GCSE Chemistry

• Acids have more H+ than OH- ions, resulting in a lower pH value.
• A solution with a pH value of 7 is neutral because it has an equal number of hydrogen (H+) and hydroxide (OH-) ions.
• atoms


  building blocks of all substances
• relative mass


  proton: 1

  neutron: 1

  electron: 0
• relative charge


  proton: +1

  neutron: 0

  electron: -1
• ion


  an atom that has a positive or negative charge
• isotopes


  atoms of the same element with different numbers of neutrons (chemically react in the same way)
• relative atomic mass


  (average mass of all the isotopes that make up an element)

  sum of isotope abundance x isotope mass/sum of abundance of all isotopes
• elements


  substances made out of one type of atom
• compounds


  made of 2 or more different elements chemically bonded together
• total mass of reactants = total mass of products
• mixture


  made up of two or more substances that are not chemically combined together, and can be separated by physical means
• fractional distillation


  (two liquids with similar boiling points)

  • uses fractionating column
  • possible because of different boiling points of the liquid in the mixture
• paper chromatography


  (identify substances from a mixture in solution)

  • separates them as they move up the paper
  • different substances are separated because of their different solubilities in the solvent used
• gain electrons -> negative ion

  lose electrons -> positive ion
• the number of electrons on the outermost shell determines how the element reacts
• metals -> lose electrons to form positive ions

  non-metals -> gain electrons to form negative ions
• group 1 properties (alkalis)


  -react with water to produce hydrogen and an alkaline solution

  -melting and boiling points decrease

  -reactivity increases

  -form +1 ions <- make ionic compounds

  -are white and dissolve in water, giving colorless solutions
• group 7 properties (halogens)


  -form ions with a single negative charge in their ionic compounds with metals

  -form covalent compounds by sharing electrons with other non-metals

  -more reactive halogens displace less reactive halogens

  -reactivity decreases

  -melting and boiling point increases

  -electrostatic attraction from the nucleus to the outer shell is weaker because the distance is further away, which makes it harder to gain one electron

  -a more reactive halogen can displace a less reactive halogen
• electrostatic attraction depends on

  -distance between outermost electrons and nucleus

  -the number of occupied inner shells of electrons which provide a shielding effect

  -the size of the positive charge on the nucleus
• electrons are easier for larger atoms to lose and harder for them to gain
• transition metals


  -can form ions with different charges in colored compounds

  -are important industrial catalysts

  -do not react vigorously with oxygen or water
• solid


  -packed closely together

  -vibrate around in fixed positions

  -fixed shape and volume

  -cannot be compressed
• liquid


  -particles are close together but free to move around

  -can slip and slide over eachother

  -occupy slightly more space than solid
• gas


  -particles have lots of space between them

  -zoom around randomly

  -can be compressed easily
• melting and boiling


  energy is transferred from surroundings to the substance
• freezing and condensing


  energy is transferred from the substance to the surroundings
• covalent bonding: sharing electrons <- makes it more stable and unreactive

  ionic bonding: transferring electrons
• ionic bonding


  compounds are held together by strong forces of attraction between their oppositely charged ions
• group 0 (noble gases)


  -full outer shells (no need to lose or gain electrons)

  -very unreactive (inert)

  -exist as single atoms and do not bond to form molecules

  -boiling points increase

  -atoms have stable arrangement of electrons
• ionic compounds


  -takes a lot of energy to break strong ionic bonds

  -electrostatic force of attraction is strong

  -hold a giant ionic lattice together

  -high melting points

  -solid at room temperature

  -conduct electricity when molten or dissolved in water because the ions can now carry charge through the liquid
• covalent bonds


  -formed when atoms of non-metals share pairs of electrons

  -each shared pair of electrons is a covalent bond

  -consist of simple molecules (many substances)

  -some have giant covalent structures
• single bond: 1 bond between 2 atoms

  double bond: 2 bonds between 2 atoms
• simple molecules


  -weak intermolecular forces

  -substances have low m.p and b.p

  -no overall charge

  -cannot carry an electrical charge

  -do not conduct electricity

  -intermolecular forces increase with the size of the molecules
• polymers are made up of many small reactive molecules that bond to eachother to form long chains
• giant covalent structures


  -high m.p and b.p

  -insoluble in water

  -apart from graphite, they're hard and cant conduct electricity

  -huge numbers of non-metal atoms

  -bonded by covalent bonds

  -arranged into regular repeating lattices

  -strong
• carbon nanotubes are cylindrical tubes formed from hexagonal rings of carbon atoms joined by covalent bonds
• properties of metals


  -pure metals are malleable because the layers can slide over each other

  -good conductors of electricity and thermal energy because delocalized electrons are free to move through the whole structure

  -high m.p and b.p because electrostatic force of attraction between metal ions and delocalized electrons is strong so lots of energy is needed to break it
• graphite


  -contains giant layers of covalently bonded carbon atoms

  -no covalent bonds between layers

  -can slide over eachother

  -making it soft and slippery

  -can conduct electricity and thermal energy because of the delocalized electrons that can move along its layers
• when carbon atoms have a rigid giant covalent structure, they become very hard substances