Atomic Structure

Atom: the smallest part of an element. They cannot be broken down into anything simpler in a                 chemical reaction.

Molecule: Two or more atoms that are covalently bonded together.

Ion: a charged particle. Usually forms when atoms gain or lose electrons.

Compound: a subsatnce containing two or more elements bonded together.

Relative atomic mas: The average mass of an atom of the element compared to 1/12 of the mass                         of one atom of carbon-12

Relative isotopic mass: the mass of one atom of the isotope compared to 1/12 of the mass                         of one atom of carbon-12

The nucleus is tiny compared to the size of the atom but it contains nearly all the mass of the atom. In an atom the number of protons is equal to the number of electons because an atom has no charge. This works because electrons and protons are oppositely charged. In negative ions the atom has gained electrons, so has more electrons than protons. In positive ions the atom has lost electrons so has more protons than electrons.

Isotopes are atoms of the same element, and therefore have the same number of protons (atomic number) but a different number of neutrons, which will affect the mass number. This does not effect how the chemical reacts because it still has the same number of neutrons in it's outer shell. Different isotopes are detected using a mass spectrometer.

• Electron shells increase in energy the further they get away from the nucleus.
• Electron shells can be split into further sub-shells: s, p and d
• Each sub-shell contains orbitals
• An orbital is a region or volume of space in an atom where two electrons with the opposite spin are found.
• S sub-shells have 1 s orbital so can hold 2 electrons
• P sub-shells have 3 p orbitals so can hold 6 electrons
• D sub-shells have 5 d orbitals so can hold 10 electrons

The amount of electrons that can be held in each shell changes a bit from GCSE:

• The 1st shell still holds 2 electrons
• The 2nd shell can have up to 8
• The 3rd shell holds up to 18 electrons

This is made clearer when we look at the sub-shell:

• The Ist shell has 1 s sub-shell
• The 2nd shell has an s sub-shell and a p sub-shell
• The 3rd shell has an s sub-shell, a p sub-shell and a d sub-shell

The sub-shells cloest to the nucleus fill up with electrons first:

• 1s
• 2s
• 2p
• 3s
• 3p
• 4s
• 3d
• 4s.... etc

Interestingly the 4s sub-shell fills before the 3d becuase it is at a lower energy level.