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Chemical calculations - both Tiers

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The mass number of an atom is its total number of protons and neutrons.

The relative formula mass of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] is found by adding together the relative atomic masses of all the atoms in the formula [formula: A formula is a combination of symbols that indicates the chemical composition of a substance. ] of the compound.

# Mass number

Structure of an atom

Each atom consists of a nucleus [nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. ] containing protons and neutrons, with electrons arranged around it.

Protonsprotons: Sub-atomic particles with a positive charge and a relative mass of 1. and neutronsneutrons: Uncharged sub-atomic particles, with a mass of 1 relative to a proton. both have a relative mass of 1 unit.

Electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] have a very small mass compared to protons and neutrons. Generally when working out the mass of atoms and molecules we can ignore the mass of the electrons.

Notice that most of the mass of an atom is found in its nucleus:

• the mass number of an atom is the total number of protons and neutrons it contains
• the atomic number (also called the proton number) is the number of protons it contains.

The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

The full chemical symbol for an element [element: A substance made of one type of atom only. ] shows its mass number at the top, and atomic number at the bottom. Here is the full symbol for carbon.

It tells us that a carbon atom has six protons. It will also have six electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 - 6 = 6 neutrons.

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