What is an Atom?
Atoms make up all the matter around us including ourselves but what is an atom? The original definition of the word is ancient Greek and it was assigned to the theoretically smallest particle that could not be divided (atoma).
These days we know that atoms are made of yet smaller particles. There are three smaller particles that make up individual atoms. These are called subatomic particles as they are below the level of the atom in size.
The three particles are different in size and charge. Neutrons have no charge and are the largest subatomic particles. They are roughly the size of both a proton and an electron put together. Protons are slightly smaller than neutrons and have a positive (+) charge. Electrons are the smallest of all and carry a negative (-) charge.
The + and – charges are simply opposites of each other. They have been labeled + and – for convenience. Of course in reality these subatomic particles do not wear labels with their charge on them but it is easiest to the atom’s structure if we keep the labels in place.
It is also important to know that like charges repel and that opposite charges attract. This is best seen with two bar magnets. If you place the two North or South ends together they push apart whereas if you place a North and South end together they pull toward each other.
The protons and neutrons are clumped together in the middle of an atom and the electrons orbit around the outside. While this seems to contradict the idea that like charges repel, scientists have established that though protons (+) do indeed repel each other, once they are very close to each other another force, called the Strong Force, takes over and glues them together. The exact mechanism behind this force is not well understood.
What is an atom — Electrons Orbiting
The electrons orbit the nucleus at great speed and distance. They are held in orbit by the pull of the oppositely charged protons in the nucleus and their speed prevents them from collapsing into the nucleus. This is similar to how satellites orbit the earth. As an example, for a Helium atom the structure is like this:
The electrons orbit the nucleus at such a great distance that 99.99% of the atom is empty space. The different numbers of protons and neutrons in the nucleus result in different elements and isotopes of those elements.