Structure of Atoms

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All substances are made of tiny particles called atoms. We cannot see atoms, but an understandingof
what atoms are and the difference between atoms is very important if we are to make sense of
The Size of Atoms
Atoms are tiny. If 100 000 000 carbon atoms were placed in a line, the line would only be 7.7 mm long.
Atomic Structure
Atoms are made up of three smaller particles called protons, neutrons and electrons. The mass is shown
as a relative mass, which is the mass of the particle compared to a proton.
At the centre of the atom is a tiny nucleus, which is far smaller than the atom. The nucleus contains
protons and neutrons and so most of the mass of the atom is in the nucleus. Outside the nucleus are the
electrons which orbit the nucleus in shells.
Atoms are neutral because they contain the same number of electrons, which are negatively charged,
and protons which are positively charged.
Atomic number and Mass Number
Atoms of different elements have different numbers of protons, neutrons and electrons. Atoms are
described by their mass number and atomic number.
Atomic number = number of protons
Mass number = mass of protons + neutrons

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These two numbers can be used to work out how many protons, neutrons and electrons there are in an
atom. Remember that because atoms are neutral, the number of electrons will always be equal to the
number of protons.…read more


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