Unit 2.2: Atomic Number and Atomic Mass
Engineering Materials → Engineering Materials → Fundamentals of Engineering Materials → Fundamentals of Engineering Materials → Atomic Structure and Bonding | Author: admin | Mar 09, 2026
Introduction
Every element in nature is made of atoms, and each atom has a unique number of protons and neutrons. These quantities determine the identity and mass of the atom.
Two important terms used to describe atoms are atomic number and atomic mass. These concepts help scientists and engineers understand different elements and their properties.
Atomic Number
Atomic number is the number of protons present in the nucleus of an atom.
Each element has a unique atomic number, which identifies the element in the periodic table.
Example:
Hydrogen → Atomic number = 1
Carbon → Atomic number = 6
Iron → Atomic number = 26
This means:
Hydrogen has 1 proton
Carbon has 6 protons
Iron has 26 protons
In a neutral atom, the number of electrons is equal to the number of protons.
Therefore:
Number of electrons = Atomic number
Key Points about Atomic Number
Atomic number determines the identity of an element.
It also determines:
Number of protons in the nucleus
Number of electrons in a neutral atom
Position of the element in the periodic table
Atomic Mass
Atomic mass (or atomic weight) is the average mass of atoms of an element.
It is approximately equal to the total number of protons and neutrons present in the nucleus.
Formula:
Atomic Mass ≈ Number of Protons + Number of Neutrons
Example:
Carbon atom
Protons = 6
Neutrons = 6
Atomic mass ≈ 12
Mass Number
Mass number is the exact total number of protons and neutrons in the nucleus of a particular atom.
Formula:
Mass Number = Protons + Neutrons
Example:
An atom with
Protons = 8
Neutrons = 8
Mass number = 16
Difference Between Atomic Number and Atomic Mass
| Property | Atomic Number | Atomic Mass |
|---|---|---|
| Meaning | Number of protons | Total mass of atom |
| Symbol | Z | A |
| Determines | Element identity | Approximate atomic weight |
| Formula | Z = number of protons | A = protons + neutrons |
Importance in Engineering Materials
Atomic number and atomic mass help in understanding:
Material composition
Atomic structure
Chemical behavior of elements
Arrangement of elements in periodic table
These factors influence material properties and bonding behavior.
Exam-Focused Points
Atomic number = number of protons.
In a neutral atom:
Number of electrons = number of protons.
Atomic mass ≈ protons + neutrons.
Mass number represents the total number of nucleons (protons + neutrons).
Example Competitive Exam Questions
What is atomic number?
Answer — Number of protons present in the nucleus.
What is atomic mass?
Answer — Total mass of an atom approximately equal to protons plus neutrons.
If an atom has 11 protons, what is its atomic number?
Answer — 11
Mass number of an atom with 7 protons and 7 neutrons is?
Answer — 14
Quick Revision
Atomic number → number of protons.
Atomic mass → total mass of atom.
Mass number → protons + neutrons.
Atomic number identifies the element, while atomic mass indicates atomic weight.