Atomic Structure

6 marks · Normal

A molecule of ammonia has the formula .

(a)

State what is meant by the term molecule.

[1 mark]

(b)

Complete the table below for the sub-atomic particles in a nitrogen atom.

Properties of sub-atomic particles in a nitrogen atom
	Relative mass	Relative charge	Position in atom
Proton
Neutron
Electron

[3 marks]

(c)

Given that the atomic number of nitrogen is 7 and the mass number of its most common isotope is 14:

(i)

Calculate the number of neutrons in a nitrogen atom.

[1 mark]

(ii)

Determine the total number of electrons in a molecule of ammonia ().

[1 mark]

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.

Show answer

(a) A molecule is a group of two or more atoms chemically bonded together.

(b) — · must show Proton: Relative mass 1, Relative charge +1, Position nucleus, Neutron: Relative mass 1, Relative charge 0, Position nucleus, Electron: Relative mass 1/1836, Relative charge -1, Position outside nucleus

(c(i)) 7

(c(ii)) 10

Common mistakes: Confusing mass number with atomic number; Including formula units instead of molecules; Mixing up properties of protons, neutrons and electrons

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Correct definition of molecule.

B2 (b) · 1 mark · Correct properties filled for proton.

B3 (b) · 1 mark · Correct properties filled for neutron.

B4 (b) · 1 mark · Correct properties filled for electron.

M1 (c(i)) · 1 mark · Method for calculating number of neutrons (mass number - atomic number).

A1 (c(ii)) · 1 mark · Correct total number of electrons in .

Worked solution

a) A molecule is a group of two or more atoms chemically bonded together.

	Relative mass	Relative charge	Position in atom
Proton	1	+1	nucleus
Neutron	1	0	nucleus
Electron	1/1836	-1	outside nucleus

c) i) Number of neutrons = mass number - atomic number = 14 - 7 = 7

ii) Ammonia () has 1 nitrogen atom and 3 hydrogen atoms.

Number of electrons = (number in N) + (number in 3 H) = 7 + (3 1) = 10