Atomic Structure & Subatomic Particles (CIE AS Chemistry)

• All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions
• Atoms are mostly made up of empty space around a very small, dense nucleus that contains protons and neutrons
• The nucleus has an overall positive charge
  • The protons have a positive charge and the neutrons have a neutral charge

• Negatively charged electrons are found in orbitals in the empty space around the nucleus

The structure of an atom

Atoms contains protons, neutrons and electrons

• The protons, neutrons and electrons that an atom is made up of are called subatomic particles
• These subatomic particles are so small that it is not possible to measure their masses and charges using conventional units (such as grams or coulombs)
• Instead, their masses and charges are compared to each other, and so are called ‘relative atomic masses’ and ‘relative atomic charges’
• These are not actual charges and masses but charges and masses of particles relative to each other
  • Protons and neutrons have a very similar mass, so each is assigned a relative mass of 1
  • Electrons are 1836 times smaller than a proton and neutron, and so their mass is often described as being negligible

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

Relative mass & charge of subatomic particles table

Subatomic particle	Relative charge	Relative mass
Proton	+1	1
Neutron	0	1
Electron	–1

• The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
  • The mass of the electron is negligible

• The nucleus is also positively charged due to the protons
• Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
• The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together

The distribution of mass in an atom

The mass of the atom is concentrated in the positively charged nucleus which is attracted to the negatively charged electrons orbiting around it

• Protons, neutrons and electrons behave differently when they move at the same velocity in an electric field
• When a beam of electrons is fired past the electrically charged plates, the electrons are deflected very easily away from the negative plate towards the positive plate
  • This proves that the electrons are negatively charged; like charges repel each other
  • It also shows that electrons have a very small mass, as they are easily deflected

• A beam of protons is deflected away from the positive plate and towards the negative plate
  • This proves that the proton is positively charged
  • As protons are deflected less than electrons, this also shows that protons are heavier than electrons

• A beam of neutrons is not deflected at all
  • Which proves that the particle is neutral in character; it is not attracted to, or repelled by, the negative or positive plate

Subatomic particles passing through an electric field

The lighter electrons undergo much more deflection than the protons