Friday 7 June 2013

chemistry GCSE revision notes - atoms and bonds

ATOMS AND BONDS

ATOMS

The nucleus
  • contains protons and neutrons
  • protons positively charged; neutrons with no charge
  • nucleus has a positive charge overall because of the positive chare of the protons
Electrons
  • negatively charged
  • occupy shells around the nucleus
  • tiny but cover a lot of space
protons = no. of electrons so atoms have no charge because the positive and negative charges cancel each other out

*if some electrons are added or removed the atom becomes charged and it is then an ion*

ELEMENTS
  • elements consist of one type of atom only
  • atoms can have different numbers of protons, neutrons and electrons
  • it's the number of protons in the nucleus that decides what type of atom it is
periodic table
  • the periodic table puts elements with similar properties together
  • all of the elements in the same group have the same number of electrons in their outer shell so they have similar properties
  • the more electrons they have in their outer shell, the less reactive and more stable they are as they don't need to steal any electrons
ELECTRON SHELLS
  • electrons occupy shells
  • the shells closest to the nucleus are filled in first and can only hold 2 electrons, the rest of the shells can carry 8 electrons

  • atoms are much happier when they have full shells
  • in most atoms the outer shell is not full and this makes them want to react to fill it. the fewer the electrons in the outer shell, the more reactive they are.
 Na
14 7
 
the bottom number of nitrogen tells us that it has 7 protons so it must have 7 electrons. we show it like the diagram above.


COMPOUNDS

when different elements react, atoms form chemical bonds with different atoms to form compounds. there are 3 different types of these chemical bonds.



an IONIC BOND is formed between metals and non-metals the metal donates one or more electron to the non-metal; making the metal positive and the non-metal negative. the attraction between the positive metal and the negative non-metal causes an ionic bond.

properties of an ionic bond

  • high melting and boiling points
  • strong bonds
  • conduct electricity when melted of dissolved in water because the ions are free to move
  • cannot conduct electricity when a solid because the ions cannot move around
an ionic structure consists of a giant lattice of these oppositely charged ions.



a COVALENT BOND is a strong bond between two non-metal atoms that have a shared pair of electrons.

a covalent molecular structure consist of these non-metal with a shared electron being held together by weak intermolecular structures. these molecular structure have low melting and boiling points and do not conduct electricity.

a covelant network structure consists of a giant lattice of covalently bonded atoms. as a result of this they have very high boiling and melting points but only some of them conduct electricity.



a METALIC BOND is formed between two metal that become positively charged.

a metallic structure is a giant lattice of these positively charged metal ions and delocalised outer electrons. these substances can be bent and shaped because the layers of atoms slide over each other and they also conduct electricity because the delocalized electrons are free to move.