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Protons, Neutrons and Isotopes

Protons and neutrons have a mass of one unit, but electrons have very little mass. The mass number of an atom is the number of protons + neutrons in the atom.

Isotopes are atoms of the same element with different mass numbers.

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Relative Atomic and Formula Mass

The relative atomic mass (Ar) of an element is the average mass of its isotopes compared with an atom of the 12C isotope.

The relative formula mass of a compound (Mr) is found by adding up the relative atomic masses of the atoms shown in its formula.

One mole of a substance is its Ar or Mr weighed out in grams.

Percentages of elements in compounds can be calculated from Ar and Mr.

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Percentages by mass can also be used to calculate empirical formulae.

Masses of reactants and products can be calculated from balanced equations.

The percentage yield of a reaction does not always equal the theoretical yield.

The atom economy of a reaction is the mass of atoms in the useful products as a percentage of the mass of the mass of all the atoms in the reactants

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Reversible Reactions

Reversible reactions can be used efficiently in industrial processes like the Haber process, in which nitrogen and hydrogen react to make ammonia.

In a closed system a reversible reaction can reach equilibrium.

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