The graph shows how a periodic property varies when plotted against atomic number for Period 3 (sodium to argon). (i) Identify the property. (ii) Explain the overall trend across the period.

(i) The property is ionisation energy. Ionisation energy is the energy required to remove an electron from the valence shell of an atom. (ii) In general, the first ionisation energy increases across Period 3 as the effective nuclear charge increases and electrons successively fill the third quantum shell. As electrons are in the same shell, the shielding effect is similar in atoms of each element. There are small 'dips' in the general trend across the period between Magnesium and Aluminium because the last electron in aluminium is in 3 p sub-shell which is slightly further away from the nucleus than 3 s sub-shell, thus ionisation energy decreases and in the case of phosphorous and sulphur, the electron removed from the phosphorous is from an orbital that contains an unpaired electron. The electron removed from the sulphur is from the orbital that contains a pair of electrons. The extra repulsion between the pair of electrons in this orbital results in less energy being needed to remove an electron. So, IE for sulphur is lower, because of spin-pair repulsion.

MEDIUM

AS and A Level

IMPORTANT

Earn 100

The chloride of an unknown element, X, is a liquid at $20 ° C$ . This chloride reacts with water, giving off white fumes and leaving an acidic solution.
i) Does element X belong to Group 1, Group 2 or Group 15 of the Periodic Table?
ii) What type of reaction takes place between X and water?
iii) Identify the white fumes given off when X reacts with water.

Important Questions on Periodicity

MEDIUM

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Question>Q 9

The chloride of an unknown element Y is a solid at

20^{°} C

. This chloride does not react with water but dissolves to give a neutral solution. Does element Y belong to Group 1, Group 14 or Group 16 of the Periodic Table?