CAIE A-Level Chemistry 1.4 Ionisation Energy

Practise definitions, gas-phase equations, periodic trends and successive-ionisation data using nuclear attraction and shielding.

Exam points

state the full first-ionisation-energy definition and construct gas-phase electron-removal equations
explain trends using nuclear charge, distance, shielding, sub-shell energy and electron pairing
use large jumps in successive values to infer valence-electron number and electronic structure

Covered syllabus points

1.4.0—Scope note 1.4.1—Terms: first ionisation energy, IE 1.4.2—Equations to represent first, second 1.4.3—The trends in ionisation energies across 1.4.4—The variation in successive ionisation 1.4.5—Ionisation energies are due to the attraction 1.4.6—Factors influencing the ionisation energies 1.4.7—The electronic configurations of elements 1.4.8—Periodic Table position from IE data

[Maximum number: 7]

(a)

Define first ionisation energy.

[ 2 ]

(b)

Successive ionisation energies for element A are shown in Table 1.1.

Table 1.1

Use Table 1.1 to deduce the group of the Periodic Table that A belongs to. Explain your answer. Group

[ 1 ]

(c)

Across Period 3 there is a general trend for first ionisation energies to increase due to the increase in attraction between the nucleus and the outer electron.

Explain why the first ionisation energy of sulfur is less than the first ionisation energy of phosphorus.

[ 2 ]

(d)

In an $\mathrm{A} l^{2+}$ ion the nuclear attraction for the outer electron is stronger than in an atom of Na .

Compare the electronic structures of $\mathrm{A} l^{2+}$ and an atom of Na and explain why the third ionisation energy of aluminium is greater than the first ionisation energy of sodium.

[ 2 ]

M1 similarity in electronic structure / shielding of $\mathrm{Al}^{2+}$ and Na both remove electron from (3)s ${ }^{1}$ / single electron in (3)s (sub-level / orbital)

OR $\mathrm{Al}^{2+}$ and Na have same electronic configuration

OR shielding (of outer electron) is the same

M2 greater nuclear charge / number of protons

$\mathrm{Al} l^{(2+)}$ has greater nuclear charge

OR 13p compared to 11p

[Maximum number: 3]

Nitrogen and phosphorus are elements in Group 15 of the Periodic Table.

(a)

Fig. 3.1 shows a sketch of some of the ionisation energies of phosphorus, P.

Fig. 3.1

[ 3 ]

(i)

Construct an equation to represent the third ionisation energy of P.

[ 1 ]

$\mathrm{P}^{2+}(\mathrm{g}) \rightarrow \mathrm{P}^{3+}(\mathrm{g})+\mathrm{e}^{(-)}$

(ii)

Complete the graph in Fig. 3.1 to show the third to sixth ionisation energies of P.

[ 2 ]

[Maximum number: 8]

The Periodic Table is arranged such that the properties of the elements show a number of trends.

(a)

A plot of the first ionisation energies for the first 18 elements is shown.

Figure for Question (a) — CAIE A-Level Chemistry AS

[ 8 ]

(i)

Explain why the values show a general increase from atomic number 11 to 18 .

[ 2 ]

(ii)

Explain the decreases in first ionisation energies between
- atomic numbers 12 and 13,
- atomic numbers 15 and 16 .

[ 4 ]

(iii)

Suggest an explanation for the trend in the first ionisation energies of the elements with atomic numbers 2, 10 and 18.

[ 2 ]

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Covered syllabus points

1.4 Ionisation energy question 1

Question (a)

Question (b)

Question (c)

Question (d)

1.4 Ionisation energy question 2

Question (a)

Question (i)

Question (ii)

1.4 Ionisation energy question 3

Question (a)

Question (i)

Question (ii)

Question (iii)