Periods and Groups

The periodic table is arranged in a way that helps scientists understand and predict the properties of elements. An element’s position is not random. The row and column in which an element appears provide important information about its atomic structure and chemical behaviour.

Periods

The horizontal rows of the periodic table are called periods.

There are 7 periods on the modern periodic table.

• Period 1 contains hydrogen (H) and helium (He).
• Period 2 begins with lithium (Li) and ends with neon (Ne).
• Period 3 begins with sodium (Na) and ends with argon (Ar).
• Periods generally become longer as you move down the periodic table.

The period number tells us how many occupied electron shells an atom has.

For example:

Therefore, sodium is in Period 3 because its electrons occupy three electron shells.

Periods and Atomic Structure

As you move from left to right across a period, the number of protons and electrons increases.

For example, consider part of Period 3:

Na → Mg → Al → Si → P → S → Cl → Ar

Each element has one more proton than the element before it. For a neutral atom, it also has one more electron.

However, all of these elements have the same number of occupied electron shells: three.

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Groups

The vertical columns of the periodic table are called groups.

There are 18 groups in the modern periodic table.

Elements in the same group often have similar chemical properties because they have similar arrangements of valence electrons.

Valence electrons are the electrons in the outermost occupied electron shell. These electrons are especially important because they are involved in chemical bonding and reactions.

For many main-group elements, the group can therefore help us predict the number of valence electrons.

For example:

*Helium has 2 electrons in its only shell; the other Group 18 elements typically have 8 valence electrons.

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Why Do Elements in the Same Group Behave Similarly?

Chemical reactions usually involve an atom's outer electrons.

Elements in the same group have similar outer-electron arrangements, so they tend to react in similar ways.

For example, lithium, sodium and potassium are all Group 1 metals.

They each have one valence electron:

• lithium: 2, 1
• sodium: 2, 8, 1
• potassium: 2, 8, 8, 1

Because each atom has one outer electron, these elements tend to lose one electron during chemical reactions.

This is why Group 1 elements share many properties and form ions with a +1 charge.

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Position and Atomic Structure

An element's position on the periodic table gives us clues about its atomic structure.

For many of the elements students encounter first:

Period → number of occupied electron shells

Group → information about the valence electrons

Consider magnesium (Mg).

Magnesium has the electron arrangement:

2, 8, 2

This tells us:

• it has 3 occupied shells, so it is in Period 3
• it has 2 valence electrons, so it is in Group 2

Now consider chlorine (Cl):

2, 8, 7

This tells us:

• it has 3 occupied shells, so it is in Period 3
• it has 7 valence electrons, so it is in Group 17

This relationship allows us to connect the periodic table directly to the structure of atoms.

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Using Groups to Predict Properties

Groups can also help us make predictions about unfamiliar elements.

Group 1 – Alkali Metals

Examples include lithium, sodium and potassium.

They:

• are metals
• have 1 valence electron
• usually lose one electron
• form +1 ions
• are chemically reactive

Group 17 – Halogens

Examples include fluorine, chlorine and bromine.

They:

• are non-metals
• have 7 valence electrons
• usually gain one electron
• form −1 ions
• are reactive

Group 18 – Noble Gases

Examples include helium, neon and argon.

They:

• have full outer electron shells
• are very stable
• are generally very unreactive

Because elements within a group have related electron arrangements, knowing the group allows scientists to make useful predictions about an element's behaviour.

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Locating an Element

We can describe an element's position using both its period and group.

For example:

Calcium (Ca) is in Period 4, Group 2.

This immediately tells us that calcium:

• has four occupied electron shells
• has two valence electrons
• is a metal
• is likely to lose two electrons during reactions
• commonly forms a Ca²⁺ ion

Similarly, bromine (Br) is in Period 4, Group 17.

We can predict that bromine:

• has four occupied electron shells
• has seven valence electrons
• is a non-metal
• tends to gain one electron
• commonly forms a Br⁻ ion

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A Useful Way to Remember

Think of the periodic table like a large apartment building:

• Periods are the floors running horizontally.
• Groups are vertical stacks of apartments.
• Elements living in the same vertical group belong to the same "family" and tend to behave similarly.

So:

Across = Period

Down = Group

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Did You Know?

The periodic table is powerful because it can be used to predict properties of elements that have not yet been studied in detail.

In the 1800s, Dmitri Mendeleev deliberately left gaps in his periodic table for elements that had not yet been discovered. He predicted several of their properties based on the groups in which the missing elements should appear. Later discoveries closely matched many of his predictions.

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Key Terms

Period – A horizontal row on the periodic table.

Group – A vertical column on the periodic table.

Valence electron – An electron in the outermost occupied electron shell.

Electron shell – A region around the nucleus occupied by electrons.

Chemical property – A characteristic describing how a substance behaves during chemical reactions.

Periodic table – An arrangement of the elements according to atomic number and repeating patterns in their properties.

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Key Takeaways

• The horizontal rows of the periodic table are called periods.
• The vertical columns are called groups.
• The period number indicates the number of occupied electron shells.
• Elements in the same group have similar outer-electron arrangements.
• Similar valence-electron arrangements cause elements in the same group to have similar chemical properties.
• An element's position can therefore provide information about its atomic structure.
• Periods and groups can be used to locate elements and predict basic chemical properties.