STUDENT SPACE · TOPIC 4

Electron shells and ions

Write electronic configurations for atoms and ions of the first 20 elements.

The essentials

Count electrons before filling shells. Use the first-20-elements pattern where appropriate.
Positive ions form by losing electrons; negative ions form by gaining electrons. The nucleus stays unchanged.
Check charge using number of protons minus number of electrons.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

An atom has 3 protons and 3 electrons. It loses one electron. State its charge and remaining electron count.

Need a hint for Question 1?

The proton number stays the same.

Check Question 1

It has 2 electrons and charge 1+, because 3 − 2 = +1.

For ²³₁₁Na⁺, give protons, neutrons, electrons and electronic configuration.

Need a hint for Question 2?

Ion formation changes electrons, not the nucleus.

Check Question 2

11 protons, 12 neutrons and 10 electrons. Na⁺ has configuration 2,8.

Calcium has Z = 20. Give the configurations of its neutral atom and Ca²⁺ ion. Explain the change.

Need a hint for Question 3?

Remove the outer electrons from the neutral-atom arrangement.

Check Question 3

Ca atom: 2,8,8,2. Ca²⁺: 2,8,8. The atom loses its 2 outer electrons. Its proton number remains 20.

Use the matching chapter worksheet. Attempt independently, compare your reasoning with your partner, then bring unresolved questions to your teacher.

Change the number of neutrons and compare isotope mixtures.