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An atom is the smallest particle of an element that still is that element.
The atom is made up of several parts. It has a center part called the nucleus. The nucleus is made up of a positively charged particle called a proton and a particle that does not have a charge called a neutron. The nucleus is surrounded by negatively charged particles called electrons that move in orbits around the nucleus. Most of the mass of the atom is contained in the nucleus and the mass of the proton and neutron are about the same. However, the nucleus is a small portion of the total amount of space the atom takes up. Different types of atoms can be identified by the numbers of protons, neutrons and electrons they contain and their atomic mass.
Atoms that have the same number of protons but different numbers of neutrons are called isotopes.
The negatively charged electrons are very light in mass. An atom is described as neutral if it has an equal number of protons and electrons or if the number of electrons is the same as the atomic number of the atom. The electrons that are outermost can combine with other atoms by giving up or taking on electrons. This can cause atoms to have an electrical charge and these kinds of atoms are called ions. Atoms can also share electrons and this is called a chemical bond.
The atomic number of an atom is the number of protons it has. The atomic weight or mass of the atom is the total number of protons and neutrons in the atom's nucleus.