Atoms, Elements and Compounds

Lessons

Senior 1 · Chemistry

Atoms, Elements and Compounds

5 lessons

Lesson 4 of 5

Why Atoms Gain or Lose Electrons

25 min

Lazy gases and dangerous metals

The glowing tubes in the photo below contain the noble gases. Pass electricity through them and they glow, but they do not burn, rust or react with anything. That is why argon fills ordinary light bulbs and welders use it as a shield gas: it protects hot metal from the air.

Sodium is the opposite. A fresh piece of sodium cut with a knife shines like silver for a few seconds, then turns dull white as it reacts with the air. It is stored under paraffin oil so that neither air nor water can reach it. Chlorine, a poisonous yellow-green gas, is just as eager to react. What makes one element so calm and another so reactive?

The secret is a full outer shell

Compare the configurations from Lesson 3. Helium (2), neon (2,8) and argon (2,8,8) all have a full outer shell: 2 electrons for helium, 8 for the others. A full outer shell is a very stable (low-energy) arrangement, so these atoms have no tendency to react.

Every other atom has an incomplete outer shell. In chemical reactions atoms lose, gain or share electrons until they have the same arrangement as the nearest noble gas. This idea is called the octet rule.

Definition

Octet and duplet: a full outer shell of 8 electrons is an octet; the full first shell of 2 electrons (as in helium) is a duplet. Atoms tend to react until they have an octet, or a duplet if they have only one shell. Example: sodium, 2,8,1, reaches the octet 2,8 by losing one electron.

Definition

Ion: an atom (or group of atoms) that has lost or gained electrons, so that it carries an electric charge. A positive ion is a cation; a negative ion is an anion. Examples: $\mathrm{Na^+}$ (cation), $\mathrm{Cl^-}$ (anion).

Lose or gain? It depends on the outer shell

An atom with 1, 2 or 3 outer electrons finds it easier to lose them than to gain 5, 6 or 7 more. It then has more protons than electrons and becomes a positive ion. Metals behave like this.
An atom with 5, 6 or 7 outer electrons finds it easier to gain 3, 2 or 1 more. It then has more electrons than protons and becomes a negative ion. Non-metals behave like this.
An atom with 4 outer electrons, such as carbon or silicon, usually shares electrons instead. You will study sharing (covalent bonding) in S2.

The number of protons never changes in a chemical reaction, so the charge on an ion is simply

$\text{charge} = \text{number of protons} - \text{number of electrons}$

Figure 4.1: Forming ions. Sodium loses its single outer electron; chlorine gains one. Both end with the configuration of a noble gas. The square brackets and the sign show the charge on the whole ion.

We show these changes with short equations, in which $\mathrm{e^-}$ stands for an electron:

$\mathrm{Na \rightarrow Na^+ + e^-} \qquad \mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$

$\mathrm{Cl + e^- \rightarrow Cl^-} \qquad \mathrm{O + 2e^- \rightarrow O^{2-}}$

The name of a simple negative ion ends in -ide: chloride, oxide, sulfide. A positive metal ion keeps the name of the metal: a sodium ion, a calcium ion.

Table 4.1: Ions formed by some of the first 20 elements

Element	Atom	Electrons	Ion	Ion's configuration	Same as
Lithium	2,1	loses 1	$\mathrm{Li^+}$	2	helium
Sodium	2,8,1	loses 1	$\mathrm{Na^+}$	2,8	neon
Potassium	2,8,8,1	loses 1	$\mathrm{K^+}$	2,8,8	argon
Magnesium	2,8,2	loses 2	$\mathrm{Mg^{2+}}$	2,8	neon
Calcium	2,8,8,2	loses 2	$\mathrm{Ca^{2+}}$	2,8,8	argon
Aluminium	2,8,3	loses 3	$\mathrm{Al^{3+}}$	2,8	neon
Nitrogen	2,5	gains 3	$\mathrm{N^{3-}}$ (nitride)	2,8	neon
Oxygen	2,6	gains 2	$\mathrm{O^{2-}}$ (oxide)	2,8	neon
Fluorine	2,7	gains 1	$\mathrm{F^-}$ (fluoride)	2,8	neon
Sulfur	2,8,6	gains 2	$\mathrm{S^{2-}}$ (sulfide)	2,8,8	argon
Chlorine	2,8,7	gains 1	$\mathrm{Cl^-}$ (chloride)	2,8,8	argon

Definition

Valency: the combining power of an element: the number of electrons one of its atoms loses, gains or shares when it forms a compound. Examples: sodium 1, magnesium 2, aluminium 3, carbon 4, nitrogen 3, oxygen 2, chlorine 1. The noble gases have valency 0.

Worked example

Problem. Magnesium burns in air with a dazzling white flame to form magnesium oxide. Describe what happens to the electrons of a magnesium atom, $\mathrm{^{24}_{12}Mg}$ , and give the charge and configuration of the ion formed.

Solution. Magnesium has 12 electrons: 2,8,2. Losing its 2 outer electrons is easier than gaining 6, so $\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$ .

	Protons	Electrons	Charge	Configuration
Mg atom	12	12	$12 - 12 = 0$	2,8,2
$\mathrm{Mg^{2+}}$ ion	12	10	$12 - 10 = + 2$	2,8 (like neon)

The number of neutrons, $24 - 12 = 12$ , does not change either.

Worked example

Problem. In the same reaction each oxygen atom, $\mathrm{^{16}_{8}O}$ , takes the electrons that magnesium gives away. Find the charge and configuration of the oxide ion.

Solution. Oxygen is 2,6. It needs 2 more electrons for an octet: $\mathrm{O + 2e^- \rightarrow O^{2-}}$ . Protons 8, electrons $8 + 2 = 10$ , charge $8 - 10 = - 2$ , configuration 2,8. One $\mathrm{Mg^{2+}}$ balances one $\mathrm{O^{2-}}$ , so the formula of magnesium oxide is $\mathrm{MgO}$ .

Worked example

Problem. Identify each particle and give its symbol with its charge: (a) 13 protons, 14 neutrons, 10 electrons; (b) 16 protons, 16 neutrons, 18 electrons; (c) 19 protons, 20 neutrons, 18 electrons.

Solution. The protons identify the element; protons minus electrons gives the charge.

(a) 13 protons: aluminium; charge $13 - 10 = + 3$ : $\mathrm{Al^{3+}}$ .
(b) 16 protons: sulfur; charge $16 - 18 = - 2$ : the sulfide ion, $\mathrm{S^{2-}}$ .
(c) 19 protons: potassium; charge $19 - 18 = + 1$ : $\mathrm{K^+}$ .

Warning

Common mistake: saying that a positive ion has "gained protons". Protons stay locked in the nucleus during chemical reactions. A positive ion has lost electrons; a negative ion has gained electrons.

Tip

Metals lose electrons and become positive; non-metals gain electrons and become negative. A memory hook: "a cation is paws-itive".

FuseSchool: What Are Ions (5 min). Watch how the charge is worked out by comparing protons with electrons.

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Practice

Why are the noble gases unreactive?
Define (a) an ion, (b) a cation, (c) an anion.
Write equations for the formation of (a) $\mathrm{K^+}$ , (b) $\mathrm{Al^{3+}}$ , (c) $\mathrm{F^-}$ , (d) $\mathrm{S^{2-}}$ .
Give the configuration of (a) $\mathrm{Ca^{2+}}$ , (b) $\mathrm{N^{3-}}$ .
How many protons and electrons are in (a) $\mathrm{Li^+}$ , (b) $\mathrm{O^{2-}}$ ?
Explain why aluminium forms $\mathrm{Al^{3+}}$ and not $\mathrm{Al^{5-}}$ .
Which ions in Table 4.1 have the same configuration as neon? Why can they still be told apart?

Show answersHide answers

Their outer shells are full (2 for helium, 8 for the others), which is a very stable arrangement, so they have no tendency to lose, gain or share electrons.
(a) An atom or group of atoms that has lost or gained electrons and carries a charge. (b) A positive ion. (c) A negative ion.
(a) $\mathrm{K \rightarrow K^+ + e^-}$ (b) $\mathrm{Al \rightarrow Al^{3+} + 3e^-}$ (c) $\mathrm{F + e^- \rightarrow F^-}$ (d) $\mathrm{S + 2e^- \rightarrow S^{2-}}$ .
(a) 2,8,8 (b) 2,8.
(a) 3 protons, 2 electrons. (b) 8 protons, 10 electrons.
Aluminium is 2,8,3. Losing 3 electrons gives the stable 2,8 and needs far less energy than gaining 5 to make 2,8,8.
$\mathrm{Na^+}$ , $\mathrm{Mg^{2+}}$ , $\mathrm{Al^{3+}}$ , $\mathrm{N^{3-}}$ , $\mathrm{O^{2-}}$ , $\mathrm{F^-}$ . They all have 10 electrons, but different numbers of protons (11, 12, 13, 7, 8, 9), so they have different charges and belong to different elements.

Sources and credits

Image: Edelgase in Entladungsroehren.jpg by Alchemist-hp (www.pse-mendelejew.de), via Wikimedia Commons, licensed under CC BY-SA 2.0 DE.
Video: What Are Ions, FuseSchool, embedded from YouTube.
Written by Ganzaa. The ion diagram, table, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Lazy gases and dangerous metals

2The secret is a full outer shell

3Lose or gain? It depends on the outer shell

Lazy gases and dangerous metals

The secret is a full outer shell

Lose or gain? It depends on the outer shell