Atomic number and mass number are terms used to describe the properties of atoms, particularly in the context of isotopes.
Atomic number (Z) represents the number of protons found in the nucleus of an atom. It defines the identity of an element and determines its position on the periodic table. For example, all hydrogen atoms have an atomic number of 1 because they have one proton in their nucleus.
Mass number (A) represents the total number of protons and neutrons in the nucleus of an atom. It is an integer value and is approximately equal to the atomic mass of an atom, although it doesn't account for the mass of electrons. For example, a helium atom has an atomic number of 2 (two protons) and a mass number of 4 (two protons plus two neutrons).
Isotopes are different forms of an element that have the same atomic number (Z) but different mass numbers (A). In other words, isotopes of an element have the same number of protons but different numbers of neutrons. Since neutrons contribute to the mass of an atom but not its charge, isotopes of an element have different atomic masses.
For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 has an atomic number of 6 (six protons) and a mass number of 12 (six protons plus six neutrons). Carbon-13 has an atomic number of 6 (six protons) and a mass number of 13 (six protons plus seven neutrons). Carbon-14 has an atomic number of 6 (six protons) and a mass number of 14 (six protons plus eight neutrons).
Isotopes often exhibit similar chemical properties due to the same number of protons, but they can have different physical properties, such as atomic mass and stability, due to varying numbers of neutrons.