Learning Objective

In this lesson we will learn about the main subatomic particles which make up atoms.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the properties of protons, neutrons and electrons.
  • Describe the arrangement of protons, neutrons and electrons in an atom.
  • Describe the distribution of mass and charge within an atom.

Structure of Atoms

1 | Atoms

2 | Protons

3 | Neutrons

4 | Electrons

5 | Nuclei

6 | Summary

 

 structure of atoms worksheet  year 9 chemistry pdf workbook  Year 9 Chemistry Print Workbook Australian Curriculum

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Atoms

  • Atoms are composed of subatomic particles called protons, neutrons and electrons.
  • The protons and neutrons form a cluster at the centre of the atom, called the nucleus.
  • The electrons move around the nucleus at high speeds, forming an electron cloud.

 
atom structure protons electrons neutrons unlabelled

Atoms are composed of protons, neutrons and electrons.

 


Protons

  • Protons are subatomic particles with a positive charge (+1).
  • Protons have a mass of 1 atomic mass unit.
  • The number of protons in an atom determines the type of atom.
  • Examples
  • ▸ If an atom has 7 protons, it is a nitrogen atom.
  • ▸ If an atom has 16 protons, it is a sulfur atom.
  • ▸ If an atom has 47 protons, it is a silver atom.

 
atom structure protons

Protons have a mass of 1 atomic mass unit and a positive charge.

 


Neutrons

  • Neutrons are subatomic particles with no charge; in other words, they are neutral.
  • Neutrons have a mass of 1 atomic mass unit.
  • The number of neutrons in a particular type of atom can vary.
  • Examples
  • ▸ 99% of carbon atoms have 6 neutrons, 1% have 7 neutrons.
  • ▸ 76% of chlorine atoms have 18 neutrons, 24% have 20 neutrons.
  • ▸ 79% of magnesium atoms have 12 neutrons, 10% have 13 neutrons, 11% have 14 neutrons.
  • The number of neutrons in an atom is similar to the number of protons, but as the size of atoms increases, the proportion of neutrons gradually increases.
  • For example, small atoms, such as helium, oxygen and neon, have a proton:neutron ratio of approximately 1:1, whereas large atoms, such as gold, mercury and lead, have a proton:neutron ratio of approximately 1:1.5.
  • This is because, as the nuclei of atoms become larger, more neutrons are required to stabilise them.

 
atom structure neutrons

Neutrons have a mass of 1 atomic mass unit and no charge.

 


Electrons

  • Electrons are subatomic particles with a negative charge (–1).
  • Electrons are much smaller than protons and neutrons.
  • They have a mass of approximately 1/1800 of an atomic mass unit.
  • Therefore, almost all of the mass of an atom is in the nucleus.
  • The number of electrons in an atom is the same as the number of protons.
  • Therefore, although atoms contain charged particles, they have no overall charge.

 
atom structure electrons

Electrons have a mass of 1/1800 of an atomic mass unit and a negative charge.

 


Nuclei

  • Since protons are positive and neutrons are neutral, the overall charge of a nucleus is positive.
  • The nucleus is extremely dense.
  • It accounts for 99.9% of the mass of an atom but takes up less than 0.001% of the volume.

 
atom structure nucleus

Protons and neutrons form a dense cluster at the centre of an atom, called a nucleus.

  


Summary

  • Atoms are composed of subatomic particles called protons, neutrons and electrons.
  • Protons have a mass of 1 atomic mass unit and a positive charge (+1).
  • The number of protons determines the type of atom.
  • Neutrons have a mass of 1 atomic mass unit and no charge.
  • Electrons have a mass of 1/1800 of an atomic mass unit and a negative charge (–1).
  • Protons and neutrons form a dense cluster at the centre of an atom, called a nucleus.
  • Electrons move around the nucleus, forming an electron cloud.

 
atom structure labelled protons electrons neutrons nucleus

 

Particle / Structure Location Charge Mass (atomic mass units)
Proton Nucleus +1 1
Neutron Nucleus 0 1
Electron Electron Cloud –1 1/1800
Nucleus Centre of atom positive
(amount depends on atom)
(depends on atom)
Electron Cloud Around the nucleus negative
(amount depends on atom)
(depends on atom, but negligible)
Atom 0 (depends on atom, approximately same as nucleus)

Summary of atomic structure

(Header image: geralt, Pixabay)

 


 

 structure of atoms worksheet  year 9 chemistry pdf workbook  Year 9 Chemistry Print Workbook Australian Curriculum

Click images to preview the worksheet for this lesson and the Year 9 Chemistry Workbook (PDF and print versions)