Fundamental Particles
Specification Reference Physical Chemistry, Atomic Structure 3.1.1.1
Quick Notes
- Atoms consist of three fundamental particles:
- Protons: +1 charge, mass ~1 amu, found in the nucleus.
- Neutrons: 0 charge, mass ~1 amu, found in the nucleus.
- Electrons: −1 charge, negligible mass (~1/1836 amu), found in orbitals around the nucleus.
- Atomic Number (Z) = Number of protons (defines the element).
- Mass Number (A) = Number of protons + neutrons.
- Neutral Atoms: Number of protons = Number of electrons.
- Ions: Atoms that gain or lose electrons to form positive (cations) or negative (anions) ions.
Full Notes
The structure of the atom has been outlined in more detail here.
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The Structure of an Atom
Atoms are the smallest, indivisible units of elements and are composed of three fundamental subatomic particles:
- Protons (p+): Positively charged particles found in the nucleus. Their number determines the element’s identity (e.g., all hydrogen atoms have 1 proton, all oxygen atoms have 8 protons).
- Neutrons (n0): Neutral particles also found in the nucleus. Neutrons add mass but do not affect chemical properties.
- Electrons (e−): Negatively charged particles that exist around the nucleus in energy levels (shells). Their arrangement determines an atom’s chemical behaviour.
Comparison of Subatomic Particles
| Particle | Relative Mass | Relative Charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 | −1 | Orbitals |
Atomic Number, Mass Number & Isotopes
The Atomic Number (Z) represents the number of protons in an atom.
The Mass Number (A) is the total number of protons and neutrons.
Isotopes are atoms of the same element with different neutron numbers.
Example: Carbon-12 and Carbon-13 both have 6 protons but different neutrons (Carbon 12 has 6 neutrons, Carbon 13 has 7 neutrons).
Ions and Their Formation
Atoms can gain or lose electrons to become ions:
- Cations: Atoms that have lost electrons to become positively charged (e.g., Na+, Mg2+).
- Anions: Atoms that have gained electrons to become negatively charged (e.g., Cl−, O2−).
Development of Atomic Models
The understanding of fundamental particles has evolved over time:
- Dalton’s Model (1803) – Atoms are indivisible, solid spheres.
- Thomson’s Plum Pudding Model (1897) – Discovery of electrons, proposed atoms were positive spheres with embedded electrons.
- Rutherford’s Nuclear Model (1911) – Gold foil experiment showed atoms have a dense, positive nucleus with electrons orbiting.
- Bohr’s Model (1913) – Electrons exist in quantized energy levels (shells).
- Quantum Mechanical Model (1926–Present) – Electrons exist in orbitals as probability clouds.