Gaining and Losing Electrons to Form Ions

Introduction

Table salt is not made of sodium atoms and chlorine atoms. It is made of ions — sodium particles that have lost an electron and chloride particles that have gained one. Ions are everywhere: in seawater, in your blood, in rocks and in batteries. The periodic table lets you predict the charge of the common ion of most main-group elements in seconds, just by looking at the group number. This page shows how.

Core explanation

What an ion is. An atom is neutral because it has equal numbers of protons (positive) and electrons (negative). If it loses or gains electrons, the numbers are no longer equal and the particle becomes charged. This charged particle is an ion . The number of protons never changes when an ion forms; only the electrons change.

Losing electrons: positive ions. Atoms with 1, 2 or 3 valence electrons — mostly metals — find it easier to lose those electrons than to gain enough to fill the shell. Losing electrons leaves more protons than electrons, so the ion is positive (a cation).

- Sodium, 2,8,1, loses 1 electron → Na⁺, arrangement 2,8 - Magnesium, 2,8,2, loses 2 electrons → Mg²⁺, arrangement 2,8 - Aluminium, 2,8,3, loses 3 electrons → Al³⁺, arrangement 2,8

Gaining electrons: negative ions. Atoms with 5, 6 or 7 valence electrons — non-metals — find it easier to gain a few electrons than to lose many. Gaining electrons gives more electrons than protons, so the ion is negative (an anion).

- Chlorine, 2,8,7, gains 1 electron → Cl⁻, arrangement 2,8,8 - Oxygen, 2,6, gains 2 electrons → O²⁻, arrangement 2,8 - Nitrogen, 2,5, gains 3 electrons → N³⁻, arrangement 2,8

In each case the ion ends up with a full outer shell, the same arrangement as a noble gas.

The pattern across the table.

Group 1 2 3 4 5 6 7 0 --- --- --- --- --- --- --- --- --- Valence electrons 1 2 3 4 5 6 7 full Common ion charge 1+ 2+ 3+ usually none 3− 2− 1− none

For Groups 1 to 3, the charge equals the group number (positive). For Groups 5 to 7, the charge is 8 minus the group number (negative). Group 4 elements such as carbon and silicon would need to lose or gain four electrons, which takes too much energy, so they usually share electrons instead of forming simple ions. Group 0 elements already have full shells and form no ions.

Naming. Positive metal ions keep the element's name: sodium ion. Simple negative ions change the ending to "-ide": chloride, oxide, nitride, sulfide.

Step-by-step reasoning

To predict the ion formed by sulfur:

1. Sulfur is in Group 6, arrangement 2,8,6. 2. Gaining 2 electrons is easier than losing 6. 3. After gaining 2 electrons: 16 protons and 18 electrons, so charge = 2−. 4. The ion is S²⁻, the sulfide ion, arrangement 2,8,8.

Visual explanation

Draw a sodium atom with one electron on its outer ring and an arrow carrying that electron across to a chlorine atom with a gap on its outer ring. After the transfer, draw both particles in square brackets with their charges, [Na]⁺ and [Cl]⁻, each with a complete outer ring. The interactive periodic table can colour each main group by the charge of its common ion.

Real-world analogy

Think of a bus with seats for eight. If only one or two passengers are on the upper deck, it is easier for them to get off and move downstairs than to fill all the empty seats. If seven are already seated, it is easier to pick up one more passenger. Atoms "choose" the smaller change in the same way.

Real-world example

Sports drinks contain dissolved sodium ions (Na⁺), potassium ions (K⁺) and chloride ions (Cl⁻) to replace those lost in sweat. These ions carry electrical signals in nerves and muscles. Calcium ions (Ca²⁺), from Group 2, are needed for strong bones and teeth.

Why?

Why does losing electrons give a positive charge? The protons stay in the nucleus, so removing negative electrons leaves an excess of positive charge. Sodium keeps 11 protons but has only 10 electrons, giving an overall charge of +1.

Common misconception

"A positive ion forms when an atom gains protons." Ions form only by losing or gaining electrons. The number of protons is fixed for each element; changing it would turn the atom into a different element.

Worked example

Question: Calcium has atomic number 20. Predict the charge on its ion and give the numbers of protons and electrons in the ion.

Reasoning: Calcium is 2,8,8,2, in Group 2. It loses 2 electrons to reach 2,8,8. Protons remain 20; electrons become 18.

Answer: Ca²⁺, with 20 protons and 18 electrons.

Quick check

1. What is the charge on the common ion of a Group 7 element? Answer: 1−, because the atom gains one electron to fill its outer shell.

Exam focus

Be ready to predict ion charges from group numbers and to explain them in terms of electrons lost or gained. Show the electron arrangement before and after, and count protons and electrons to justify the charge. Remember that metals form positive ions and non-metals form negative ions.

Advanced insight

The energy needed to remove electrons rises steeply once a noble-gas arrangement is reached. Removing the first electron from sodium is relatively easy, but removing a second electron from the full 2,8 shell needs roughly nine times as much energy. This sharp jump is why sodium forms Na⁺ and never Na²⁺ in ordinary chemistry.

Summary

Ions form when atoms lose or gain valence electrons. Metals in Groups 1, 2 and 3 lose their valence electrons to form 1+, 2+ and 3+ ions. Non-metals in Groups 5, 6 and 7 gain electrons to form 3−, 2− and 1− ions. In each case, the ion has a full outer shell like a noble gas. Only electrons change; the number of protons stays the same.

Practice questions

1. Explain, in terms of electrons, how a magnesium atom becomes a magnesium ion. Answer: The magnesium atom (2,8,2) loses its two outer electrons, giving Mg²⁺ with the arrangement 2,8. 2. Predict the charge on the common ion of potassium and of oxygen. Answer: Potassium, Group 1, forms K⁺; oxygen, Group 6, forms O²⁻. 3. A fluoride ion has 9 protons. How many electrons does it have, and what is its charge? Answer: 10 electrons; charge 1− (F⁻). 4. Why does carbon not usually form a simple ion? Answer: It has four valence electrons, and losing or gaining four electrons needs too much energy, so it shares electrons instead. 5. Name the ion formed by nitrogen and give its formula. Answer: The nitride ion, N³⁻.