What is Group 14?
In the periodic table, the elements carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) are in one column called Group 14 (older name: Group IVA). Each atom has 4 electrons in its outer shell, so each can form 4 bonds. Elements in one group behave in a similar way, but they change step by step going down.
- Carbon: non-metal.
- Silicon and germanium: metalloids (in between; they are semiconductors).
- Tin and lead: metals.
Bigger atoms hold the outer electrons less tightly, so they lose them more easily. That is why the metal character grows down the group.
Forms of carbon and adsorption
Carbon exists in different forms called allotropes: diamond (very hard, every carbon joined to 4 others), graphite (soft, slippery layers, conducts electricity), and fullerenes and graphene. Full details are in the lesson Allotropes of carbon.
Adsorption means that gases or dissolved particles stick to the surface of a solid (not inside it). Activated charcoal has a huge surface full of tiny holes, so it adsorbs smells, colours and poisons. It is used in water filters, gas masks and for removing colour from sugar.
Oxides of carbon and the greenhouse effect
Carbon forms carbon monoxide (CO), a poisonous gas made when fuel burns without enough air, and carbon dioxide (CO₂), made when fuel burns with enough air and when we breathe out. See Oxides of carbon.
CO₂ lets sunlight in but traps some of the heat that tries to leave the Earth. This is the greenhouse effect. A little keeps the Earth warm; too much, from burning coal, oil and gas and cutting forests, makes the planet warmer. Methane and water vapour also trap heat. See The greenhouse effect.
Carbonates and hydrogen carbonates
Carbonates contain the CO₃²⁻ ion (example: calcium carbonate CaCO₃, in limestone, marble, chalk, shells). Hydrogen carbonates contain HCO₃⁻ (example: sodium hydrogen carbonate NaHCO₃, baking soda).
- With acid: they fizz and give CO₂. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Pass the gas through lime water and it turns milky.
- On heating: CaCO₃ → CaO + CO₂ (quicklime). Baking soda gives off CO₂, so cakes rise: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
- Solubility: most carbonates do not dissolve in water, except those of sodium and potassium. Hydrogen carbonates dissolve more easily, which is why limestone caves form: rain with CO₂ slowly turns CaCO₃ into soluble Ca(HCO₃)₂.
Silicon and its dioxide
After oxygen, silicon is the most common element in the Earth's crust. It is a grey, shiny metalloid and a semiconductor, so it is used for computer chips and solar cells. In nature it is found as silicon dioxide (SiO₂, silica) in sand and quartz, and as silicates.
Why CO₂ is a gas but SiO₂ is a solid: in CO₂ each carbon is joined to two oxygens by double bonds and the molecule stands alone; molecules attract each other only weakly. In SiO₂ each silicon is joined to four oxygens, and each oxygen to two silicons, forming a giant network of strong bonds. To melt it you must break these bonds, so its melting point is very high (about 1700 °C) and it is hard.
Silica does not react with water or most acids. It dissolves in hot concentrated alkali (making silicates).
Silicates, glass, cement and ceramics
Silicates contain silicon and oxygen with metals (for example sodium silicate, clay). Many materials we use are silicate-based.
- Glass: sand + sodium carbonate + limestone are melted at a high temperature and cooled quickly. It has no regular crystal pattern. See Glass.
- Cement: limestone and clay are heated and ground. With water it sets and hardens. See Cement.
- Ceramics: clay is shaped and fired in a kiln to make bricks, tiles and pots. See Ceramics.
Key formulas and definitions
- Group 14 atoms: 4 outer electrons
- Trend down: size up, metallic character up
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- CaCO₃ → CaO + CO₂ (on heating)
- 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (on heating)
- CO₂ test: lime water turns milky
Worked examples
1. How many outer electrons does silicon have? How do you know?
4, because silicon is in Group 14, just like carbon.
2. Why is CO2 a gas but SiO2 a hard solid at room temperature?
CO2 is made of separate small molecules with weak attraction between them. SiO2 is a giant network of strong Si-O bonds, so it needs a lot of heat to melt.
3. A white powder fizzes with dilute acid and the gas turns lime water milky. What is it?
A carbonate or hydrogen carbonate, because the gas is CO2.
4. Write the equation for heating limestone.
CaCO3 → CaO + CO2. The CaO is quicklime.
Common mistakes
- Saying carbon is a metal. Carbon is a non-metal; metal character grows down the group.
- Thinking SiO2 is made of small molecules like CO2. It is a giant network.
- Calling any fizzing a sign of CO2 without testing. Confirm with lime water turning milky.
- Mixing up carbonate (CO3 2-) and hydrogen carbonate (HCO3 -).