Atomic structure: Important Definitions:

Atom The unit particle of an element
Proton Number The number of protons in one atom of an element
Nucleon Number The number of protons and neutrons in an atom of an element
Isotopes Atoms of the same element, that have different number of neutrons

The Atom

An atom is the simplest unit of an element. It consists of:

  • A nucleus: containing protons and neutrons
  • Several electrons orbiting the nucleus, arranged in energy levels or shells
Particle Relative mass Relative charge
Proton 1 Positive (+1)
Electron 1/1840 Negative (-1)
Neutron 1 No charge

Every atom has equal numbers of protons and electrons; hence the atom has no overall charge!

Structure of an atom

Discovering the Atomic Structure

The ways in which the structure of an atom was discovered has been explained in depth by the simulation by PHET Colorado. Please scroll down below.:


(credits for the simulation above go to PHET.Colorado.Edu)

Isotopes are atoms with different nucleon numbers of the same element.

To make that a little simple, isotopes are atoms with the same number of protons and different numbers of neutrons.

Isotopes are usually written in the following format:


A is the nucleon number
Z is the proton number
X is the symbol of the element


Let’s consider the following example:

Isotopes of Hydrogen
Isotope name Components Structures
Protium 1 proton

1 electron

0 neutron

Deuterium 1 proton

1 electron

1 neutron

Tritium 1 proton

1 electron

2 neutrons

As you can see, the different atoms of hydrogen all have one proton and one electron but the numbers of neutrons are different.

Radioactive Isotopes

Some isotopes are radioactive; for example, in the above example, tritium is radioactive whereas the other 2 are not. The nuclei of radioactive isotopes are unstable and give off radiation. Take a look at an example below, focusing on radioactive isotopes of uranium:


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