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IGCSE CHEMISTRY (demo)

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  1. 2. Atoms, Elements & Compounds (DEMO)
    7 Topics
  2. 3. Stoichiometry (DEMO)
    1 Topic
  3. 6. Chemical Reactions (DEMO)
    6 Topics
IGCSE CHEMISTRY (demo) 2. Atoms, Elements & Compounds (DEMO) 2.2.2 The Proton Number and Mass Number (Demo)
Lesson 1, Topic 7
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2.2.2 The Proton Number and Mass Number (Demo)

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  • Define proton number/atomic number as the number of protons in the nucleus of an atom
  • Define mass number/nucleon number as the total number of protons and neutrons in the nucleus of an atom
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Recall Protons and neutrons are located in the nucleus of an atom.
Take a look at the following diagrams representing different atoms. Count how many protons and neutrons are in the nucleus of each atom.

Apply what you learned from the previous tab by completing the following table.

Click the tabs to learn more about proton number and mass number.

Proton Number
Proton number (Z) is the number of protons in the nucleus of an atom. The proton number is also known as the atomic number.
Mass Number
Mass number (A) is the total number of protons and neutrons in the nucleus of an atom. The mass number is also known as the nucleon number.

You can represent the proton number (Z), mass number (A), and the element symbol as shown in the diagram.

NOTE In all versions of the periodic table, the largest number represents the mass number. Mass number > Proton number

Move the slider to explore.

For example, in this periodic table, the proton number for carbon is 6 (on the top), and its mass number is 12 (at the bottom).

Neutron Number

The short way of describing an element only shows the atomic number (proton number) and nucleon number (mass number). It does not show the number of neutrons.

However, you can calculate the number of neutrons using the proton number and mass number:

Example Calculate the number of neutrons in the nucleus of a lithium (Li) atom.

  • mass number = 7
  • proton number = 3

number of neutrons = 7 – 3 = 4 neutrons

Check your understanding of proton number and mass number

Use the periodic table to identify the element. Then, determine its proton number and the number of neutrons.