Covalent and metallic bonding — GCSE Chemistry practice questions

Question 1

Hydrogen gas, H₂, is used as the fuel in some buses. The two atoms in a hydrogen molecule are held together by a covalent bond.

What is a covalent bond?

Accepted Answer

B: a pair of electrons shared between two atoms. A covalent bond is a pair of electrons shared between two atoms. In H₂ each atom gives one electron to the shared pair, so both atoms have two outer electrons, like helium.

Question 2

Labels have fallen off four jars of solids in a school laboratory store. A technician measures some of their properties.

| solid | melting point / °C | conducts when solid? | conducts when molten? |
|---|---|---|---|
| W | 1650 | no | no |
| X | 714 | no | yes |
| Y | 44 | no | no |
| Z | 1455 | yes | yes |

Which solid has a giant ionic structure?

Accepted Answer

B: X. X has a high melting point and conducts only when molten, when its ions are free to move: it is ionic. W is giant covalent, Y is simple molecular and Z, which conducts as a solid, is a metal.

Question 3

Methanal is used to preserve biological specimens. Fig. 1 shows the displayed formula of a methanal molecule.

Which row gives the number of electrons shared in covalent bonds and the number of outer shell electrons NOT used in bonding, in one molecule of methanal?

Accepted Answer

D: 8 · 4. Two C–H single bonds and one C=O double bond make 4 shared pairs, which is 8 shared electrons. Oxygen uses 2 of its 6 outer electrons in the double bond, so it keeps two non-bonding pairs: 4 electrons. Carbon and hydrogen have none left over.