Elements

• Atoms can have different numbers of protons, neutrons, and electrons.
• All the atoms of a particular element have the same number of protons, and different elements have atoms with different numbers of protons.
• Isotopes are different forms of the same element, which have the same number of protons but a different number of protons.
• Isotopes have the same atomic number but a different mass number.
• Because many elements can exist as different isotopes, relative atomic mass (Ar) is used instead of mass number when referring to the element as a whole.
• Relative Atomic Mass (Ar) is an average mass taking into account the different masses and abundances of the isotopes that make up the element.
• The formula for relative atomic mass is:

  Ar = sum of (isotope abundance x isotope mass number) / sum of abundances of all the isotopes.
• An ion is an atom or group of atoms that has gained or lost electrons so it now has a positive or negative charge.
• A molecule is two or more atoms chemically bonded together.