Now, due to the amazing techniques created, it's possible to get a metal only made from its pure element! But, the problem with this is that they're not that useful because their properties aren't that great, so we need something which can make the weakness of a metal be less.
First lets look at Iron:
Iron straight from the furnace is only 96%. The other 4% is impurities such as carbon. This impure iron is used as a product called 'cast iron'. It's handy for making ornamental railings but that's really all it's useful for because it's properties makes it to brittle to be used for anything else.
So, if you remove all the impurities (or as much as you can) the iron loses the brittle quality and becomes soft and flexible (all the atoms are the same shape and size and in a regular arrangement, this allows the atoms of the metal slide over each other) which then creates another problem, if it's too soft, it can't be used for anything because as soon as it comes under pressure, it will bend and re-shape.
Iron Into Steel
Now, when iron is produced it's normally never left as 'iron' and is normally turned into a compound (alloy) called steel. Now there is no one use for steel due to the fact it can have other elements in it which give it different properties like:
Low carbon steel (0.1% carbon) is easily shaped but still has a bit more strength that pure iron. This steel tends to be used for car bodies.
High carbon steel (1.5% carbon) is far more stronger than iron and low carbon steel. It's properties are that it's very hard and inflexible meaning it won't bend shape if it's put under a significant amount of pressure. This type of steel tends to be used for cutting tools and bridges.
Stainless steel (chromium added, something nickel) isn't very strong but has other properties such as it's corrosion-resistant which means it can be used for cutlery and used as containers for corrosive materials.
Alloys Stronger The Metals?
First thing you need to realise is that every element has a different size atom i.e. hydrogen (being the first element on the periodic table) has the smallest atoms whilst roentgenium (one of the last elements on the periodic table) is one of the biggest. An easy way to remember this is looking at an elements atoms mass! The larger the number (atomic mass) the larger that elements atom is.
Now if you were to add carbon to a block of a pure metal block (which has a larger atomic mass than carbon), you would give the metal different properties because the carbon atoms are smaller which will upset the regularity of the metal atoms. This causes them to change there layering, which makes them considerably stronger. So, alloys are stronger than the metal in its pure element form.
Some alloys you may need to know:
Bronze = Copper + Tin (Bronze is far stronger than copper)
Cupronickel = Copper + Nickel (This is hard and corrosive resistant)
Gold tends to be an alloy due to the fact that gold as a pure metal is to soft. Metals such as zinc, copper, silver and palladium is used to harden gold.
Aluminium alloys are used to make aircraft because aluminium is a low density metal which can be alloyed with other metals to make it a lot stronger!