Atomic number and mass number

4 marks · Normal

Use the periodic table and your knowledge to answer the following questions about atomic numbers and mass numbers.

(a)

Define mass number.

[1 mark]

(b)

An atom of element X has atomic number 9 and mass number 19.

State how many protons, neutrons and electrons are present in a neutral atom of X.

[3 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) Mass number is the total number of protons and neutrons in the nucleus of an atom.

(b) Protons: 9, Electrons: 9, Neutrons: 10

Common mistakes: Mass number confused with atomic number; Neutron calculation error

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Defines mass number as the total number of protons and neutrons in the nucleus.

B2 (b) · 1 mark · Number of protons given as 9.

B3 (b) · 1 mark · Number of electrons given as 9 (for a neutral atom).

B4 (b) · 1 mark · Number of neutrons calculated as 10 (mass number – atomic number).

Worked solution

a) Mass number is the total number of protons and neutrons in the nucleus of an atom.

b) Protons = atomic number = 9. Electrons = 9 (as atom is neutral). Neutrons = mass number – atomic number = 19 – 9 = 10.