Metals and non-metals (part II)
Francine Taylor-Campbell, Contributor

IMPORTANT POINTS

Non-metals:

• are generally found between groups five and seven of the periodic table
• form ions by gaining electrons and are said to be electronegative
• are oxidising agents as they react by gaining electrons N + n e == N^n-
• react with air or oxygen to form acidic oxides, which can dissolve in water to form acids
• do not react with dilute acids, water or steam
• form covalent compounds with other non-metals and ionic compounds with metals.

The reactivity of a non-metal is based upon how readily it gains electrons or its affinity for electrons. For example, in Group 7 of the periodic table, the halogens, the most reactive elements, are at the top of the group. This is because the outer shell is closer to the nucleus than the element at the bottom of the group, hence, its attracting power for electrons is greater. So, fluorine is more reactive and electronegative than chlorine, which is more reactive than bromine.

Practice questions

Q1. Use the list of elements to answer the questions that follow.

Bromine	Carbon	Chromium	Helium
Magnesium	Phosphorous	Sodium	Sulphur

Choose one element which

a. Forms a basic oxide

b. Has a high density and forms coloured compounds

c. Is composed of diatomic molecules

d. Is a liquid at room temperature and pressure

e. Exists as allotropes

Q2. The effect of heat on compounds of metals may be predicted from the position of the metals in the reactivity series.

a. How would the action of heat on potassium carbonate differ from the action of heat on copper (II) carbonate? Give the relevant equation(s).

b. When most nitrates are heated, one or two gases may be given off. From the nitrates of the metals sodium, calcium, magnesium and zinc, select one nitrate that (i) gives off one gas (ii) gives off two gases.

Give the equation of the reaction in (ii) above.

c. In the reactivity series, Metal X is placed below zinc but above copper

i. What happens when the hydroxide of copper is heated?

ii. What products would you expect to obtain when the hydroxide of X is heated in a dry test tube?

iii. Metal X forms the X2+ ion. Write a balanced equation, using X as the symbol for the metal, for the reaction that you would expect to take place when the hydroxide of X is heated in a dry test tube.

ANSWERS

1. a. Magnesium, chromium and sodium are all metals, hence, they react with oxygen to form basic oxides

b. Chromium, as it is a transition metal. Metals have high density and coloured compounds are formed by transition metals.

c. Bromine - the formula is Br2, that is, two atoms of bromine.

d. Bromine is a liquid at room temperature and pressure.

e. Carbon, phosphorous and sulphur exist as allotropes. Remember allotropes are different forms of an element in the same state having different physical properties but the same chemical properties. The allotropes of carbon are diamond, graphite and buckminsterfullerene (C60). Allotropes of phosphorous are white and red phosphorous and monoclinic and rhombic sulphur are allotropes of sulphur.

2. (a) Potasium carbonate is stable and not decomposed by heat, while copper (II) carbonate would be decomposed by heat.

K₂CO₃ (s) == K₂CO₃ (s) (stable to heat)

CuCO₃ == CuO (s) + CO₂ (g) (decomposed by heat)

(b) (i) Sodium nitrate gives off only one gas (oxygen) when heated.

(ii) Calcium, magnesium and zinc nitrates produce two gases (oxygen and nitrogen dioxide) when heated.

2Mg (NO₃)₂ (s) == 2MgO (s) + 4NO₂ (g) + O₂ (g)

(c) When copper hydroxide is heated it decomposes to form copper oxide and steam.

(ii) If the hydroxide of X is heated in a dry test tube, one would expect to obtain the oxide of X and steam.

(iii) If the ion of X is X₂+ then the formula of its hydroxide is X(OH)₂.

Therefore, X(OH)₂ (s) == XO (s) + H₂O (g)

Jeffrey Webster (right), a final-year chemistry student at the University of the West Indies, Mona, engages Professor Robert Lancashire, professor of computational chemistry, in a discussion. - Nathaniel Stewart/Freelance Photographer

Francine Taylor-Campbell is an independent contributor.