Chemistry
Atoms, moles and reactions — the counting rules behind every equation.
Key ideas
The atom in three particles
Protons fix which element it is, neutrons add mass, electrons do all the chemistry. The periodic table is organised by proton count — that is what atomic number means.
The mole is a counting unit
One mole is 6.022 × 10²³ particles — chemistry's "dozen". It connects the invisible (atoms) to the weighable (grams): one mole of carbon-12 is exactly 12 g.
Reactions conserve atoms
Atoms are never created or destroyed in a reaction, only rearranged. Balancing an equation is bookkeeping: the same count of each element on both sides.
Flashcards
Quick quiz
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Balance: __ H₂ + O₂ → __ H₂O
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