Chemical Formulae and the Mole Concept
Writing chemical formulae is like balancing a budget - the charges must always cancel out to zero. Group numbers tell you an element's charge (Group 1 = +1 charge), and don't forget about polyatomic ions like hydroxide (OH⁻) and nitrate (NO₃⁻).
The mole is chemistry's way of counting particles - one mole contains 6.022 × 10²³ particles (Avogadro's number). Think of it like a dozen, but much bigger! You'll use the formula N = n × Nₐ to convert between moles and actual particle numbers.
Relative atomic mass (Ar) is the number at the top of each element on the periodic table, whilst relative formula mass (Mr) is the total mass of all atoms in a compound. For H₂O: Mr = 16 + (1×2) = 18 g/mol.
Key Formula Triangle: Remember Mass = Moles × Mr. Cover what you want to find, and the triangle shows you the calculation!
The conservation of mass means atoms can't disappear - whatever goes into a reaction must come out, just rearranged differently.
