A hydrogen atom has the main energy levels (shells) shown in the diagram below. $\mathrm{n}$ refers to the shell number. This is the Bohr model of the atom.

List these electron transitions in order of increasing wavelength of light absorbed or emitted: 

1. $\mathrm{n}=1$ to $\mathrm{n}=2$
2. $\mathrm{n}=3$ to $\mathrm{n}=2$ 
3. $\mathrm{n}=2$ to $\mathrm{n}=4$
4. $\mathrm{n}=3$ to $\mathrm{n}=1$ 

State electron arrangements of the main energy levels (shells) of atoms of two noble gas elements.

State electron arrangements of the main energy levels (shells) of atoms of a nitride ion. 

State electron arrangements of the main energy levels (shells) of atoms of a magnesium ion. 

Interpret the information below about four metals A, B, C and D to judge their order of reactivity. 

• Metal A and metal B react slowly with water. 
• Metal C displaces metal B from an aqueous solution of its chloride salt.
• Metal D does not react with dilute acids. 
• Metal B does not displace metal A from an aqueous solution of its sulphate salt. 

In the following reaction, identify the substance being oxidized and the substance being reduced.

$\mathrm{Zn} \left(\mathrm{s}\right) + 2{\mathrm{H}}^{+} \left(\mathrm{aq}\right)\to {\mathrm{Zn}}^{2+} \left(\mathrm{aq}\right) +{\mathrm{H}}_2 \left(\mathrm{g}\right)$

In the following reaction, identify the substance being oxidized and the substance being reduced.

$2\mathrm{KI} \left(\mathrm{aq}\right) + {\mathrm{Cl}}_2 \left(\mathrm{g}\right)\to 2\mathrm{KCl} \left(\mathrm{aq}\right) + {\mathrm{I}}_2 \left(\mathrm{aq}\right)$