Edexcel Chemistry- C2, Topic 1

Atoms

The nucleus of an atom:

• In the middle of the atom
• Contains protons and neutrons
• Has an overall positive charge as protons are positive and neutrons neutral
• Almost the whole mass of the atom is concentrated in the nucleus
• It is small compared to the whole atom

Electrons:

• Move around the nucleus
• Have a negative charge
• Tiny compared to the nucleus but cover a large area as they move around
• The size of their orbits determine the atom's size
• They have virtually no mass
• Occupy shells (energy levels) around the nucleus

Neutral atoms have no overall charge. The number of protons and electrons in an atom is always the same, as they have the same amount of charge, but it is opposite. The number of neutrons is not fixed, however it is normally a similar amount to the protons.

Particle                                      Relative Mass                                         Relative Charge

Proton                                                    1                                                              +1

Neutron                                                  1                                                                0

Electron                                             1/2000                                                            -1

Electron shells

• Electrons always occupy shells
• The lowest energy levels…