Atom Timeline

Angelea Francesco

1803 - Dalton's Atom Model

His atomic theory stated:


1. Matter is made up of atoms that are indivisible and indestructible.

2. All atoms of an element are identical.

3. Atoms of different elements have different weights and different chemical properties.

4. Atoms of different elements combine in simple whole numbers to form compounds.

5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged.

1897 - Thomson's Atom Model

His atomic theory stated:


1. An atom consists of a sphere of positive charge with negatively charged electron embedded in it,

2. The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge.

1909 - Rutherford Atom Model

His atomic theory states:


1. Major portion of the atom is empty.

2. The whole mass of the atom is concentrated in the center of atom called nucleus.

3. The positively charged particles are present in the nucleus of atom.

4. The charge on the nucleus of an atom is equal to (+z.e) where Z= charge number, e = charge of proton.

5. The electrons revolve around the nucleus in different circular orbits.

6. Size of nucleus is very small as compare to the size of atom.

1913 - Bohr Atom Model

His atomic theory states:



1. Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits.

2. Each orbit has an energy associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on.

3. Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbedwhen it jumps from a lower to higher orbit.

4. The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies, e.g.,

Current Atom Model

Modern theory states:


1. Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits.

2. Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.

3. Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.

4. The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.