Dalton's Atomic Theory and the Evolution of Atomic Models

Dalton's Atomic Theory

John Dalton proposed that elements were made up of little solid spheres without internal structure. Atoms of the same element were equal. Atoms combine, forming compounds. Depending on the ratio, we can have different compounds.

J.J. Thomson

First model of the atom (1904). He discovered/proved the existence of tiny, negatively charged particles: electrons.

Rutherford's Model of the Atom

Gold foil experiment, Ernest Rutherford (1911). Alpha particles are tiny, positively charged particles much smaller than an atom. An atom is mostly empty space. Some particles will bounce back, and some others will pass but in a different way. The nucleus is something dense and positively charged.

Thomson's Atomic Model

In 1904, Thomson's atomic model showed the atom as a positively charged ball of matter with electrons floating freely around inside of it. Positive and negative charges were balanced.

The Composition of the Nucleus

Positive charges are concentrated in a very small fraction of the atom's volume. In 1918, Rutherford discovered other elementary particles, protons, and suggested that the nucleus should also contain other particles: neutrons. In 1932, James Chadwick was able to prove experimentally that these neutral particles do exist.

Bohr's Atomic Model

He used the same basic structure as Rutherford but suggested that the motion of electrons had to be restricted to certain special orbits in which electrons did not fall towards the nucleus. Each orbit has a fixed radius and energy level and can accommodate only a certain number of electrons. The energy of an electron depends on the size of its orbit and is lower for small orbits. Electrons could jump between orbits and would emit or absorb energy in the form of electromagnetic radiation (light).

Orbital: It is a 3D region where we can find the electron.

Orbit: Fixed path (circular or elliptical).

Ions are atoms with an electric charge.

Anions are atoms that have gained electrons (e-) and get negatively charged.

Cations are atoms that have lost electrons and get positively charged.

Isotopes

Isotopes are atoms of the same element (same number of protons) with different mass numbers, which means that isotopes have different numbers of neutrons.