What's for the last 1?

Explanation:

(i) Which one of these structures A to E represents a noble gas?

Noble gases have completely filled outer most orbital. It means they have 8 valence electrons. Picture D have 8 valence electrons thus it represent noble gas.

(ii)Which two of these structures represent atoms from the same group of the periodic table?

Atoms present in same group having same number of valence electrons.

Picture A and C both have one valence electron thus both are present in same group.

(iii)Which one of these structures represents an atom with an atomic number of 8?

Picture B have total 8 number of electrons it means this atom have atomic number 8. So answer is picture B.

(iv)Which one of these structures forms a stable ion by gaining one electron?

Ions are formed by gaining or losing the electrons. When an atom gain electron anion is formed. There are 7 electron in outer orbital of picture E when it gain one electron outer orbital become completely filled thus form stable ion.

(v)Which one of these structures is in period 3 of the periodic table?

Picture C have 11 electron it means this atom have atomic number 11 which represent sodium. Sodium is present in period 3. Thus answer is picture C.