Ancient Greek
Ancient Greek atomic theory, pioneered by Leucippus and Democritus in the 5th century BC, proposed that all matter is composed of indivisible particles called atoms. These atoms were thought to be eternal, unchangeable, and in constant motion within empty space. While this theory was a significant early contribution.
- The core idea: Leucippus and Democritus believed that all matter is made up of tiny, indivisible particles, which they termed “atomos” (meaning uncuttable or indivisible).
- Key properties of atoms: These atoms were thought to be eternal, unchanging, and in constant motion.
- Different materials: Democritus and Leucippus suggested that different materials were formed by different arrangements and shapes of these atoms
Dalton’s Atomic Theory
The English chemist John Dalton suggested that all matter is made up of atoms, which were indivisible and indestructible. He also stated that all the atoms of an element were exactly the same, but the atoms of different elements differ in size and mass. Chemical reactions, according to Dalton’s atomic theory, involve a rearrangement of atoms to form products. According to the postulates proposed by Dalton, the atomic structure comprises atoms, the smallest particle responsible for the chemical reactions to occur.
- Every matter is made up of atoms.
- Atoms are indivisible.
- Specific elements have only one type of atom in them.
- Each atom has its own constant mass that varies from element to element.
- Atoms undergo rearrangement during a chemical reaction.
- Atoms can neither be created nor destroyed but can be transformed from one form to another.
Demerits of Dalton’s Atomic Theory
1.The theory was unable to explain the existence of isotopes.
2.Nothing about the structure of the atom was appropriately explained.
3.Later, scientists discovered particles inside the atom that proved the atoms are divisible. The discovery of particles inside atoms led to a better understanding of chemical species; these particles inside the atoms are called subatomic particles.
The discovery of various subatomic particles is as follows
Thomson Atomic Model
The English chemist Sir Joseph John Thomson put forth his model describing the atomic structure in the early 1900s. He was later awarded the Nobel Prize for the discovery of “electrons”. His work is based on an experiment called the cathode ray experiment.
The construction of working of the experiment is as follows:
- Cathode Ray Experiment It has a tube made of glass which has two openings, one for the vacuum pump and the other for the inlet through which a gas is pumped in. The role of the vacuum pump is to maintain a “partial vacuum” inside the glass chamber. A high-voltage power supply is connected using electrodes, i.e., cathode and anode, which are fitted inside the glass tube.
Rutherford Atomic Theory
Rutherford, a student of J. J. Thomson, modified the atomic structure with the discovery of another subatomic particle called “Nucleus”. His atomic model is based on the Alpha ray scattering experiment. Alpha Ray Scattering Experiment
1. A very thin gold foil of 1000 atoms thick is taken.
2. Alpha rays (doubly charged Helium He2+) were made to bombard the gold foil.
3. Zn S screen is placed behind the gold foil.
Rutherford’s Structure of Atom Based on the above observations and conclusions, Rutherford proposed his own atomic structure, which is as follows.
- The nucleus is at the center of an atom, where most of the charge and mass is concentrated.
- The atomic structure is spherical.
- Electrons revolve around the nucleus in a circular orbit, similar to the way planets orbit the sun.