Atomic number = No. of protons
Mass number = No. protons + neutrons
Neutrons: relative charge = 0, relative mass = 1
Protons: relative charge = +1, relative mass = 1
Electrons: relative charge = -1, relative mass: 1/1836 (negligable)
Isotopes = atoms that have the same atominc number but different mass numbers.
no. electrons = no. protons
Atoms are arranged in periodic table in order of increasing atomic number.
No. electrons in outer shell is the same as the group number (for groups 1-7)
Elements in group 0 (noble gases) have 8 electrons in their outer shell, except for helium.