Atomic structure

Atoms

All substances are made up of tiny particles called atoms. There are about 100 different types of atom found naturally on Earth and they can combine in a huge variety of ways, giving all those different substances. They have a central nucleus with electrons orbiting around it.

• Elements are made up of one type of atom.
• Elements can have different properties: silver, gold, copper, and chromium are solid, shiny metals, but other elements like argon, chlorine, nitrogen, and argon are non-metals and are gases at room temperature.
• There are symbols for these elements and they are shown in the periodic table (below).

The symbols in the periodic table represent atoms. 

• Elements in the table are arranged in columns, called groups. Each group contains elements with similar chemical properties.
• The staircase drawn on the right of the periodic table is the dividing line between metals and non-metals. The elements to the left of the line are metals; the elements to the right of the line are non-metals. Some elements lying next to the dividing line are metalloids (they contain some metallic and non-metallic properties).

The vast majority of substances you come across are not elements:

• Compounds are different types of atoms bonded together.
• Chemical bonds hold the atoms tightly together in compounds.

Chemical equations

Chemical equations show the reactants and products in a reaction.

• You can write a word equation. 
• In chemical reactions, atoms get rearranged.
• Symbol equations helps you to see how much of each substance is involved in a reaction.
• Balanced means that there is the same number of each type of atom on both sides of the equation.

Atoms cannot be created or destroyed in a chemical reaction: the total mass of the reactants is equal to the total mass of the products. 

• This is called the Law of conservation of mass.
• This rule may appear to be broken when reactions are carried out in open containers, like test tubes or conical flasks. However, gas can escape into the air (so it appears to lose mass) and extra mass from gases can react to increase mass.

State symbols can be added to a balanced symbol equation to give extra information.

• (s) for solids
• (l) for liquids
• (g) for gases
• (aq) for aqueous solutions

When balancing an equation:

• Write the formula for each reactant and product. 
• Count the atoms on either side of the equation.
• Never change a chemical formula.
• Add superscripts to balance the equation.
• Count the atoms again on either side of the equation to check that it is balanced.

Separating mixtures

A mixture is two or more substances (elements or compounds) not chemically combined together. 

Compounds                                                                                   Mixtures

have a fixed composition.              …