Fundamental Particles

The atomic model has evolved over time as scientific discoveries have improved our understanding of atomic structure. Modern atomic theory describes atoms as consisting of a dense, positively charged nucleus surrounded by negatively charged electrons in specific energy levels.

• Solid Sphere Model, John Dalton in 1803 : Atoms theorised as indivisible spheres.

• Plum Pudding Model, JJ Thomson in 1904 : Initially thought that atoms consisted of a sphere of positive charge with small negative charges (electrons) distributed evenly within it.

• Rutherford’s Nuclear Model, 1911: Gold foil experiment resulted in the theory that atoms have a small, dense, positively charged nucleus at the centre with electrons moving around it. Most of the atom is empty space.

• Bohr’s Quantised Shell Model, 1913: Refinement to Rutherford’s model proposing electrons orbit the nucleus in fixed energy levels –shells –and can move between these energy levels by absorbing or emitting energy.

• Quantum model, 1926: Schrodinger and Heisenberg further developed atomic theory by introducing quantum mechanics. Instead of fixed orbits, electrons are wave-particles that exist in probabilistic regions called orbitals.

Fundamental Particles

• Protons determine the atomic number and the identity of the element.

• Neutrons contribute to the mass of the atom but do not affect chemical properties.

• Electrons are responsible for chemical bonding and reactivity.

• Atomic Number (Z) = Number of protons in the nucleus.

• Mass Number (A) = Total number of protons and neutrons in the nucleus.

• Number of Neutrons = Mass Number – Atomic Number

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

• Since they have the same number of electrons, isotopes exhibit identical chemical properties but may have different physical properties such as mass and density.