Ion Formation

Atoms can gain or lose electrons to form ions.

An ion is a charged particle formed when an atom gains or loses one or more electrons.

Atoms form ions because gaining or losing electrons can give them a more stable outer electron arrangement, often similar to that of a noble gas.

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Why Do Atoms Form Ions?

Atoms are most stable when their outer electron shell is full.

The noble gases already have full outer shells, so they are generally very unreactive.

Other atoms may gain or lose electrons to achieve a more stable arrangement.

For many main-group elements:

• atoms with only a few outer-shell electrons tend to lose electrons
• atoms with nearly full outer shells tend to gain electrons

This often produces the same electron arrangement as a nearby noble gas.

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Valence Electrons

The electrons in the outermost occupied shell are called valence electrons.

Valence electrons are especially important because they determine how an atom behaves chemically.

For main-group elements, the group number helps us predict the number of valence electrons.

For introductory chemistry, these are sometimes described as Groups I–VIII.

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Losing Electrons

When an atom loses electrons, it has more protons than electrons.

This gives the particle a positive charge.

A positively charged ion is called a: cation

For example, a sodium atom has: 11 protons

and: 11 electrons

Its electron arrangement is: 2, 8, 1

If sodium loses its one outer electron: Na → Na⁺ + e^-

the resulting sodium ion has: 11 protons

but only: 10 electrons

Its charge is therefore: 11(+1) + 10(-1) = +1

so: Na⁺ is formed.

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Gaining Electrons

When an atom gains electrons, it has more electrons than protons.

This gives the particle a negative charge.

A negatively charged ion is called an: anion

For example, chlorine has the electron arrangement: 2, 8, 7

It has seven outer-shell electrons.

Chlorine can gain one electron: Cl + e^- → Cl^-

The chloride ion now has the electron arrangement: 2, 8, 8

and a charge of: -1

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Cations and Anions

The two main types of ions are:

A useful way to remember this is:

Cation = positive ion

Anion = negative ion

The nucleus itself does not normally change during ion formation.

Only the number of electrons changes.

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Ion Formation and the Periodic Table

The periodic table can help us predict which ions many elements form.

For the main-group elements:

Group 14 is more complicated because gaining or losing four electrons is usually not favourable for simple monatomic ions.

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Group 1 Ions

Group 1 elements have one valence electron.

Examples include:

• lithium
• sodium
• potassium

They usually lose one electron.

Lithium

Electron arrangement: 2, 1

Ion formation: Li → Li⁺ + e^-

Ion: Li⁺

Sodium

2, 8, 1 → 2, 8

Ion: Na⁺

Potassium

2, 8, 8, 1 → 2, 8, 8

Ion: K⁺

Therefore, Group 1 elements generally form: 1+ ions.

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Group 2 Ions

Group 2 elements have two outer-shell electrons.

They usually lose both.

For example, magnesium has: 2, 8, 2

It loses two electrons: Mg → Mg²⁺ + 2e^-

The magnesium ion has: 2, 8

Therefore: Mg²⁺ is formed.

Other common Group 2 ions include: Ca²⁺

and: Ba²⁺

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Group 13 Ions

Many Group 13 metals have three valence electrons and can lose all three.

For example, aluminium has: 2, 8, 3

It loses three electrons: Al → Al³⁺ + 3e^-

The aluminium ion is: Al³⁺

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Group 17 Ions

Group 17 elements have seven valence electrons.

They need only one more electron to fill their outer shell.

Therefore, they usually gain one electron and form: 1- ions.

Examples include:

F^-

Cl^-

Br^-

I^-

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Group 16 Ions

Group 16 elements have six valence electrons.

They can gain two electrons.

For example, oxygen has: 2, 6

It gains two electrons: O + 2e^- → O^2-

The oxide ion has: 2, 8

Therefore: O^2- is formed.

Sulfur similarly forms: S^2-

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Group 15 Ions

Group 15 elements have five valence electrons.

They may gain three electrons to achieve a full outer shell.

For example, nitrogen can form: N^3-

This ion is called the nitride ion.

Phosphorus can form: P^3- called the phosphide ion.

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Why Do Metals Usually Form Positive Ions?

Metals are generally found on the left side of the periodic table.

They often have:

• one
• two
• or three

valence electrons.

It is usually easier for them to lose these few electrons than to gain enough electrons to fill the shell.

Therefore: Metals usually form cations

For example:

Na⁺

Mg²⁺

Al³⁺

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Why Do Non-Metals Usually Form Negative Ions?

Many non-metals are located on the right side of the periodic table.

They often have:

• five
• six
• or seven

valence electrons.

It is usually easier for them to gain a small number of electrons than to lose most of their outer shell.

Therefore: Non-metals often form anions

Examples include:

N^3-

O^2-

Cl^-

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Electron Arrangements Before and After Ion Formation

Ion formation can be understood clearly by comparing electron arrangements.

A major pattern appears:

$\boxed{\text{Many ions have full outer electron shells}}$

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How Ion Charges Are Determined

Ion charge depends on the difference between:

• number of protons
• number of electrons

Remember: charge = protons - electrons

For example, magnesium has: 12 protons

A magnesium ion has: 10 electrons

Therefore: 12 - 10 = +2

so: Mg²⁺

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Worked Example: Predict the Calcium Ion

Calcium is in Group 2.

Its electron arrangement is: 2, 8, 8, 2

It has two valence electrons.

Calcium loses both: Ca → Ca²⁺ + 2e^-

The ion has: 2, 8, 8

Therefore: Ca²⁺

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Worked Example: Predict the Sulfur Ion

Sulfur is in Group 16.

It has six valence electrons.

It needs two additional electrons to fill its outer shell.

Therefore:

$\text{S}+2e^-\rightarrow\text{S}^{2-}$

The ion formed is:

$\boxed{\text{S}^{2-}}$

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Worked Example: Predict the Aluminium Ion

Aluminium is in Group 13.

It has three valence electrons.

It loses all three:

$\text{Al}\rightarrow\text{Al}^{3+}+3e^-$

Therefore:

$\boxed{\text{Al}^{3+}}$

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Predicting Ion Charge from Group Number

For many main-group elements, you can use this simple pattern:

Metals

Group 1:

$\boxed{+1}$

Group 2:

$\boxed{+2}$

Group 13:

$\boxed{+3}$

Non-Metals

Group 15:

$\boxed{-3}$

Group 16:

$\boxed{-2}$

Group 17:

$\boxed{-1}$

Notice that the non-metal ion charge can often be predicted by asking:

How many electrons are needed to reach eight outer-shell electrons?

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Noble Gases

Group 18 elements already have full outer electron shells.

Examples include:

• helium
• neon
• argon

Because they are already stable, they generally do not need to gain or lose electrons.

Therefore, noble gases rarely form simple ions.

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What About Transition Metals?

Transition metals are more complicated.

They can sometimes form ions with different charges.

For example, iron can form:

$\text{Fe}^{2+}$

or:

$\text{Fe}^{3+}$

Copper commonly forms:

$\text{Cu}^+$

or:

$\text{Cu}^{2+}$

Because their charges cannot always be predicted from the group number alone, the charge may be given in a compound name.

For example:

iron(III) oxide

The Roman numeral III tells us that iron has a charge of:

$\boxed{3+}$

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Naming Simple Anions

When many non-metals form negative ions, the ending of the element name changes to -ide.

Examples:

Positive metal ions usually keep the element name.

For example:

$\text{Na}^+$

is the sodium ion.

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Ion Formation and Ionic Bonding

Ion formation is the first step in understanding ionic bonding.

Consider sodium and chlorine.

Sodium has:

$2,8,1$

Chlorine has:

$2,8,7$

Sodium can transfer one electron to chlorine.

This produces:

$\text{Na}^+$

and:

$\text{Cl}^-$

The oppositely charged ions then attract each other because of electrostatic attraction.

This forms an ionic compound:

$\boxed{\text{NaCl}}$

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Another Example: Magnesium and Oxygen

Magnesium has:

$2,8,2$

Oxygen has:

$2,6$

Magnesium loses two electrons:

$\text{Mg}\rightarrow\text{Mg}^{2+}+2e^-$

Oxygen gains two electrons:

$\text{O}+2e^-\rightarrow\text{O}^{2-}$

The ions formed are:

$\text{Mg}^{2+}$

and:

$\text{O}^{2-}$

These oppositely charged ions attract to form:

$\boxed{\text{MgO}}$

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Atoms and Ions Are Different

An atom is electrically neutral because it contains equal numbers of:

$\text{protons and electrons}$

An ion is charged because the numbers are no longer equal.

For example:

Sodium Atom

$11p,\quad11e^-$

Charge:

$0$

Sodium Ion

$11p,\quad10e^-$

Charge:

$+1$

The number of protons has not changed.

Only the number of electrons has changed.

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Do Atoms Gain or Lose Protons?

No.

In ordinary chemical reactions, atoms gain or lose electrons, not protons.

Changing the number of protons would change the identity of the element.

For example:

• 11 protons = sodium
• 12 protons = magnesium

Ion formation does not change one element into another.

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Cation or Anion?

Consider:

$\text{X}^{2+}$

The positive charge means the atom has lost two electrons.

Therefore it is a:

$\boxed{\text{cation}}$

Now consider:

$\text{Y}^{3-}$

The negative charge means the atom has gained three electrons.

Therefore it is an:

$\boxed{\text{anion}}$

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Using the Periodic Table to Predict an Unknown Ion

Suppose element X is in Group 2.

We can predict:

• 2 valence electrons
• loses 2 electrons
• forms a$2+$ion

Therefore:

$\boxed{\text{X}^{2+}}$

Now suppose element Y is in Group 17.

We predict:

• 7 valence electrons
• gains 1 electron
• forms a$1-$ion

Therefore:

$\boxed{\text{Y}^-}$

This means the periodic table allows us to make predictions even if we are unfamiliar with the particular element.

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A Useful Pattern

For many main-group elements:

$\boxed{\text{Group position}\rightarrow\text{valence electrons}\rightarrow\text{ion charge}}$

For example:

Magnesium

Group 2

$\downarrow$

2 valence electrons

$\downarrow$

loses 2 electrons

$\downarrow$$\boxed{\text{Mg}^{2+}}$

Chlorine

Group 17

$\downarrow$

7 valence electrons

$\downarrow$

gains 1 electron

$\downarrow$$\boxed{\text{Cl}^-}$

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Common Ions to Know

Some particularly important ions include:

Learning these patterns makes predicting ionic compounds much easier.

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Common Misconceptions

Positive ions have gained protons.

Incorrect. Positive ions form because atoms lose electrons.

Negative ions have lost electrons.

Incorrect. Negative ions form because atoms gain electrons.

The nucleus changes when an ion forms.

It does not. The number of protons normally remains unchanged.

All elements form ions with only one possible charge.

Not always. Many transition metals can form several different ions.

An ion is a different element.

It is still the same element because it has the same number of protons.

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Did You Know?

Ions are essential in both living organisms and technology.

For example, ions such as:

$\text{Na}^+,\quad\text{K}^+,\quad\text{Ca}^{2+},\quad\text{Cl}^-$

are involved in:

• nerve impulses
• muscle contraction
• fluid balance
• cellular communication

Ions are also important in:

• batteries
• electrolysis
• water treatment
• fertilizers
• ionic compounds

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Key Terms

Ion – A charged particle formed when an atom gains or loses electrons.

Cation – A positively charged ion.

Anion – A negatively charged ion.

Valence electron – An electron in the outermost occupied shell.

Electron arrangement – The distribution of electrons among electron shells.

Ion charge – The electrical charge produced when the number of electrons differs from the number of protons.

Noble gas configuration – A stable electron arrangement with a full outer shell.

Electrostatic attraction – Attraction between particles with opposite electrical charges.

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Key Takeaways

• Atoms form ions by gaining or losing electrons.
• Ion formation often produces a more stable full outer electron shell.
• Metals generally lose electrons and form positive ions called cations.
• Non-metals generally gain electrons and form negative ions called anions.
• The number of protons does not change during ordinary ion formation.
• Group 1 elements generally form:

$\boxed{1+}$

ions.

• Group 2 elements generally form:

$\boxed{2+}$

ions.

• Group 13 elements commonly form:

$\boxed{3+}$

ions.

• Group 15 elements can form:

$\boxed{3-}$

ions.

• Group 16 elements commonly form:

$\boxed{2-}$

ions.

• Group 17 elements commonly form:

$\boxed{1-}$

ions.

• The periodic table allows us to predict valence electrons and common ion charges.
• Transition metals can form ions with more than one possible charge.
• Understanding ion formation is essential for explaining ionic bonding and ionic compounds.