Atomic Number, Mass Number, Atomic Mass and Isotopes Atomic Number: is the number of protons in the nucleus of the atom. All atoms of the same element have the same atomic number and the same protons. If the protons change, the element is also not the same. Atoms have no overall electrical charge so, an atom must have as many electrons as there are protons in its nucleus. Atomic number = Protons = electrons A=P=E Ex: Sodium (Na) What is the atomic number of Sodium? How many protons does sodium have? How many electrons does sodium have? Mass Number: • The sum of the protons and neutrons in the nucleus. • Mass – Atomic = Neutrons M–A=N Nuclear Notation NUCLEAR NOTATION : Gives specific information about the nucleus Other ways to write elements: Mass Number Mass Number 36 17Cl Atomic Number Cl-36 HYPHEN NOTATION: Gives only specific mass number. Nucleons: protons and neutrons Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they have the same atomic number. What type of notation is used above? Example: Isotopes of Carbon and Hydrogen Isotopes of Hydrogen protium deuterium H H Isotopes of Carbon tritium H Atomic Mass Unit is a unit used to compare the masses of atoms and has the symbol u or amu. 1 amu or u is approximately equal to the mass of a single proton or neutron. Atomic Mass is the weighted average mass of all the naturally occurring isotopes of that element.