The atom is the smallest and most stable particle that maintains all the properties of an element . That is, the smallest part of matter that can be measured.

Each atom, in turn, has smaller parts, known as subatomic particles. These include protons, neutrons and electrons.

Atoms combine to form molecules that then interact to form solids, liquids and gases.

Structure of the atom

The atom is composed of two different regions: the nucleus, where the protons and neutrons are, and the cloud or electronic orbitals, where the electrons are located.

In the nucleus of the atom protons and neutrons are concentrated, while the electrons move in an electronic cloud.


The nucleus is the center or heart of the atom that is composed of protons and neutrons. Both protons and neutrons have mass, but only protons have a positive charge.

All the atoms of an element have the same number of protons, but the number of neutrons can change. These atoms with different number of neutrons are known as isotopes of an element.

For example, carbon-12 and carbon-14 are isotopes of the element carbon, which has 6 protons in its nucleus, but carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.

Electronic cloud

Surrounding the nucleus are the electrons in a kind of cloud. The electrons have a negative charge.

Atoms transform into ions when they gain or lose electrons.

If we made the comparison of the structure of the atom with a football field, the electronic cloud would occupy the entire football field, while the nucleus of the atom would be a grain of sand in the middle of the field.

The atom is composed of three subatomic particles: protons, neutrons and electrons.


It is the positively charged particle that is found in the nucleus of the atom. It has a mass of approximately 1.67 x 10 -24 grams, which is equal to 1 dalton.

The number of protons in an atom determines the atomic number of the element. For example, hydrogen has only one proton, so its atomic number equals 1.


It is the neutral particle of the nucleus of the atom with mass equal to that of a proton, that is, a dalton.


Electrons are the subatomic particles that orbit around the nucleus . The electron has a mass of 9.11 x 10 -28 grams, this is 1/1800 the mass of the proton. Its mass is so small that it is considered negligible.

Each electron has a negative charge. The charge of an atom is normally neutral, because it has as many protons as electrons, causing the positive charges of one to be canceled with the negative charges of the other.

History of the atom

The concept of atom as an indivisible unit dates from the fifth century BC. of C. The Greeks Leucippus and Democritus called atoms the smallest and indivisible particles of which they had knowledge. In fact, Democritus proposed that different types and combinations of these particles were responsible for all matter.

The chemist and meteorologist John Dalton (1766-1844) is considered the first to formulate the atomic theory. Dalton conducted experiments with gases that led him to formulate the concept of atomic structure in the early 1800s. Dalton’s atomic theory states the following ideas:

  • All the atoms of an element are equal.
  • The atoms of different elements vary in size and mass.
  • The atoms are indestructible. A chemical reaction results in the rearrangement of the atoms between reactants and products.

Discovery of the parts of the atom

The electrons were discovered by means of the experiments of JJ Thomson (1856-1940) studying the cathode ray tubes between 1898 and 1903. Thompson’s finding knocked down the theory of the indivisibility of the Dalton atom.

The Thomson model is known as “raisin pudding” where the atom is like a mass of positive and negative charges.

The nuclear atom

Ernest Rutherford (1871-1937) was a student of Thomson in 1911. Using radioactivity, he bombarded elements with alpha particles and discovered that the largest mass of the atom was concentrated in a tiny nucleus with a charge proportional to the atomic mass, thus demonstrating that the atom was not homogeneous.

Atomic model of Bohr

Niels Bohr (1885-1962) went to the University of Manchester to study with Rutherford. Bohr proposed that the electron in a hydrogen atom moves around the nucleus in certain circular orbits. However, the Bohr model applied to other atoms did not work.

In the current model of the atom, the electrons behave like waves but their position is unknown.

The three physicists featured in this model are Werner Heisenberg (1901-1976), Louis de Broglie (1892-1987) and Erwin Schrödinger (1887-1961). The approach that these researchers developed is better known as quantum mechanics .

In the quantum mechanical model of the atom, the electron behaves like a standing wave. The orbits are possible spatial distributions of the electron, in a kind of density map or electronic cloud.

Due to Heisenberg’s uncertainty principle, the movement of the electron around the nucleus can not be determined accurately, so Bohr’s orbits are incorrect.

As we can see, the history of the atom represents a chain of admirable discoveries, made possible by the work of great scientists.

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