Atomic Structure (CIE AS Physics)

• The atoms of all elements are made up of three types of particles: protons, neutrons and electrons.

Structure of an atom 

Protons and neutrons are found in the nucleus of an atom while electrons orbit the nucleus

Table of properties of subatomic particles

•  A stable atom is neutral (it has no charge)
• Since protons and electrons have the same charge, but opposite signs, a stable atom has an equal number of both for the overall charge to remain neutral

• We live in a universe made up of matter particles (protons, neutrons, electrons etc.)
• All matter particles have antimatter counterparts
  • Antimatter particles are identical to their matter counterpart but with the opposite charge

• This means if a particle is positive, its antimatter particle is negative and vice versa
• Common matter-antimatter pairs are shown in the table below:

Charges of matter and antimatter particles

Matter	Charge	Antimatter	Charge
Electron,	-1	Positron,	+1
Proton,	+1	Anti-proton,	-1
Neutron,	0	Anti-neutron,	0

• Apart from electrons, the corresponding antiparticle pair has:
  • the same name with the prefix ‘anti-’
  • a line above the corresponding matter particle symbol

• The unified atomic mass unit (u) is roughly equal to the mass of one proton or neutron:
  • 1 u = 1.66 × 10⁻²⁷ kg

• The atomic mass unit is sometimes abbreviated to a.m.u
• This value will be given on your data sheet in the exam
• The a.m.u is commonly used in nuclear physics to express the mass of subatomic particles. It is equal to 1/12 of the mass of the carbon-12 atom

Table of common particles with mass in a.m.u

• The mass of an atom in a.m.u is roughly equal to the sum of its protons and neutrons (nucleon number)
  • For example, the mass of Uranium-235 is roughly 235u