Group properties and the charge of ions

Same outer electrons, same behaviour

Chemical properties depend mostly on the outer electrons. Every element in a group has the same number of them, so the whole group reacts in similar ways.

Group 1 metals have one outer electron and lose it easily to form 1+ ions, so they are called electropositive. They react vigorously with water to give hydrogen and an alkali, their oxides dissolve to give alkalis, and they react with halogens to form halides. Reactivity increases down the group.

Group 2 metals lose both outer electrons to form 2+ ions, such as Mg²⁺, and this gets easier down the group as atoms get bigger. Their oxides are basic too: CaO and water give Ca(OH)₂.

Group 17, the halogens (s² p⁵), gain one electron to form 1− ions; they are very reactive non-metals that form salts with metals, and their reactivity decreases down the group. Group 16 (s² p⁴) accepts two electrons to form 2− ions.

So cations carry a charge equal to their number of outer electrons, and anions carry a charge equal to the electrons they need to complete an octet.

Metals on the left lose their outer electrons; non-metals on the right gain enough to reach eight.

Key terms

Electropositive

Group 1 and 2 metals.

Electronegative

Halogens, oxygen.

Halogen

Making salts with metals.

Dipositive ion

Group 2 metals.

Dinegative ion

Group 16 elements.

Short questions with model answers

1. Q1. Write the ions formed by aluminium (group 13) and sulphur (group 16).

   • Aluminium
   • Sulphur

   Al³⁺ and S²⁻.

2. Q2. Do you expect calcium to be more reactive than sodium? (Short question v, p. 117)

   • Electrons to lose
   • Ease

   No. Sodium is more reactive because it only has to lose one electron.

Common mistakes

• ✗ Halogens get more reactive down the group, like alkali metals.

  ✓ Halogen reactivity decreases down the group (p. 110).

• ✗ Group 17 elements form 17− ions.

  ✓ They need one electron for an octet: 1− (p. 110).

• ✗ Metals gain electrons to form negative ions.

  ✓ Metals lose electrons to form positive ions.

MCQs

1. 1. Group 2 elements form ions with charge:

   1. (a) 1+
   2. (b) 2+
   3. (c) 1−
   4. (d) 2−
   Show answer

   (b) They lose both outer electrons (p. 109).

2. 2. How many electrons can nitrogen accept in its outer shell? (Exercise v, p. 117)

   1. (a) 2
   2. (b) 3
   3. (c) 4
   4. (d) 5
   Show answer

   (b) ns² np³ needs three more for an octet.

3. 3. Sodium reacts with water to give:

   1. (a) NaOH and H₂
   2. (b) NaCl and O₂
   3. (c) Na₂O only
   4. (d) No reaction
   Show answer

   (a) p. 109.

4. 4. Down group 17, reactivity:

   1. (a) Increases
   2. (b) Decreases
   3. (c) Stays the same
   4. (d) Becomes zero
   Show answer

   (b) p. 110.

5. 5. Which is the most reactive element? (Exercise vi, p. 117)

   1. (a) Oxygen
   2. (b) Chlorine
   3. (c) Fluorine
   4. (d) Nitrogen
   Show answer

   (c) Top of group 17; halogen reactivity falls down the group.

Quick revision

• A group reacts alike because its members share the same outer electrons.
• Group 1 forms 1+, group 2 forms 2+; both get more reactive down the group.
• Group 17 forms 1−, group 16 forms 2−; halogens get less reactive down the group.