# What is the difference between atomic number and mass number?

The atomic number of an atom is the number of protons that resides in the nucleus, while the mass number is the number of protons and neutrons combined. The atomic number determines the type of element an atom, while the number of neutrons determines the isotope of the element. Many elements naturally occur in the form of several different isotopes.

All atoms have a nucleus, composed of a cluster of protons and neutrons, as well as an electron “cloud” that surrounds the nucleus. Protons are positively charged particles, neutrons are particles that have no charge, and electrons are particles that have a negative charge.

In most circumstances, the net charge of an atom is neutral because the protons and neutrons are found in identical quantities. When an atom gains or loses an electron, the atom becomes a charged ion. If an electron is lost, the atom then becomes positively charged; however, if it gains an electron, the atom becomes negatively charged.

If an atom loses a neutron and becomes an isotope, its properties may differ from those that are in the natural configuration. Often, atoms that are radioactive undergo a decay process that results in the atom losing its neutrons.

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