Atomic number and mass number: counting the particles

Two numbers identify an atom

Every atom is built from the same three particles: electrons, protons and neutrons. One element differs from another because its atoms contain different numbers of these particles.

The number of protons in an atom is its atomic number, Z. It is fixed and unique to each element. A neutral atom has as many electrons as protons. The periodic table lists elements in ascending order of Z.

Protons plus neutrons give almost all of an atom's mass, so their total is the mass number, A, also called the nucleon number. Electrons are too light to be counted in it.

In a symbol, A is written as a left superscript and Z as a left subscript: ¹⁶₈O is oxygen with 8 protons and mass number 16. The neutron number is N = A − Z, so ³²₁₆S has 32 − 16 = 16 neutrons.

Your book adds: changing the number of electrons forms ions, while changing the number of neutrons forms isotopes. Changing the protons would give a different element.

Z = protons = electrons in a neutral atom. A = protons + neutrons. Neutrons N = A − Z.

Key terms

Atomic number (Z)

Identifying the element and placing it in the periodic table.

Mass number (A)

Counting protons and neutrons together; electrons are left out.

Neutron number (N)

Finding the neutrons from the two numbers in a symbol.

Nuclide symbol

Writing A as a left superscript and Z as a left subscript on the element's symbol.

Short questions with model answers

1. Q1. Sample Example (p. 23): find the neutrons, protons and electrons in barium, ¹³⁷₅₆Ba.

   • Z = 56, A = 137
   • p = 56, e = 56
   • N = A − Z = 137 − 56 = 81

   81 neutrons, 56 protons and 56 electrons.

2. Q2. Exercise (p. 24): find the neutrons, protons and electrons in platinum, ¹⁹⁵₇₈Pt.

   • Z = 78, A = 195
   • p = e = 78
   • N = 195 − 78 = 117

   117 neutrons, 78 protons and 78 electrons.

Common mistakes

• ✗ Mass number = protons + neutrons + electrons.

  ✓ Electrons are too light to count. A is protons plus neutrons only.

• ✗ In ¹³⁷₅₆Ba, the small bottom number 56 is the mass number.

  ✓ The bottom number is Z (protons). The top number is A, and it is always the larger one.

• ✗ An atom always has as many neutrons as protons.

  ✓ Only sometimes: sulphur-32 has 16 and 16, but barium-137 has 81 neutrons and 56 protons. Always work out A − Z.

MCQs

1. 1. The atomic number of an element is the number of its:

   1. (a) Neutrons
   2. (b) Protons and neutrons
   3. (c) Protons
   4. (d) Electrons and neutrons
   Show answer

   (c) p. 23: the number of protons is the atomic number, Z.

2. 2. How many neutrons are in ³²₁₆S?

   1. (a) 16
   2. (b) 32
   3. (c) 48
   4. (d) 8
   Show answer

   (a) p. 23: N = A − Z = 32 − 16 = 16.

3. 3. How many electrons are in a neutral atom of ¹³⁷₅₆Ba?

   1. (a) 137
   2. (b) 81
   3. (c) 193
   4. (d) 56
   Show answer

   (d) Sample Example (p. 23): electrons equal protons, which equal Z = 56.

4. 4. How many neutrons are in ⁵⁵₂₅Mn?

   1. (a) 25
   2. (b) 30
   3. (c) 55
   4. (d) 80
   Show answer

   (b) Exercise (p. 24): N = 55 − 25 = 30.

5. 5. How many neutrons are present in ²¹⁰₈₃Bi?

   1. (a) 83
   2. (b) 210
   3. (c) 127
   4. (d) 293
   Show answer

   (c) Short question iii (p. 31): N = 210 − 83 = 127.

Quick revision

• Atomic number Z is the number of protons; it identifies the element and equals the electrons in a neutral atom.
• Mass number A is protons plus neutrons; electrons are not counted.
• Read A (top) and Z (bottom) from the symbol, then neutrons N = A − Z.