4.6 - Elements in Period 3

Period 3 Elements and Uses

• Period 3 contains sodium, Na; magnesium, Mg; aluminium, Al; silicon, Si; phosphorus, P; sulphur, S; chlorine, Cl; and argon, Ar.
• Magnesium is used in lighters, aluminium in cans, phosphorus in fireworks and sulphur in fungicides.

Physical Trends Across Period 3

Across Period 3 from left to right:

• atomic radius decreases because the increasing proton number increases nuclear charge;
• nuclear attraction for electrons increases, so electronegativity increases;
• physical state changes from solid to gas;
• character changes from metals, through the metalloid silicon, to non-metals.

Classification	Elements
Metals	Na, Mg, Al
Metalloid	Si
Non-metals	P, S, Cl, Ar

Element	Atomic radius (nm)	Electronegativity
Na	0.186	0.9
Mg	0.160	1.2
Al	0.143	1.5
Si	0.118	1.8
P	0.110	2.1
S	0.104	2.5
Cl	0.100	3.0
Ar	0.094	—

Experiment 4.2: Properties of Period 3 Oxides

Aim: Study the change in chemical properties of oxides across Period 3.

Problem statement: How do the chemical properties of oxides change across Period 3?

Materials: $\mathrm{N}{\mathrm{a}}_2\mathrm{O}$, $\mathrm{MgO}$, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, $\mathrm{S}{\mathrm{O}}_2$ gas, $\mathrm{Si}{\mathrm{O}}_2$, distilled water, $2.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ $\mathrm{NaOH}$ and $2.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ $\mathrm{HN}{\mathrm{O}}_3$.

Apparatus: Test tubes and stoppers, test-tube holder, $10\mathrm{c}{\mathrm{m}}^3$ measuring cylinder, pH meter, Bunsen burner, glass rod and spatula.

Part A — reaction with water

• Hypothesis: Across Period 3, the oxides change from basic to acidic.
• Manipulated variable: type of Period 3 oxide; responding variable: change in oxide property; fixed variable: volume of water.

1. Add $10.0\mathrm{c}{\mathrm{m}}^3$ distilled water to half a spatula of $\mathrm{N}{\mathrm{a}}_2\mathrm{O}$ in a test tube and shake.
2. Measure the solution’s pH and record the observation.
3. Repeat with $\mathrm{MgO}$, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ and $\mathrm{S}{\mathrm{O}}_2$.

Part B — reaction with sodium hydroxide and nitric acid

1. Put $\frac{1}{4}$ spatula of $\mathrm{MgO}$ into each of two test tubes.
2. Add $5.0\mathrm{c}{\mathrm{m}}^3$ of $2.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ $\mathrm{NaOH}$ to the first and $5.0\mathrm{c}{\mathrm{m}}^3$ of $2.0\mathrm{mol}\mathrm{d}{\mathrm{m}}^{-3}$ $\mathrm{HN}{\mathrm{O}}_3$ to the second.
3. Heat gently, stir and record the solubility in each solution.
4. Repeat with $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ and $\mathrm{Si}{\mathrm{O}}_2$.

Trend in Oxide Properties

Across Period 3, oxide character changes:

Type	Period 3 oxides and behaviour
Basic	$\mathrm{N}{\mathrm{a}}_2\mathrm{O}$ and $\mathrm{MgO}$; metal oxides that produce alkaline solutions in water and react with acids to form salt and water
Amphoteric	$\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$; reacts with both acids and alkalis to form salt and water
Acidic	$\mathrm{Si}{\mathrm{O}}_2$, ${\mathrm{P}}_4{\mathrm{O}}_{10}$, $\mathrm{S}{\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_7$; non-metal oxides that react with alkalis to form salt and water, while those soluble in water produce acidic solutions

Metalloids in Microelectronics

• Metalloids have properties of both metals and non-metals.

• They are weak electrical conductors at ordinary temperatures but conduct electricity better at high temperatures.

• Silicon is used as a semiconductor in electronic microchips for devices such as computers and mobile phones.

• Si and Ge are metalloids considered in microelectronics.