The History of the Atom, Atomic Structure, and Subatomic Particles

John Dalton's Atomic Model

Described atoms as indivisible solid spheres, proposing that different spheres make up different chemical elements.

J.J. Thomson's Discovery (1897)

Identified the electron using charge and mass measurements, disproving the solid sphere theory and introducing the plum pudding model.

Plum Pudding Model

An early atomic model depicting the atom as a sphere of positive charge with negatively charged electrons embedded throughout it.

Gold Foil Experiment

An experiment by Rutherford, Geiger, and Marsden where alpha particles were fired at thin gold foil; deflections showed that mass and positive charge are concentrated in a central nucleus.

Rutherford's Nuclear Model

A model featuring a tiny, positively charged nucleus surrounded by a cloud of negative electrons, with most of the atom being empty space.

Bohr Model

A model proposing that electrons orbit the nucleus only within fixed shells at specific, discrete energy levels, preventing the atom from collapsing.

Peer Review

The scientific process where research and theories are submitted to other scientists to scrutinize findings, detect errors, and validate conclusions.

Proton

A heavy subatomic particle located in the nucleus with a relative mass of $1$ and a relative charge of $+1$.

Neutron

A heavy subatomic particle located in the nucleus with a relative mass of $1$ and an electrical charge of $0$.

Electron

A subatomic particle that orbits the nucleus in shells, possessing a relative mass of $0.0005$ and a relative charge of $-1$.

Atomic Radius

The approximate size of an atom, determined by the outer spread of electron shells, measuring roughly $10^{-10}\text{ m}$.

Electrical Neutrality of Atoms

The state of having no overall electrical charge because the number of positive protons equals the number of negative electrons.

Ion

An atom or group of atoms that has gained or lost electrons, resulting in an imbalance between protons and electrons and an overall electrical charge.

Atomic Number ($Z$)

The total number of protons in the nucleus of an atom, uniquely defining which element it is.

Mass Number ($A$)

The total number of protons and neutrons combined within the nucleus of an atom.

Neutron Count Calculation

Calculated by subtracting the atomic number from the mass number: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$.