# 1.2 The mole concept

• Particles are classified as either: Atoms, ions, molecules or formula units
• To perform chemistry, moles of substance are used, and this allows us to make comparisons between chemical species.

### The Mole

• Avogadro’s constant NA = 6.02 x 1023 mol-1
• Mole: a fixed number of particles and refers to the amount, n, of substance
• Molar mass: mass of 1 mole of a substance (g mol-1)
• Number prefixes which are important to know:
• ### Mole Calculations ### Relative atomic mass & molar mass

• Isotopes: Atoms of the same element which have same number of protons
• Isotopes of an element have different mass numbers
• Relative abundance: Measure of percentage of isotopes present in element
• Relative atomic mass (Ar): weighted average of the atomic masses of its isotopes and their relative abundances
• Relative because compared to 1 atom of carbon-12 (12C) which is 12 units
• Relative molecular mass (Mr): Combining individuals Ar values of atoms in molecule or formula unit

### Empirical and molecular formula determination

• Empirical formula: simplest whole number ratio of atoms or amount (in mol) of each element present in a compound
• Molecular Formula: the actual number of atoms or amount (in mol) of elements in one structural unit or one mole of the compound