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In general the atom is made from two groups of particles, nucleons and electrons. Nucleons are the protons and neutrons that are found in the nucleus of an atoms. (Hence the name "nucleon.") Electrons are found outside of the nucleus.

Electrons can be removed from the atom. Protons can be removed from the nucleus. Neutrons cannot be removed from the nucleus. When they are removed, neutrons are unstable and decay into something else. Neutrons cannot be removed from the nucleus and remain stable. The number of protons in an atom determine the element. The number of neutrons and electrons can vary. But if the number of protons changes, then the element has also changed. For example if carbon with 6 protons, increases the number of protons to 7 then the element is no longer carbon. It is now changed to nitrogen.

If an atom is "neutral," then number of charges must equal each other. Neutrons carry no charge. Protons carry a charge of +1.6x10^-19C. Electrons carry a charge of -1.6x10^-19C. If an atom is neutrally charged, then the number of protons and neutrons must equal each other. (In the diagram above, the atom is not neutral. It would be considered positively charged because there are more protons than neutrons.

An isotope is when the number of nucleons has changed but the number of protons has stayed the same. For example, Carbon has 3 isotopes, carbon-12, carbon-13 and carbon-14. Each of these elements has 6 protons. But the number of nucleons is different between them, (12, 13, and 14 nucleons.)

Carbon-14 is the only unstable isotope of carbon. After 5730 years, half of the carbon-14 that was on the planet will change to nitrogen-14. 5730 years is consider to be half-life of carbon-14. Different isotopes of different elements will have varying half lives. A half-life determines the time it takes for half of a sample to under go radioactive decay into a a new material. Sometimes the new material is a new element. Other times the new material is a different isotope.

Fundamental particles are the smallest possible particle that make up the parts of an atom. An electron is a fundamental particle. Scientists have not been able to crack it open to see if anything smaller exists inside of it.

Protons and neutrons are NOT fundamental particles. They can be broken up into smaller particles. They are made of particles called "quarks." The study of quarks is in itself another course or series of courses. Suffice it to say there are 6 basic quarks. there are several characteristics that describe the quarks. Below are quarks and their respective charges.

A proton is made from 2 up quarks and 1 down quark. A neutron is made from 1 up quark and 2 down quarks.

Now the model of an atom is looks more detailed that before.