2.3 Atomic Symbolism

All atoms can be identified by the number of protons and neutrons they contain. The atomic number (Z) is the number of protons in the nucleus.

Atoms are neutral, so it’s also the number of electrons.
Protons determine the identity of an element.

For example, nitrogen’s atomic number is 7, so every nitrogen has 7 protons.

The mass number (A) is the total number of protons and neutrons. Protons and neutrons are collectively referred to as nucleons.

For atoms: we use small units of measurement.

1 amu = 1 atomic mass unit = 1/12 the mass of 1 carbon 12 atom
1 amu = 1.6605×10^–24 g

Practice

Determine the numbers of protons, neutrons, and electrons in each of the following species:

A. ²⁸Si
B. ¹⁹F
C. ⁵⁷Fe
D. carbon–13

Solution

A: ^{28^Si}

Z = 14 (14 protons)
A = 28 (28-14 = 14 neutrons)
e^– = 14 (equal to the number of protons)

B: ^{19^F}

Z = 9 (9 protons)
A = 19 (19-9 = 10 neutrons)
e^– = 9 (equal to the number of protons)

C: ^{57^Fe}

Z = 26 (26 protons)
A = 57 (57-26 = 31 neutrons)
e^– = 26 (equal to the number of protons)

D: carbon-13

Z = 6 (6 protons)
A = 13 (13-6 = 7 neutrons)
e^– = 6 (equal to the number of protons)

Practice

Identify the # of protons, neutrons and electrons given the following information:

A. ¹⁹⁷Au
B. ⁴⁸Ti
C. ⁵⁵Mn

Solution

A: ¹⁹⁷Au

p⁺ = 79
n = 118
e^– = 79

B: ⁴⁸Ti

p⁺ = 22
n = 26
e^– = 22

C: ⁵⁵Mn

p⁺ = 25
n = 30
e^– = 25

Practice

Which neutral element are the following?

A. 26 protons, 56 neutrons
B. 20 neutrons, 18 electrons, no charge
C. 76 neutrons, 52 protons

Solution

A: Fe

B: Ar

C: Te

Practice

Pick the correct description for ⁵⁹Co.

A. 59 protons, 27 neutrons
B. 27 protons, 59 neutrons
C. 32 protons, 27 neutrons
D. 27 protons, 32 neutrons

Solution

2.3.1 Isotopes

Most elements have two or more isotopes, atoms that have the same atomic number (Z) but different mass numbers (A). This is due to each isotope having a different number of neutrons. Isotopes are atoms of the same element with different A (mass).

equal numbers of p⁺
different numbers of n

Isotopes of the same element typically can exhibit very similar chemical properties.