Formulae, Equations and Hazards
- Word Equations show the names of reactants and products
- Symbol Equations must be balanced and may require state symbols of solid (s), liquid (l), gas (g) or aqueous (aq), meaning dissolved in water
- Ionic equations show only the reacting particles and the products they form. To write an ionic equation, remove any spectator ions present and show electron transfer
Atomic Structure
In 1805, John Dalton described atoms as solid spheres which formed different elements with different spheres, which could not be created or destroyed. In 1897, JJ Thompson’s measurements of shape and mass proved the existence of electrons and included them in the Plum-Pudding model. In 1909, Ernest Rutherford conducted an experiment in which alpha particles were fired at thin gold foil, which surprisingly led to some being deflected, proving that there was a small positive nucleus which repelled like-charged particles.
We now know that an atom is made of a nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus of the atom is very small compared to the overall size of the atom, and most of the atom’s mass is concentrated in the nucleus. The mass number of an atom is the total number of protons and neutrons, while atomic number is the number of protons or electrons in a neutral atom. The number of protons and electrons is the same in an atom as the -1 and +1 charges cancel out, leaving the atom neutral.
This symbol tells us that the number of protons/electrons is 17, while number of neutrons is 18 (Mass Number – Atomic Number)
Isotopes are different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei. In the calculation of relative atomic mass, the abundance of different isotopes must be factored in, meaning sometimes the number is not an integer.
The Periodic Table
Dmitri Mendeleev arranged elements known in the 19th Century in a table in order of increasing relative atomic mass, not always correct as abundances of isotopes had not been considered. Now we know that it is increasing atomic number, the number of protons in the atom. He left gaps in the table for elements which would be discovered in the future and predicting that they would share chemical
- Never soluble in water and as there are no charged particles, do not conduct electricity
- Allotropes are different forms of the same element
- Diamond is made if a network of carbon atoms that each form four covalent bonds. It has a tetrahedral arrangement, meaning it has a high melting point, it does not conduct electricity. Its rigid lattice structure makes it hard, so it is used in cutting tools
- Graphite forms when carbon atoms form three covalent bonds, creating sheets of carbon atoms in hexagons. The many bonds that need to be broken means that it has a high melting point, and since there are no covalent bonds between layers, they are held together weakly and can slide over each other, meaning it is a good lubricant. Its one delocalised electrons allows it to conduct electricity and it is therefore used in electrodes. A single layer is called graphene
- Fullerenes are molecules of carbon shaped like tubes or balls, used to cage other molecules and have a huge surface area, making them useful catalysts. Nanotubes conduct electricity and have high tensile strength. Buckminsterfullerene (C60) is a hollow sphere due to the shapes the carbon atoms bond in
Metallic Bonding
- Electrons in the outer shell are delocalised. There are strong forces of attraction between positive metal ions and shared negative electrons, meaning there is a high melting point
- Solid at rtp and insoluble in water
- High density and malleable as layers of pure metal can slide over each other
- Sea of delocalised electrons can conduct electric current and heat
Calculations
The Relative Formula Mass of a compound is the mass number of all the atoms in the formula added together.
A mole is a constant for number of molecules. One mole of atoms of any substance will have a mass in grams equal to the relative formula mass for that substance. One mole equals Avogadro’s Constant of 6.022 × 1023
The concentration is the mass of a substance in a given volume
The empirical formula is the smallest integer ratio of atoms in a given compound
e.g. Glucose [C6H12O6] has an empirical formula of CH2O
If you are given the relative formula mass and the empirical formula, then divide the given mass by the formula mass of the empirical formula to get the multiplier. Use this multiplier to work out the number of each type of atom in the molecule to get the molecular formula.
You must be able to describe an experiment which you can use to determine the empirical formula of magnesium oxide. First, weigh the crucible then add magnesium. Reweigh to find mass of Mg. Then heat over a Bunsen burner, lifting the lid to allow oxygen in. Once there is no further change, measure the mass of the crucible with the compound. The mass of oxygen reacted is this mass minus the mass of the crucible and Mg, since in a closed system, the mass of reactants is conserved, meaning the mass of the products is the same. Then calculate the empirical formula by finding the ratio of moles of Mg and O.
In a reaction, the mass of product formed is controlled by the mass of the reactant which is not in excess. To work out the limiting reagent, calculate the number of moles of each of the reactants. Using the molar ratio, work out the ratio of each product using the number of moles. If the ratio has a is too low for one part, then that substance is the limiting reagent.
To balance equations using masses, find the moles of each substance, and divide the moles for each substance by the smallest number of moles in the reactions, and multiply to make all the numbers integers. Write the balanced symbol equation using these numbers as the coefficients.