All atoms of the same element have the same number of protons in their nucleus.
For neutral atoms, which have no overall charge, the number of electrons is the same as the number of protons. The number of neutrons is just the mass number minus the atomic number.
Atoms form ions by gaining or losing electrons.
Isotopes- atoms with the same number of protons but different numbers of neutrons.
The number and arrangement of the electrons decides the chemical properties of an element. Physical properties tend to depend more on the mass of the atom.
Relative atomic mass- (Ar) the average mass of one atom of an element relative to 1/12 the mass of Carbon-12
Relative isotopic mass- the mass of an atom of an isotope of an element relative to 1/12 the mass of Carbon-12
Relative molecular mass- (Mr) the average mass of a molecule relative to 1/12 the mass of Carbon-12
The mass number of an atom is the number of protons plus the number of neutrons.
It is often given the symbol A.
The proton number of an atom is the number of protons in its nucleus.
