Chemistry · Ch 2 — Structure of Atom

Atomic Number and Mass Number

2.2.3

Atomic Number and Mass Number

The Nucleus: Protons and Atomic Number

The positive charge of an atom's nucleus comes entirely from the protons packed inside it. Each proton carries a charge exactly equal in magnitude to that of an electron, but opposite in sign. The number of protons in the nucleus defines the atomic number of the element, denoted by the symbol $Z$ .

For a neutral atom, the total positive charge from the protons must be balanced by an equal total negative charge from the electrons. This gives a direct and fundamental relationship:

Important

For a neutral atom:

$\text{Atomic number } (Z) = \text{Number of protons in the nucleus} = \text{Number of electrons in the atom}$

Consider hydrogen: its nucleus contains exactly 1 proton, so $Z = 1$ , and a neutral hydrogen atom has 1 electron. Sodium has 11 protons in its nucleus, so $Z = 11$ , and a neutral sodium atom has 11 electrons. This single number, $Z$ , is the identity of the element — change the number of protons and you change the element itself.

The Nucleus: Protons, Neutrons, and Mass Number

While protons provide the charge of the nucleus, they do not account for its entire mass. The nucleus also contains neutrons — particles with a mass nearly identical to that of a proton but carrying no electric charge. Together, protons and neutrons are called nucleons.

The total number of nucleons in a nucleus is the mass number of the atom, denoted by $A$ . If the number of neutrons is represented by $n$ , then:

\text{Mass number } (A) = \text{Number of protons } (Z) + \text{Number of neutrons } (n)

Start your 14-day free trial to unlock the full solution →

← Previous · 2.2.2Rutherford's Nuclear Model of Atom Next · 2.2.3 →Problem 2.1 — Calculate the number of protons, neutrons and electrons in ⁸⁰₃₅Br.