The History of the Atom
The Early Models of the Atom
- Democritus (460-370 BC) first introduced the concept of the atom, which he thought to be an indivisible unit from which everything was composed.
- John Dalton (1803) introduced his atomic theory, suggesting that all matter is composed of atoms, which are indivisible and indestructible particles.
- Dalton proposed that all atoms of a given element are identical in mass and properties.
Discovery of Sub-Atomic Particles
- J.J. Thompson's experiments (1897) led to the discovery of electrons – very small, negatively charged particles present in an atom.
- Thompson developed the "plum pudding" model of the atom, wherein negative electrons were embedded in a spread of positive charge, like plums in a pudding.
- Rutherford’s alpha scattering experiment (1909) showed that most of the mass and positive charge is concentrated at the centre (the nucleus) of the atom.
- This led to the development of the "nuclear model" of the atom.
- Niels Bohr (1913) further improved this model by introducing energy levels or shells for electrons.
- James Chadwick (1932) is credited with discovering the neutron – a neutral particle found in the nucleus along with protons (positively charged particles).
Modern Understanding of the Atom
- Today, atoms are known to consist of a nucleus (containing protons and neutrons) and electrons.
- Electrons move around the nucleus in energy levels or shells.
- The number of protons (atomic number) identifies each element.
- The total number of protons and neutrons (mass number) identifies each isotope of the element.
Key Concepts and Terminology
- Isotopes: These are atoms of the same element (same number of protons) but with different numbers of neutrons (different mass numbers).
- Ions: These are charged atoms or groups of atoms. They are formed when atoms lose or gain electrons.
- Orbitals: These are regions of space where there is a high probability of finding an electron. They are grouped into energy levels or electron shells.
Make sure you have a thorough understanding of these concepts as they form the foundation of the atomic theory.