Metallic character, reactivity and density

How easily electrons leave

Metallic character is an element's tendency to lose electrons and form positive ions. It is tied to ionization energy: the less energy needed to remove an electron, the more metallic the element.

Down a group, ionization energy falls, so elements lose electrons more easily: metallic character and reactivity both increase. Across a period, nuclear charge rises and holds the outer electrons more strongly, so metallic character decreases from left to right.

Reactivity across a period first falls and then rises. Sodium is very reactive, magnesium less so, and aluminium and silicon less again, because more valence electrons are harder to lose. Further right, among the non-metals, reactivity rises again as atoms gain electrons more readily.

Density is mass per unit volume, in g/cm³. It generally increases down a group and changes less across a period. Table 8.11 gives group 1: Li 0.53, Na 0.97, K 0.89, Rb 1.63 and Cs 1.879 g/cm³. Potassium breaks the pattern slightly, which is why the book says 'generally'.

Low ionization energy means high metallic character: most metallic at the bottom left.

Key terms

Metallic character

Comparing metals.

Reactivity

Predicting reactions.

Valence electrons

Deciding how ions form.

Density

Comparing heaviness.

Short questions with model answers

1. Q1. In what order does metallic character change in group 2? (Exercise viii, p. 117)

   • Order down group 2
   • Trend

   Ba > Sr > Ca > Mg.

2. Q2. An element is in period 3, group 1. Is it a metal, and what is its oxide like? (based on Exercise 1, p. 115)

   • Identify
   • Properties

   A metal; electropositive; its oxide Na₂O is basic; it is a solid.

Common mistakes

• ✗ Metallic character increases from left to right.

  ✓ It decreases: nuclear charge holds electrons more tightly (p. 114).

• ✗ Reactivity falls steadily all the way across a period.

  ✓ It falls, then rises again towards the non-metals (p. 114).

• ✗ Density rises with every step down group 1.

  ✓ Generally, but K (0.89) is less dense than Na (0.97) (Table 8.11).

MCQs

1. 1. Metallic character in group 2 changes as: (Exercise viii, p. 117)

   1. (a) Mg > Ca > Ba > Sr
   2. (b) Sr > Ba > Ca > Mg
   3. (c) Mg > Sr > Ca > Ba
   4. (d) Ba > Sr > Ca > Mg
   Show answer

   (d) Increases down the group (p. 114).

2. 2. Metallic character is the tendency to:

   1. (a) Gain electrons
   2. (b) Lose electrons and form cations
   3. (c) Share electrons
   4. (d) Form anions
   Show answer

   (b) p. 114.

3. 3. Which group 1 element in Table 8.11 is densest?

   1. (a) Li
   2. (b) Na
   3. (c) K
   4. (d) Cs
   Show answer

   (d) Cs 1.879 g/cm³.

4. 4. Across a period from Na to Si, reactivity:

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

   (b) More valence electrons are harder to lose (p. 114).

5. 5. Where would you expect the element with the largest atomic radius? (Exercise 2, p. 115)

   1. (a) Group 1, last period
   2. (b) Group 17, period 2
   3. (c) Group 18, period 1
   4. (d) Group 2, period 2
   Show answer

   (a) Radius grows down a group and shrinks across a period.

Quick revision

• Metallic character is the tendency to lose electrons; it follows low ionization energy.
• Metallic character and metal reactivity increase down a group and decrease across a period.
• Across a period reactivity falls, then rises again among the non-metals.
• Density generally increases down a group (K is an exception in Table 8.11).