, we can sometimes predict possible applications.

For example:

High electrical conductivity + ductility

Possible application:

electrical wiring.

Copper is a familiar example.

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Low Density + High Strength

Suppose a metal has:

• high strength
• relatively low density
• good corrosion resistance

These properties would be useful in applications where reducing mass matters.

Possible applications include:

• aircraft
• spacecraft
• high-performance vehicles

Titanium alloys are a familiar example.

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Low Reactivity

Suppose a gas is:

• chemically very unreactive
• non-flammable

It may be useful where reactions with surrounding substances need to be minimized.

Examples include:

• shielding atmospheres
• protective gas environments
• some types of lighting

Argon is a familiar example.

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High Reactivity Can Also Be Useful

High reactivity is not always undesirable.

Reactive elements and their compounds can be useful in:

• chemical synthesis
• batteries
• industrial processes

However, highly reactive elements are often used in the form of compounds rather than as pure elements.

Lithium compounds in battery technologies are an important example.

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Predicting from Neighbouring Elements

Properties often change gradually across parts of the periodic table.

Therefore, neighbouring elements can provide useful evidence.

If an unfamiliar element lies between two known elements in the same group or period, its properties may often fall within or follow the trend established by those neighbours.

This idea was historically important in predicting undiscovered elements.

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Mendeleev and Undiscovered Elements

Before all modern elements were known, Dmitri Mendeleev arranged elements according to repeating chemical patterns.

He deliberately left gaps where he believed elements had not yet been discovered.

He then predicted properties of some missing elements using the properties of neighbouring elements.

Later discoveries supported several of these predictions.

This demonstrated the extraordinary predictive power of periodic patterns.

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Example: Predicting an Undiscovered Group 1 Element

Imagine an undiscovered element X below potassium.

From its position alone, we could predict:

Family: alkali metal

Valence electrons: 1

Likely ion: X⁺

Bonding with halogens: ionic

Reactivity: greater than potassium following the group trend

Metallic behaviour: strong

Atomic radius: larger than potassium

Ionization energy: generally lower than potassium

This is much more information than simply saying "X is a metal."

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Example: Predicting an Undiscovered Halogen

Suppose Y is an unfamiliar element below bromine in Group 17.

Predict:

Valence electrons: 7

Ion: Y⁻

Element type: non-metal

Elemental form: likely Y₂

Reactivity: lower than bromine

Atomic radius: larger

Melting and boiling points: generally higher

Bonding with Group 1 metals: ionic

A Group 1 metal M would form:

MY

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Example: Predicting a Compound

Suppose:

X is an unfamiliar Group 2 element.

Y is an unfamiliar Group 16 element.

X forms:

X²⁺

Y forms:

Y²⁻

Charges balance 1:1.

Therefore:

XY

Because X is metallic and Y is non-metallic, we predict the compound to be predominantly:

ionic

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Example: Another Compound

X belongs to Group 1.

Y belongs to Group 17.

Predict:

X → X⁺

Y → Y⁻

Therefore:

XY

Again:

metal + non-metal → usually ionic.

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Example: A More Difficult Compound

X belongs to Group 13.

Y belongs to Group 16.

Predict:

X → X³⁺

Y → Y²⁻

To balance charges:

2 X³⁺ → +6

3 Y²⁻ → −6

Therefore:

X₂Y₃

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Example: Predicting Reactivity

Element X and element Y are both Group 1 metals.

X is below Y.

Which should be more reactive?

X

Why?

X has:

• more occupied electron shells
• greater atomic radius
• greater shielding
• weaker effective attraction to its outer electron

The electron is therefore easier to remove.

This makes X more reactive.

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Example: Predicting Halogen Reactivity

Halogen A is above halogen B.

Which should be more reactive?

A

A has:

• fewer occupied shells
• smaller atomic radius
• less shielding
• stronger effective attraction for an incoming electron

Therefore, A gains an electron more readily.

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Example: Predicting a Displacement Reaction

Suppose halogen X is above halogen Y in Group 17.

X should be more reactive.

Therefore, X₂ should be able to displace Y⁻ from an appropriate halide compound.

General form:

X₂ + 2MY → 2MX + Y₂

where M is a suitable +1 metal ion.

This allows us to predict reactions involving completely unfamiliar element symbols.

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Example: Working Backward from Properties

Sometimes we are given properties and asked to identify the likely family.

An unknown element:

• is a soft metal
• forms X⁺
• reacts vigorously with water
• has one valence electron

Most likely family:

Group 1 – alkali metals

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Another Identification Problem

An unknown element:

• is a non-metal
• exists as X₂
• forms X⁻
• can participate in displacement reactions
• has seven valence electrons

Most likely:

Group 17 – halogens

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Another Identification Problem

An unknown element:

• exists as individual atoms
• is a gas
• has a complete outer shell
• is extremely unreactive

Most likely:

Group 18 – noble gases

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Transition-Metal Identification

An unknown element:

• is a dense metal
• has a high melting point
• forms X²⁺ and X³⁺
• forms coloured compounds
• can act as a catalyst

This strongly suggests:

a transition metal

The variable oxidation states are especially useful evidence.

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Use More Than One Piece of Evidence

A good prediction should not rely on one clue when several are available.

Suppose an unknown element is described as:

• Group 17
• Period 4
• non-metal
• forms a −1 ion

Each clue supports the others.

Group 17 predicts seven valence electrons.

The −1 ion is consistent with gaining one electron.

Period 4 tells us the atom has more occupied shells than chlorine.

Therefore, we would expect it to be larger and less reactive than chlorine.

The actual element is bromine, but the reasoning works even if its name is hidden.

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Group and Period Work Together

Consider two unfamiliar elements:

X = Group 1, Period 3

Y = Group 1, Period 5

Both should:

• have one valence electron
• form +1 ions
• show alkali-metal chemistry

But Y should generally have:

• more occupied shells
• larger atomic radius
• greater shielding
• easier electron removal
• greater reactivity

The group predicts similarity.

The period position helps explain differences.

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Predicting Across a Period

Across a period from left to right:

• atomic radius generally decreases
• ionization energy generally increases
• electronegativity generally increases
• metallic character generally decreases

This allows us to predict broad changes from reactive metals on the left toward non-metals and noble gases on the right.

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Predicting Down a Group

Down a group:

• number of occupied shells increases
• atomic radius generally increases
• shielding generally increases
• first ionization energy generally decreases

However, reactivity does not follow the same direction in every group.

Group 1:

reactivity increases down

Group 17:

reactivity decreases down

This is because Group 1 reacts by losing electrons while Group 17 reacts by gaining electrons.

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Applying Periodic Patterns to Problem Solving

When given an unfamiliar element, use this approach.

Step 1: Locate it.

Identify its group and period.

Step 2: Identify the family.

Alkali metal?

Alkaline earth metal?

Halogen?

Noble gas?

Transition metal?

Step 3: Determine valence-electron information.

How many outer electrons?

Step 4: Predict metal or non-metal behaviour.

Will it tend to lose, gain, or share electrons?

Step 5: Predict ion charge.

Use group patterns where appropriate.

Step 6: Predict bonding.

Metal + non-metal → usually ionic.

Non-metal + non-metal → usually covalent.

Step 7: Apply periodic trends.

Consider radius, shielding, ionization energy, electronegativity, and metallic character.

Step 8: Predict physical properties.

Consider known family trends.

Step 9: Predict chemical behaviour.

Consider reactivity and likely reactions.

Step 10: Justify your prediction.

Connect your answer to periodic evidence.

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Worked Problem 1

Element X is in Group 1 and Period 4.

Predict its properties.

We can infer:

• 1 valence electron
• metal
• forms X⁺
• good conductor
• relatively soft
• highly reactive
• reacts through electron loss
• likely forms ionic compounds with non-metals

Because it is lower than lithium and sodium, it also has more occupied shells and greater shielding.

Therefore, its outer electron is relatively easy to remove.

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Worked Problem 2

Element Y is in Group 17 and Period 4.

Predict its behaviour.

We can infer:

• non-metal
• 7 valence electrons
• forms Y⁻
• elemental form likely Y₂
• reacts by gaining electrons
• forms ionic compounds with metals
• forms covalent compounds with non-metals

Because it is below chlorine, it has greater radius and shielding and should be less reactive than chlorine.

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Worked Problem 3

Element Z is in Group 18.

Predict its properties.

We can infer:

• complete outer shell
• very low chemical reactivity
• monatomic
• no common simple ion
• unlikely to form ordinary ionic compounds

If Z is farther down the group, we would also expect greater atomic size and generally higher boiling point.

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Worked Problem 4

Element A is Group 2.

Element B is Group 17.

Predict the compound.

A:

A²⁺

B:

B⁻

Two B⁻ ions balance one A²⁺.

Therefore:

AB₂

Because A is metallic and B is non-metallic:

ionic bonding is expected.

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Worked Problem 5

An unfamiliar element is between known metallic and non-metallic regions near the staircase boundary.

We should be cautious about predicting purely metallic or non-metallic behaviour.

The element may be a:

metalloid

and may show properties intermediate between metals and non-metals.

This is an important reminder that periodic predictions are patterns, not rigid rules.

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Worked Problem 6

Element X forms two common positive ions and several coloured compounds.

It also acts as a catalyst.

Where is X likely located?

These properties are characteristic of many:

transition metals

Therefore, X is likely located in the:

central d-block

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Worked Problem 7

An unfamiliar element has:

• one valence electron
• a large atomic radius
• low first ionization energy
• strong metallic character

We would predict that it:

• loses an electron readily
• forms a +1 ion
• is chemically reactive

These properties strongly suggest a:

Group 1 metal

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Worked Problem 8

An unfamiliar element has:

• seven valence electrons
• high electronegativity
• non-metallic behaviour
• tendency to form −1 ions

We would predict that it belongs to:

Group 17

If it is near the top of the group, it should also be especially effective at attracting electrons.

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From Prediction to Application

Periodic trends do not tell us every engineering property of an element.

However, they provide an important starting point.

If an unfamiliar element is predicted to be:

highly conductive and metallic

we might investigate it for electrical applications.

If it is:

strong, high-melting, and corrosion-resistant

we might investigate structural or high-temperature applications.

If it is:

chemically unreactive

we might investigate protective-atmosphere applications.

If it has:

multiple oxidation states

we might investigate catalytic or electrochemical applications.

The periodic table therefore helps guide scientific investigation.

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Did You Know?

One of the most famous examples of periodic prediction involved an element Mendeleev called eka-silicon.

Mendeleev predicted that an undiscovered element should exist below silicon and estimated several of its properties from periodic patterns.

The element germanium was later discovered.

Its properties showed important similarities to those Mendeleev had predicted.

This demonstrated that periodic patterns could be used not only to organize known chemistry, but also to predict the behaviour of previously unknown elements.

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Common Mistakes

Mistake 1: Using only the period to predict ion charge

Ion charge is more directly related to group and valence-electron behaviour for main-group elements.

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Mistake 2: Assuming every element in the same period behaves similarly

Elements in the same period can range from reactive metals to non-metals and noble gases.

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Mistake 3: Assuming reactivity always increases down a group

Group 1 reactivity increases down the group.

Group 17 reactivity decreases.

Always consider how the element reacts.

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Mistake 4: Treating transition metals exactly like Group 1 metals

Transition metals commonly show variable oxidation states and more complicated chemistry.

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Mistake 5: Assuming every metal forms only one ion

Many transition metals form several oxidation states.

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Mistake 6: Predicting physical properties from ion charge alone

Ion charge provides chemical information but does not directly determine properties such as density or melting point.

Use family and periodic trends as well.

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Mistake 7: Treating periodic trends as absolute rules

Periodic trends are strong general patterns, but exceptions occur.

Use appropriate caution, especially with transition metals and elements near the metal–non-metal boundary.

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Mistake 8: Making a prediction without explaining it

A strong answer should contain:

prediction + periodic evidence + atomic explanation

For example:

"X should be more reactive than Y because X is lower in Group 1, has greater shielding and a larger atomic radius, so its outer electron is easier to remove."

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Key Terms

Element family: A group of elements with related chemical properties.

Group: A vertical column of the periodic table.

Period: A horizontal row of the periodic table.

Valence electron: An electron in the outermost occupied shell.

Periodic trend: A repeating pattern in element properties across the periodic table.

Atomic radius: A measure of atomic size.

Ionization energy: Energy required to remove an electron from an isolated gaseous atom.

Electronegativity: An atom's tendency to attract bonding electrons.

Shielding: Reduction in effective nuclear attraction caused by inner electrons.

Metallic character: The degree to which an element shows characteristic metallic behaviour.

Metalloid: An element with properties intermediate between typical metals and non-metals.

Cation: A positively charged ion.

Anion: A negatively charged ion.

Oxidation state: A value describing the formal distribution of electrons in a substance.

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Key Periodic Patterns

Across a period →

Atomic radius generally:

decreases

Ionization energy generally:

increases

Electronegativity generally:

increases

Metallic character generally:

decreases

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Down a group ↓

Atomic radius generally:

increases

Shielding:

increases

First ionization energy generally:

decreases

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Group 1

Ion:

+1

Reactivity:

increases down the group

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Group 2

Ion:

+2

Metallic behaviour.

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Group 17

Ion:

−1

Reactivity:

decreases down the group

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Group 18

Complete outer shell.

Generally:

very unreactive

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Transition metals

Positive ions.

Often:

• variable oxidation states
• coloured compounds
• catalytic behaviour

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Key Takeaways

• The periodic table can be used to predict the properties of unfamiliar elements.
• The group provides important information about valence electrons and chemical behaviour.
• The period provides information about the number of occupied electron shells.
• Elements in the same group often have similar chemical properties because they have similar valence-electron arrangements.
• Metals are concentrated on the left and centre of the periodic table.
• Non-metals are concentrated toward the upper-right.
• Metalloids lie near the boundary between these regions.
• Group 1 elements commonly form +1 ions and become more reactive down the group.
• Group 2 elements commonly form +2 ions.
• Group 17 elements commonly form −1 ions and become less reactive down the group.
• Group 18 elements have complete outer shells and are generally very unreactive.
• Transition metals often form multiple oxidation states, coloured compounds, and useful catalysts.
• Atomic radius generally increases down groups and decreases across periods.
• Ionization energy generally increases across a period and decreases down a group.
• Electronegativity generally increases toward the upper-right of the periodic table.
• Metallic character generally increases toward the lower-left.
• Periodic position can help predict whether an element will gain, lose, or share electrons.
• Ion charges can be used to predict the formulas of unfamiliar ionic compounds.
• Periodic patterns can also help estimate physical properties such as melting point, boiling point, state, and metallic behaviour when a clear family trend exists.
• Properties can provide clues about possible technological applications.
• Predictions become stronger when several pieces of evidence agree.
• Periodic trends are general patterns rather than absolute rules, so unusual elements may require additional evidence.
• Mendeleev demonstrated the predictive power of periodic patterns by successfully estimating properties of elements that had not yet been discovered.
• For unfamiliar-element problems, use:

group + period → electron arrangement → periodic trends → metal/non-metal character → ion formation → bonding → reactivity → physical properties → likely behaviour and applications.