The structure of the atom
An atom is mainly empty space
It is thought that atoms are made of even smaller particles called subatomic particles.
The centre of an atom
Contains protons and neutrons so is positively charged
Occupies a small volume of the atoms, but is very dense
Electrons orbit the nucleus in energy levels or shells
Carry a negative charge and are held in orbit by electrostatic attraction
Atomic or Proton number
Number of protons in the nucleus
The number of protons in an atom determines what element it is
No. of protons (+) = No. of electrons (-) so an atom has no overall charge on it
The total number of protons and neutrons found in the nucleus.
Electron arrangement in an atom
Electrons move very fast around the nucleus in energy levels or shells
The innermost energy level can only hold 2 electrons
The next energy level can hold up to 8 electrons
The third energy level can hold up to 18 electrons
The electrons always fill the lowest available energy levels, so the innermost available shells are always filled before the outer ones.
The way the electrons are arranged is called the electron structure or electron configuration.
Atoms with a full outer shell of electrons are very unreactive.
Atoms of the same element which have different numbers of neutrons.
Most isotopes are stable, but a few have unstable nuclei.
If a nucleus has too many or too few neutrons it may split up or disintegrate.
If the unstable nucleus breaks up, the isotope is said to be radioactive.
When the nucleus breaks up it is said to decay, so the process is often referred to as radioactive decay.
Elements in the same group have similar properties because they have a similar electron arrangement
Group I or Alkali metals - Elements whose atoms have 1 outer-shell electron
Group II or Alkaline Earth Metals - Elements whose atoms have 2 outer-shell electrons
Group III - Elements whose atoms have 3 outer-shell electrons
Group IV - Elements whose atoms have 4 outer-shell electrons
Group V - Elements whose atoms have 5 outer-shell electrons
Group VI - Elements whose atoms have 6 outer-shell electrons
Group VII or Halogens - Elements whose atoms have 7 outer-shell electrons
Group 0, sometimes called group 8 or Noble Gases - Elements whose atoms have full outer shells so they are very unreactive.
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