Principles of Chemistry

7 marks · Easy

This question is about the relationship between atomic number, number of electrons, and the periodic table.

(a)

The atomic number of magnesium is 12. State the number of electrons in a neutral atom of magnesium and explain your answer.

[2 marks]

(b)

Potassium (K) is found in Period 4, Group 1 of the Periodic Table. Deduce the electronic configuration for a potassium atom. Explain your reasoning.

[3 marks]

(c)

Explain how the electronic configuration you wrote for potassium determines its position in the Periodic Table.

[2 marks]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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(a) 12 electrons. In a neutral atom, number of electrons = atomic number.

(b) 2, 8, 8, 1. (First shell: 2 electrons; second: 8; third: 8; fourth: 1). Each shell fills according to rules: 2 in first, up to 8 in second and third, remainder in the fourth.

(c) Potassium has one electron in its outer shell (fourth shell), placing it in Group 1. It has four shells occupied, so is in Period 4.

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States 12 electrons.

B2 (a) · 1 mark · Explains that number of electrons equals atomic number in a neutral atom.

B3 (b) · 1 mark · Applies shell filling rules (2, 8, 8, ...).

B4 (b) · 1 mark · Correct final configuration: 2, 8, 8, 1.

B5 (b) · 1 mark · States which number of electrons in each shell.

B6 (c) · 1 mark · Relates outer electron to group.

B7 (c) · 1 mark · Relates number of shells to period.

Worked solution

a) The atomic number of magnesium is 12, so it has 12 protons. In a neutral atom, number of protons = number of electrons, so 12 electrons.

b) Potassium's atomic number is 19. Filling electrons: 2 in 1st shell, 8 in 2nd (total 10), 8 in 3rd (total 18), 1 left for the 4th shell. So configuration is 2, 8, 8, 1. The rules: fill inner shells first, 2 in 1st shell, up to 8 in 2nd and 3rd for first 20 elements.

c) Potassium has one electron in its outer shell (Group 1), and four shells occupied (Period 4).