Electropositive metals, electronegative non-metals

Givers and takers

Metals tend to lose electrons and form positive ions, called cations. This is their electropositive character. The more easily a metal loses electrons, the more reactive it is.

Alkali metals such as sodium and potassium are highly electropositive. They react vigorously with water and halogens, giving hydroxides and halides, and with acids, giving salts and hydrogen.

Alkaline earth metals such as magnesium and calcium lose their outer electrons less easily, so they react less vigorously. Aluminium is also highly electropositive and reacts readily with mineral acids.

Non-metals attract electrons. They gain electrons and form negative ions, called anions, so they are electronegative. Your book names fluorine as the most electronegative element.

Non-metals form ionic bonds with metals and covalent molecules with other non-metals.

Key terms

Cation

What a metal atom becomes after losing electrons.

Anion

What a non-metal atom becomes after gaining electrons.

Electropositive

Describing metals and their reactivity.

Electronegative

Describing non-metals.

Short questions with model answers

1. Q1. Which type of elements lose outer electrons easily, and which gain them? (Short Q i, p. 50)

   • Metals: few outer electrons, held loosely
   • Non-metals: nearly full outer shells

   Metals (electropositive) lose electrons; non-metals (electronegative) gain them.

2. Q2. Compare how sodium and magnesium react with water. (p. 43)

   • Na: alkali metal, loses 1 electron easily
   • Mg: alkaline earth metal, loses electrons less easily

   Sodium reacts more vigorously because it is more electropositive.

Common mistakes

• ✗ A cation is a negative ion.

  ✓ A cation is positive (from a metal); an anion is negative (from a non-metal).

• ✗ Magnesium reacts with water more vigorously than sodium because it loses two electrons.

  ✓ Losing electrons less easily makes it less electropositive and less vigorous (p. 43).

• ✗ Non-metals can bond only with metals.

  ✓ They also join other non-metals to form many molecular substances.

MCQs

1. 1. Which element is the most electronegative?

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

   (d) p. 43: fluorine is the most electronegative element in the periodic table.

2. 2. Sodium reacts with water to give:

   1. (a) A hydroxide and hydrogen
   2. (b) An oxide only
   3. (c) Chlorine gas
   4. (d) No reaction
   Show answer

   (a) p. 43: sodium and potassium react vigorously with water to give their hydroxides (and hydrogen).

3. 3. Why is magnesium less electropositive than sodium?

   1. (a) It is a non-metal
   2. (b) It loses its outer electrons less easily
   3. (c) It gains electrons
   4. (d) It has no outer electrons
   Show answer

   (b) p. 43: alkaline earth metals lose their outer electrons less easily than alkali metals.

4. 4. A non-metal atom that gains electrons becomes:

   1. (a) A cation
   2. (b) A neutral molecule
   3. (c) An anion
   4. (d) A metal
   Show answer

   (c) p. 43: non-metals gain electrons and become negatively charged ions called anions.

5. 5. When a non-metal combines with another non-metal, it forms:

   1. (a) Molecular substances
   2. (b) Ionic crystals only
   3. (c) Metals
   4. (d) Nothing: they never combine
   Show answer

   (a) p. 43: non-metals also combine with other non-metals to form a wide variety of molecular substances.

Quick revision

• Metals are electropositive: they lose electrons and form cations.
• Alkali metals (Na, K) are more electropositive and reactive than alkaline earth metals (Mg, Ca).
• Non-metals are electronegative: they gain electrons and form anions; fluorine is the most electronegative.