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Not until around 460 B.C., did a Greek philosopher, Democritus, develop the idea of atoms. He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

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In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms).

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Isotopes are atoms with the same number of protons but different number of neutrons

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After the discovery of the electron, it was realized that there must be positive charge centers within the atom to balance the negative electrons and create electrically neutral atoms.

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The term isotope was first used by Scottish doctor Margaret Todd in a conversation with her cousin Soddy in 1913. Soddy was the first to isolate the isotope by breaking down uranium and he saw that they had different masses.  These components were sometimes heavier in mass than the main chemical.

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atom n. A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, the entire structure having an approximate diameter of 10-8 centimeter and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

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The word proton is Greek for "first", this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental particle, and hence a building block of nitrogen and all other heavier atomic nuclei.

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A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collectively referred to as "nucleons".

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A neutron is a particle with very similar mass to a proton but with no charge

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alpha particles

beta particles (electron emission, positron emission)

neutrino

positron

lepton

photon

quark

antiparticle

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The reason protons and neutrons stick together is because of the strong force. The strong force is much more powerful than the electrical repulsion between protons, but the particles have to be close to each other for it to stick them together.

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Protons and neutrons are made up of smaller subatomic particles. When protons or neutrons get close enough to each other, they exchange particles (mesons), binding them together. Once they are bound, it takes considerable energy to break them apart. To add protons or neutrons, the nucleons either have to be moving at high speed or they need to be forced together under great pressure.

Although the strong force overcomes electrostatic repulsion, protons do repel each other. For this reason, it's usually easier to add neutrons to an atom than to add protons.

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Leucippus, Democritus, Epicurcurus and Lucretius are the first greek philosophers which theorized the idea of atomism, and where convinced that matter was made of discrete units. This was happening as early as the 5th century B.C.

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Leucippus, Democritus, Epicurcurus and Lucretius are the first greek philosophers which theorized the idea of atomism, and where convinced that matter was made of discrete units. This was happening as early as the 5th century B.C.

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Other atomism "hotspots" could be found in India more or less in the same century as the Greek philosophers.

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The neutron was discovered in 1932 by James Chadwick

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Rutherford discovered that the atomic mass is a bit more than twice the atomic number. By setting the DeBroglie wavelength it shows no electrons were in the nucleus. Jame Chadwick placed a polonium source releasing alfa particles hitting the boron target. The alfa particles knocked off neutrons from the boron. As Chadwick couldn't detect any charge he placed wax behind the boron and detected beta particles were being released form it. Chadwick deduced that a new particle without charge was hitting the wax.

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By and experiment consisting in firing alpha particles at a very thin sheet of gold foil. The aim of the experiment was to measure how much the alpha particles were deflected from their original course when passing through the gold foil.

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It is found at the center of the atom and it is made of neutrons and protons held together by "strong forces". These force between the protons and the neutrons, overcomes the repulsive electrical force that would, according to the rules of electricity, puch the protons apart otherwise.

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The term isotope comes from the Greek word Isos that means equal and Topos, that means place.
The discovery of isotopes was not only useful to chemistry but for many other things. For example nuclear weapons and energy. In medicine, isotopes are used in photosynthesis to study the effect of animal metabolism in food. They are also used for radiation therapy to treat cancer.
Each isotope had its own properties and different use.

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Creating brand new atoms requires loading the atom's nucleus with more protons. The transuranium elements (95 to 100) were forged by bombarding uranium with neutrons and waiting for the nucleus to become radioactive and convert its extra neutron into a proton, electron and antineutrino. After fermium this technique stopped working.
The scientist started to get the nuclei of two atoms and fuse them together to one giant new unique atom. Ex. neon + uranium = nobelium

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An electron is a stable subatomic particle (the first to be discovered) with a charge of negative electricity found in all atoms. Joseph J. Thomson discovered the electron in 1897 at the Cavendish laboratory in Cambridge. It was known that the glow emitted when a high voltage was applied in a gas volume at low pressure was because of the rays coming from the cathode, the negative pole of the applied high voltage. Thomson therefore decided to conduct a series of experiments to investigate the properties of the rays. Thomson placed a cathode in electric and magnetic fields. He knew that these fields would not have much effect of waves but would bend a stream of particles. Thomson observed that the rays moved from side to side and so concluded that the cathode emitted negatively charged particles which he called "corpuscles". Interestingly enough, Thomson thought these particles wouldn't be very useful outside of a science lab.