Mr. Ehinger's Chemistry

1. States of Matter. The Atom and the Periodic Table

1.5. Atomic Number, Mass Number, and Atomic Mass

Last Updated: August 18 2019 | Print

Video

Text

Atomic number

Each element has its own number: an atom number.

The number of protons in the nucleus decides which element it is.
The atom number = the number of protons.

Examples

Element	Atomic number	Number of p⁺
Hydrogen (H)	1	1
Helium (He)	2	2
Carbon (C)	6	6
Chlorine (Cl)	17	17

Mass number

Every element (except hydrogen) also contains neutrons.

Mass number = number of protons + number of neutrons
- \(A = Z + N\)

Two definitions

Nuclide: An atomic species with a certain number of p⁺ and n.
Isotopes: Different nuclides of the same element

The universe's most simple nuclide.

Some isotopes and their mass numbers

Isotope	Nuclide	Atomic nr.	p⁺	n	mass number
Hydrogen	\({\sf ^1_1\text{H}}\)	1	1	0	1
Deuterium	\({\sf ^2_1\text{H}}\)	1	1	1	2
Tritium	\({\sf ^3_1\text{H}}\)	1	1	2	3
Helium	\({\sf ^4_2\text{He}}\)	2	2	2	4
Carbon-12	\({\sf ^{12}_{\text{ }\text{ }6}\text{C}}\)	6	6	6	12
Carbon-13	\({\sf ^{13}_{\text{ }\text{ }6}\text{C}}\)	6	6	7	13
Carbon-14	\({\sf ^{14}_{\text{ }\text{ }6}\text{C}}\)	6	6	8	14
Chlorine-35	\({\sf ^{35}_{17}\text{Cl}}\)	17	17	18	35
Chlorine-37	\({\sf ^{37}_{17}\text{Cl}}\)	17	17	20	37

Atomic mass

A single carbon-12 atom weighs 1.994✕10^–23 g

Cumbersome figure!

The unified atomic mass unit ”u” (sometimes "amu") has been defined:

A single carbon-12 atom weighs exactly 12u:
\(m_{^{12}_{\text{ }\text{ }6}\text{C}}\equiv 12\text{u}\)

Also note: 1u = 1Da

How much do the building blocks of the atom weigh?

1 proton weighs approx. 1 u
1 neutron weighs approx. 1 u
1 electron weighs approx. 1/1800 u

The atomic mass of chlorine

The atomic mass of chlorine: 35.453u ≈ 35.5u
There are two chlorine isotopes: chlorine-35 (75 %) and chlorine-37 (25 %)
The average atomic mass of chlorine:

\[0.75 \times 35\text{u} + 0.25 \times 37\text{u} = 35.5\text{u}\]

You may also be interested in:

Learn more:

Contents

Admin area