# 4.1.4 Isotopes

### Isotopes

• Although the number of protons in a particular element is always the same, the number of neutrons can be different
• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
• In the diagram below are three isotopes of Hydrogen: Hydrogen has three isotopes, each with a different number of neutrons

• Isotopes occur naturally, but some are more rare than others
• For example, about 2 in every 10,000 Hydrogen atoms is Deuterium
• Tritium is even more rare (about 1 in every billion billion hydrogen atoms)

#### Exam Tip

This topic is also covered in Chemistry, although some of the terminologies may be a little different. However, in Physics you must refer to neutrons when explaining isotopes.

### Differences Between Isotopes

• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
• This is because neutrons have no charge but do have mass
• In the periodic table, the mass number of Chlorine is often given as 35.5 This section of a periodic table shows Chlorine as having a mass number of 35.5, but other elements have an integer mass number

• The mass number of Chlorine is given as 35.5 because it has roughly equal numbers of isotopes with a mass number of 35, and of 36
• The number of electrons and protons in different isotopes remains the same
• Isotopes tend to be more unstable due to the imbalance of protons and neutrons

#### Worked Example

State the number of protons, neutrons and electrons in Chlorine-35 and Chlorine-36 atoms.

Step 1: Determine the number of protons

• The atomic number is the number of protons
• Both Chlorine-35 and Chlorine-36 have 17 protons

Step 2: Determine the number of neutrons

• The mass number is the number of protons and neutrons
• Chlorine-35 neutrons: 35 – 17 = 18 neutrons
• Chlorine-36 neutrons: 36 – 17 = 19 neutrons

Step 3: Determine the number of electrons

• The number of electrons is equal to the number of protons
• Both Chlorine-35 and Chlorine-36 have 17 electrons ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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