Notes

1. IONS
An ion is a charged particle formed when an atom gains or loses electrons from its outer shell. The inert
gases do not form ions because their outer shell already contain the maximum number of electrons
making these element stable or unreactive.
Atoms lose or gain electrons in order to get the stable electronic structure of the nearest noble gas.
There are two main types of ions:
i. Cations – positively charged ions
ii. Anions – negatively charged ions
Formation of Cations
A cation is formed when a metal atom loses the one, two or three electrons that were on its outer
shell. When this happens, the remaining particle becomes positively charged because the nucleus now
has more protons than there are electrons orbiting it. The charge on the cation formed is equal to the
number of electrons lost.
Example: Formation of sodium ion (cation)
Sodium atom Sodium ion
Na Na+
EC: 2, 8, 1 EC: 2,8
+
Sodium atom is neutral since it contains an Sodium cation carries a one positive (+)
equal number of protons and electron s charge because it has lost one outer electron.
(11P, 11E) It now has one more proton than electrons
(11P, 10E) It now has the stable EC of
Neon (the nearest noble gas)
Formation of Anions
An anion is formed when a non-metal atom gains one, two or three electrons on its outermost shell.
When this happens, the remaining particle becomes negatively charged because the nucleus now has
less protons than there are electrons orbiting it. The charge on the anion formed is equal to the
number of electrons gained
Example: Formation of oxygen ion (anion)
Oxygen atom Oxygen ion
O O2−
EC: 2,6 EC: 2, 8
2−
The oxygen atom is neutral because it contains an The oxygen ion carries a 2− charge because
equal number of protons and electrons (8P, 8e) it has gained 2 more outer electrons (8P, 10e).
It now has the stable EC of Neon (the nearest noble
gas)
11P
12N
11P
12N
23
11
8P
8N
16
8
8P
8N