Use the concept of atomic structure to determine the atomic and mass numbers of an element

The atomic number (also called proton number) is the total number of protons in the nucleus of an atom. For a neutral atom, the number of protons equals the number of electrons.

Key relationship:

• Atomic number = number of protons = number of electrons

For example, carbon has atomic number 6, meaning every carbon atom has 6 protons and 6 electrons.

The mass number (also called nucleon number) is the total number of protons plus neutrons in the nucleus. Since electrons have negligible mass, the mass number represents the mass of the atom.

Key relationship:

• Mass number = number of protons + number of neutrons

Once you know the atomic number (Z) and mass number (A), you can find the number of neutrons using:

Number of neutrons = Mass number − Atomic number $N = A - Z$

For any neutral atom:

• Protons = Atomic number (Z)
• Electrons = Atomic number (Z)
• Neutrons = Mass number (A) − Atomic number (Z)

An atom of magnesium has atomic number 12 and mass number 24.

(a) How many protons does it have?
(b) How many electrons does it have?
(c) How many neutrons does it have?

Solution

(a) Number of protons = atomic number = 12

(b) Number of electrons = number of protons = 12

(c) Number of neutrons = mass number − atomic number
= 24 − 12 = 12

The nuclide notation of an atom is written as $_ {17}^{37}$Cl.

(a) What is the atomic number?
(b) What is the mass number?
(c) How many neutrons does it have?

Solution

(a) Atomic number = lower left number = 17

(b) Mass number = upper left number = 37

(c) Number of neutrons = 37 − 17 = 20

Atoms are represented using nuclide notation: the mass number is written at the top left of the element symbol, and the atomic number is written at the bottom left.

$_ {Z}^{A} X$

For example, $_ {6}^{14}$C represents carbon-14 with 6 protons, 8 neutrons (14 − 6 = 8), and 6 electrons.

In Tanzanian hospitals, doctors use iodine-131 (a radioactive isotope) to check how the thyroid gland is working. Knowing the atomic number (53) and mass number (131) of iodine-131 helps medical professionals track the isotope safely and use it for diagnosis and treatment. This knowledge is also applied in agriculture, where phosphorus-32 is used as a radioactive tracer to study how crops absorb nutrients from fertilizers, helping farmers improve soil management and crop yields.