Atomic Structure

5 marks · Easy

Use your knowledge of isotopes and atomic structure to answer the following questions about phosphorus.

(a)

Describe two ways in which two isotopes of phosphorus are the same.

[2 marks]

(b)

Describe one way in which two isotopes of phosphorus are different.

[1 mark]

(c)

The atomic number of phosphorus is 15. State the number of electrons in a neutral atom of phosphorus.

[1 mark]

(d)

State the number of electrons in a phosphorus ion with a 3– charge.

[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.

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(a) Both isotopes have the same number of protons and same number of electrons.

(b) Different number of neutrons.

(d) 18

Also accept: Part a: Both have same number of protons AND both have same number of electrons; Part b: Different number of neutrons

Common mistakes: Confusing isotope similarity with difference; Counting electrons for ions incorrectly

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States isotopes have same number of protons (or atomic number).

B2 (a) · 1 mark · States isotopes have same number of electrons (or same chemical properties).

B3 (b) · 1 mark · States isotopes have different number of neutrons (or different mass numbers).

A1 (c) · 1 mark · Gives 15 as number of electrons in neutral phosphorus atom.

A2 (d) · 1 mark · Gives 18 (15+3) as number of electrons in a phosphorus ion with 3– charge.

Worked solution

a) Both isotopes have the same number of protons (atomic number) and same number of electrons.

b) They have different numbers of neutrons.

c) The number of electrons in a neutral phosphorus atom is 15.

d) A phosphorus ion with a 3– charge has 3 more electrons than protons: 15 + 3 = 18.