Brief History of The Development of The Modern Atomic Theory

Throughout history, people have tried to find the ultimate substance that makes up matter. Thales of Miletus (634-546 BC) proposed that all things came from one prime material – water. Empedocles (490-430 BC), a Greek philosopher, proposed that matter is made up of four elements, namely, earth, water, air and fire. Democritus (460-370 BC), a famous Greek philosopher, develops the theory of atoms. He theorized that matter is made up of tiny indivisible atoms (in Greek, atomous means ‘uncut’). He believed that the variety of observable things in the world can be explained as combinations of atoms of various sizes and shapes.

Aristotle (384-322 BC), the most famous Greek philosopher, rejected Democritus’ idea of theatomic nature of matter. He thought that matter could be subdivided into minute particles. He said that matter could be divided further and further with each becoming smaller and smaller without end.

Many centuries later, John Dalton (1766-1844), an English chemist and physicist, revived the idea of Democritus on the nature of matter. He pictured and atom as a tiny, indestructible sphere endowed with mass.

Joseph John Thompson (1856-1940), an English physicist, proposed another model of the atom. He saw an atom as a positive sphere with negative electrons stuck in it like raisins in a loaf of bread.

Ernest Rutherford (1871-1937) later performed an experiment that tested Thompson’s model and proved that it was wrong. He concluded -and today’s atomic physicists agree – that an atom is mostly empty space. It has a tiny positive core, called the nucleus. The mass of an atom is concentrated in its nucleus. The electrons of an atom are smaller spheres revolving around the nucleus. His model is known as the nucleus model (or nuclear model) of the atom.

Rutherford predicted that there must be another particle in the nucleus of an atom with a mass and no charge. The particle was later discovered by Sir James Chadwick (1891-1974), and called it the neutron. This view prevails today.

Based on these discoveries, it can be said that the proton, the electron and the neutron are the building block of an atom.

The unified atomic mass unit (u) is the unit in which atomic masses are measured. It is defined as 1/12 the mass of the carbon-12 atom. The number of positive charges (protons) determines the element and is equal of its atomic number. Since an atom is electrically neutral, the number of protons in it must be equal to the number of electrons.

The mass of an atom is concentrated in its nucleus. Therefore, the mass number of an atom is equal to the sum of the masses of the protons and the neutrons.

In 1913, Niels Bohr (1885-1962), a Danish nuclear and theoretical physicist, modified Rutherford’s theory of the atom. He saw the atom as a miniature solar system with its nucleus as the sun and the electrons whirling around it as the planets that orbit around the sun.

The electrons are arranged around the nucleus in what are called energy levels or shells. Each electron in each level contains a definite amount of energy.

Studies made on the behavior of electrons showed that each main energy level or shell in an atom can have what are known as sublevels or sub shells. These are denoted by the letters s, p, d, and f, representing the lines that appear when light is emitted by an atom.

Arnold Sommerfeld modified Bohr’s model of an atom by adding the now accepted elliptical orbits to explain certain experimental data.