Nuclear Model of the Atom (DP IB Chemistry): Revision Note

The nuclear atom

What are subatomic particles?

• Atoms are made up of protons, neutrons, and electrons, which are called subatomic particles

• These particles are extremely small, so their masses and charges are not measured in conventional units like grams or coulombs

• Instead, we use relative atomic mass and relative charge to compare them.

• These values are not actual measurements, but values relative to one another:

  • Protons and neutrons have very similar masses and are each given a relative mass of 1

  • Electrons are about 1836 times lighter than protons or neutrons, so their mass is considered negligible

• The relative mass and charge of the subatomic particles are:

Relative mass & charge of subatomic particles

• Proton

  • Relative charge: +1

  • Relative mass: 1

• Neutron

  • Relative charge: 0

  • Relative mass: 1

• Electron

  • Relative charge: –1

  • Relative mass: negligible

Where are the subatomic particles located?

• Atoms contain a positively charged, dense nucleus:

  • The nucleus is positively charged because it contains protons

  • It is dense because most of the atom’s mass is concentrated in the nucleus, where the heaviest subatomic particles (protons and neutrons) are found

  • These particles are collectively called nucleons

• Electrons, which are negatively charged, occupy the space outside the nucleus:

  • They move around the nucleus in a region sometimes described as a ‘cloud’ of negative charge

• The atom is held together by the electrostatic attraction between the positive nucleus and the negatively charged electrons surrounding it

Atomic structure diagram