Atom
An atom is the basic type of particle that makes up matter. It is the smallest part of an element, which has the properties of that element.Atoms are made up of 3 sub-atomic particles - protons, neutrons and electrons. The centre of an atom is a very small and dense region called the nucleus. The nucleus contains protons and neutrons, which are closely packed together.
The nucleus is very small even compared with the atom so the atom is mostly empty space.
There are only 94 different types of atoms that occur naturally in the world. (A few more have been made artificially in a laboratory). Every different kind of atom has a name and every name has an abbreviation. The 94 kinds of atoms are usually listed in a table. The table is called the periodic table. A scientist named Dmitri Mendeleev was the first to think of the table. In recognition of this, and other accomplishments, element 101 was named Mendelevium when it was discovered.
The first 11 kinds of atoms (also called elements) are called hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon and sodium. Their abbreviations are H, He, Li, Be, B, C, N, O, F, Ne, Na. You may notice that Sodium does not fit with Na. This is because sodium was most commonly called natrium in the past. This has carried over into other fields of science as well. Hyponatremia is a medical condition where the body lacks enough salt to function properly.
All matter and all substances are made up of these 94 kinds of atoms. Most things contain far fewer types of atoms. Water, for example, contains only atoms of hydrogen and oxygen. In water these atoms form combinations called molecules.