Democritus was a Greek philosopher, 460 BC
Roger Joseph Boscovich. A Croatian Jesuit who provided a prototype of the atomic theory
John Dalton (1766–1844), English chemist and physicist
Sir Joseph John Thomson (1856–1940), English physicist, discovered the electron and its negative charge. He got the Nobel Prize in Physics
In chemistry and physics, the **atomic theory** explains how our understanding of the atom has changed over time. Atoms were once thought to be the smallest pieces of matter. However, it is now known that atoms are made of protons, neutrons, and electrons. These subatomic particles are made of quarks. The first idea of the atom came from the Greek philosopher Democritus. A lot of the ideas in the modern theory came from John Dalton, a British chemist and physicist.

The theory applies to solids, liquids and gases. It does not apply in the same way to plasmas or neutron stars.

Democritus' atomic theory
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Democritus thought that if you cut something in half again and again, you would at last have to stop. He said that this last piece of matter could not be cut any smaller. Democritus called these small pieces of matter atoms, which means "indivisible". He thought that atoms would last forever, never change and could not be destroyed. Democritus thought that there was nothing between the atoms and that everything around us could be explained if we could understand how atoms worked. 

Some other philosophers agreed, and others disagreed. They had no way to experiment to show whether his theory was true or not.

Boscovich's atomic theory
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In 1758, Roger Joseph Boscovich described a precursor of the atomic theory.

Dalton's atomic theory
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In 1803, the English scientist John Dalton reworked Democritus' theory, as follows:

1. All matter is formed of atoms.
2. That atoms are indivisible and invisible particles.
3. That atoms of the same element are of the same type and mass.
4. The atoms that make chemical compounds are present in set proportions.
5. Chemical changes correspond to a reorganization of the atoms taking part in the chemical reaction.

Dalton defined the atom as the *basic unit of an element that can take part in a chemical combination*. Dalton and another chemist, Joseph Proust had discovered two laws: the law of definite composition and the law of multiple proportions. They discovered these laws when they measured how elements combined into compounds. These were the basis for Dalton's theory.

* The law of definite composition says that the masses of elements that join to form a compound are always in the same ratio, or proportions.
* The law of multiple proportions says that when two chemical elements combine with each other to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.

Together, these were good evidence that atoms existed.

Thomson's atomic model
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Schematic representation of the Thomson model.
In 1850, Sir William Crookes constructed a 'discharge tube', that is a glass tube with the air removed and metallic electrodes at its ends, connected to a high-voltage source. When creating a vacuum in the tube, a light discharge can be seen that goes from the cathode (negatively-charged electrode) to the anode (positively-charged electrode). Crookes named the emission 'cathode rays'.

After the cathode ray experiments, Sir Joseph John Thomson established that the emitted ray was formed by negative charges, because they were attracted by the positive pole. Thomson knew that the atoms were electrically neutral. He established that, for this to occur, an atom should have the same quantity of negative and positive charges. The negative charges were named *electrons* (*e*-).

According to the assumptions established about the atom's neutral charge, Thomson proposed the first atomic model. It was described as *a positively-charged sphere in which the electrons were inlaid (with negative charges)*. It is known as the *plum pudding model*.

In 1906, Robert Millikan determined that the electrons had a Coulomb (C) charge of -1.6 \* 10−19. This allowed calculation of its mass as tiny, equal to 9.109 \* 10−31 kg.

In the same time, experiments by Eugene Goldstein in 1886 with cathode discharge tubes allowed him to establish that the positive charges had a mass of 1.6726 \* 10−27 kg and an electrical charge of +1,6 \* 10 −19 C. Lord Ernest Rutherford later named these positively charged particles *protons*

Rutherford's atomic model
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Atomic experiment of Lord Ernest Rutherford
In 1910, the New Zealand physicist Ernest Rutherford put forward the idea that the positive charges of the atom were found mostly in its center, in the nucleus, and the electrons (*e*-) moved around it.

Rutherford showed this when he used an alpha radiation source (from helium) to hit the very thin gold sheets, surrounded by a Zinc sulphide lampshade that produced visible light when hit by alpha emissions. This experiment was called the Geiger–Marsden experiment or the Gold Foil Experiment.

By this stage the main parts of the atom were clear. It was known that atoms of an element may occur in isotopes. Isotopes vary in the number of neutrons present in the nucleus. Although this model was well understood, modern physics has developed further. Present-day ideas cannot be made easy to understand. Some idea of present-day atomic physics can be found in the links in the table below.

Modern physics
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Atoms are *not* elementary particles, because they are made of subatomic particles like protons and neutrons. Protons and neutrons are also not elementary particles. They are made of even smaller particles called quarks joined together by other particles called gluons (because they "glue" the quarks together in the atom). Quarks *are* elementary because quarks cannot be broken down any further.