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They are numerically ordered by their atomic number, which is the number of protons in the nucleus of an atom. The elements are arranged so they are in columns with similar chemical properties. These columns are called groups. Rows are called periods, and display properties known as periodicity.

The Merriam-Webster dictionary defines stoichiometry as "a branch of chemistry that deals with the application of the laws of definite proportions and of the conservation of mass and energy to chemical activity". Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry. The most common stoichiometric problem will present you with a certain amount of a reactant and then ask how much of a product can be formed. Ex:: [math]2A + 3B \to 3C[/math], Given 25 grams B and unlimited A how much C will be produced. This is called a mass-mass problem. These problems can be solved in 4 simple steps.

There are five main types of reactions (single displacement, double displacement, combustion, decomposition, and synthesis). Each of them has a specific form that they take. If you encounter problems dealing with reactions on the test, knowing the basic types can be very helpful because then you will be more likely to see the pattern and understand how to complete the reaction.

Double displacement reactions are similar to single replacement, but they are usually not oxidation-reduction reactions. Instead of just one element being traded, double displacement reactions have two similarly formed compounds reacting to form other compounds.

This type of reaction is especially important in aqueous solutions, since most precipitation reactions are double displacement reactions. Precipitation occurs when AB and CD are both soluble in water, and when put together, either AD or BC is an insoluble compound and thus precipitates out of the solution. For more information about solubility, see Chem Lab/Aqueous Solutions.

Combustion reactions are redox reactions that produce fuel. Thus, combustion reactions are also exothermic reactions since they give off heat. The most common combustion reactions form carbon dioxide, water, and energy. For example, here is the combustion reaction for methane:

Since combustion reactions are among the most common exothermic reactions, it is a good idea to know the combustion reactions of several important compounds, or at least know how to go about finding it quickly. Since all of them have a similar form, you can guess what the products will be, which will make it easier. For example, for the combustion of a hydrocarbon in oxygen, the two products will be water and carbon dioxide, since the oxygen attaches itself to both carbon and hydrogen parts.

Oxidation and reduction reactions are those which involve a net gain or loss of electrons – typically, they occur in pairs, with the electrons lost in one reaction being gained by the other. To remember which is which, just remember the simple acronym OIL RIG – Oxidation is Loss, Reduction is Gain. This is a simple way of remembering that whatever is oxidized loses electrons and whatever is reduced gains electrons. The phrase LEO the lion says GER also works (Lose Electrons-Oxidation, Gain Electrons-Reduction)

• Make sure to work not just quickly, but efficiently, on the labs. Do them as quickly as you can, but if you end up with inaccurate results then going quickly didn’t help you a whole lot. Also, do not take shortcuts unless you are absolutely, positively sure you can.