Introduction to Chemistry - Constitution of Matter


All matter is made up of very small particles. These particles we call atoms.

Atom - It is an indivisible particle.

About 2,500 years ago, the Greek philosopher Democritus said that if we divide matter into smaller and smaller pieces, we will come to indivisible grains, which are atoms (The = no and tome = part). In 1897, the English physicist Joseph Thompson (1856-1940) discovered that atoms were divisible: inside was the electron, the negatively charged particle.

In 1911, the New Zealander Ernest Rutherford (1871-1937) showed that atoms had a compact central region called the nucleus and that inside were the protons, positively charged particles.

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In 1932, the English physicist James Chadwick (1891-1974) discovered the neutron, neutral particle, proton companion in the atomic nucleus.

By the early 1960s, scientists already thought that protons and neutrons were even smaller particles. Murray Gell-Mann, born in 1929 suggests the existence of quarks, which would be these smaller particles. Quarks are held together by other particles called gluons.

In ancient times it was believed that atoms were indivisible and massive. In the twentieth century it was proved that atoms are formed by other particles. There are three fundamental particles: electrons, protons and neutrons.
The atom is divided into two parts: the nucleus and the electrosphere. Protons and neutrons are in the nucleus of the atom and electrons are in the electrosphere.

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These particles are characterized by their electric charges. The electron has a -1 charge and negligible mass (approximately 1/1836 being the proton mass). The mass of the proton would then be equal to 1 and the charge +1. The neutron has no electric charge and its mass is equal to that of the proton.

Look at the table between the mass ratios of the fundamental particles of the atom. The proton with a mass equal to 1 is adopted as standard:











Note that the electron's mass is 1,836 times smaller than that of the proton, so its mass is disregarded.

Atom Size

The size of the atom is measured in angstrons (Å).

1 angstron = 10-10meters

The average nucleus diameter of an atom is between 10-4 Å and 10-5 Å and that of the electrosphere is 1Å.

The electrosphere of an atom is between 10,000 and 100,000 times larger than its nucleus. This difference in size leads us to admit that the atom is almost made of empty space. In practical terms, if the nucleus were the size of a tennis ball, the first electron would be at a distance of 1 km.