The Atomic Structure:
- An atom is made up of a nucleus, which is made up of protons and neutrons, and electrons, orbiting the nucleus.
- A proton is positively charged, 1+, and has a mass of 1.0.
- A neutron has no charge, 0, and has a mass of 1.0.
- An electron is negatively charged, 1-, and has a mass of 1/2000.
- The atomic number of an atom tells you how many protons and electrons a particular atom carries. If there is a charge on the atom i.e. 1-, just add another number to the atomic number and that will give you the electron number. Otherwise, vice versa.
- The mass number is the total number of protons and neutrons in an atom. So to find the number of neutons, just simply deduct the atomic number by the mass number.
- Mass number - Atomic number = Neutron number.
Isotopes and Relative masses:
- Isotopes have the same number of protons but different number of neutrons, therefore have a different mass number.
- Relative isotopic mass is the mass of an isotope compared with 1/12th of the mass of an atom of C-12.
- Relative atomic mass, RAM, is the weighted mean mass of an atom compared with 1/12th of the mass of an atom of C-12.
e.g. A sample of Ca contains 63% of Ca-39 and 37% of Ca-41, determine the relative atomic mass of calcium:
RAM (Ca) = (63x39)+(37x41)/100 = 3974/100 = 39.74
- Relative molecular mass is the weighted mean mass of a molecule compared with 1/12th of the mass of an atom of C-12.
The Mole and formulas:
- The Avogadro constant is the number of atoms per mole of the C-12 isotope (6.022x10^23)
- A mole is the amount of substance containing as many particles as there are in carbon atoms in exactly 12g of the C-12 isotope. Unit = mol.
- Molar mass, M, is the mass per mole of a substance. Unit = g mol^-1
- The empirical formula is the simplest whole number ratio of each element present in a compound.
- The molecular formula is the actual number of atoms of each element present in a molecule.
- n = m/M
n = number of moles (mol), m = mass (g), M = molar mass (g mol^-1).
- Moles in solutions, n = c x V (if volume is in dm^3), n = c x V/1000 (if volume is in cm^3)
n = number of moles (mol), c = concentration (mol dm^3), V = volume (dm^3 or cm^3).
- Moles in gases, n = V / 24 (if volume is in dm^3), n = V / 24000 (if volume is in cm^3)
n = number of moles (mol), V = gas volume (dm^3 or cm^3), 24 = room temperature and pressure, RTP, (if volume is in dm^3), 24000 = room temperature and pressure, RTP, (if volume is in cm^3).
Acids and Bases:
- An acid is proton donor.
All acids contain hydrgen. When added to water, acids release this hydrogen as H+ ions (protons).
- A base is a proton acceptor. They neutralise acids.
- An alkali is a type of base that dissolves in water forming hydroxides ions, OH- ions.
- Important formulae to learn:
NaOH (sodium hydroxide), KOH (potassium hydroxide), NH3 (ammonia).
Salts and reactions:
- A salt is chemical compound formed form an acid when a H+ ion from the acid has been replaced by a metal ion.
- A cation is a positively charged ion.
- An anion is a negatively charged ion.
- Salts can br produced by neutralising acids with carbonates, bases or alkalis.
- When an acid reacts with a base or alkali, salt and water is produced only.
- When an acid reacts with carbonates, salt, water and carbon dioxide gas is produced.
Crystallisation and Oxidation numbers:
- Hydrated refers to a compound containing water.
- Anhydrous refers to a compound that contains no water molecules.
- Water of crystallisation refers to water molecules that from an essential part of the crystalline structure of a compound.
- An oxidation number indicates the formal charge of a chemically combined particle in a compound.
- The oxidation number of metals usually equals the group number (as a positive value) and minus (8 – group number) for non-metals.
- An element has been oxidised if the oxidation number increases, and reduced if the oxidation number decreases.
- When they react, metals are normally oxidised (they lose electrons), whereas non-metals gain electrons and are reduced.
- Memorise these oxidation numbers:
Uncombined element = 0, combined with oxygen = -2, combined with hydrogen = +1, simple ion = charge on ion, combined with fluorine = -1.
Oxidation numbers (continued):
- Oxidation is loss of electrons, therefore an increase in oxidation number.
- Reduction is gain of electrons, therefore a decrease in oxidation number.
- Remember this by OILRIG = Oxidation Is Loss, Reduction Is Gain.
- In redox reaction, both reduction and oxidation take place.
- Disproportionation is the oxidation and reduction of the same element in a redox reaction.