Fundamental Ideas

Atoms, Elements and Compounds + Atomic Structure

The periodic table is a list of the elements. Each element is one type of atom. Atoms are represented by chemical symbols, e.g O for Oxygen. The elements in the periodic table are arranged in columns, called groups. The elements in a group usually have similar properties.

Atoms have tiny nucleus surrounded by electrons. When elements react, their atoms join with atoms of other elements.Compounds are formed when two or more elements combine together.

The nucleus at the centre of an atom contain two types of particle, protons (Posotive Charge) and neutrons (No Charge).

the number of protons in the nucleusdetermines which element the atom is. We can find the number of protons, neutrons and electrons in an element using the periodic table.

The number above the element is the 'Mass Number' = Protons and neutrons and the number below is the 'Proton Number' = Protons.

In an atom the number of electrons is always the same as the number of protons. This is so the positive and negative charges cancel out.

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Electron Arrangement In Atoms

Each electron in an atom is an energy level, these can be represented as shells, with electrons in the lowest energy level closest to the nucleus. The lowest energy level or first shell can hold two electrons, and the second energy level can hold eight. Electrons occupy the lowest possible energy levels. The electronic structure of neon with 10 electrons is 2, 8. Argon with 18 electrons, has the electronic structure of 2, 8, 8.

Elements in the same group of the periodic table have the same number of electrons in their highest energy level. E.g. Group 1 elements have one electron in their highest energy level. As a result of this, Sodium would be in group 1.

The other group one elements are lithium and potassium, these react quickly with water and oxygen. The atoms of the unreactive noble gases (in group 0) all have very stable arrangements of electrons.

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Forming bonds

When different elements combine they form compounds. When a metal raects with a non-metal, ions are formed. Metal atoms lose one or more electrons to form positivley charged ions. Non-metal atoms gain electrons to form negativley charged ions. The oppositley charged ions attract each other strongly and the compound has ionic bonds.

The chemical formula of an ionic compound tells us the simplest ratio of ions in the compound. For example, NaCl shows that sodium chloride is made from equal numbers of sodium ions and chloride ions.

When non metal elements combine, their atoms share electrons to from covalent bonds and molecules are also formed.

The chemical formula of a molecule shows us the number of atoms which have been bonded together in the molecule. For example, H2o shows that a water molecule contains two hydrogen atoms and one oxygen atom. Covalemt bonds can be shown as lines betweenbthe atoms that are bonded together.

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Chemical Equations

In chemical reactions the atoms reactants rearange themselves to form new substances, the products. Atoms are neither created nor destroyed in a reaction. So the number and type of atoms stays the same as before and after the reaction.

This means that the mass of products equals the mass of reactants. It also means that we can write chemical equations to represent reactions. 

Word equations only give the name sof the reactants and products. Symbol equations show the numbers and types of atoms in the reactants and products.  When symbol equations are written they should always be balanced. This means that the numbers of each type of atom should be the same on both sides of a symbol equation.

Making an equation balance:

Symbol equations are balanced by changing the large numbers in front of the formulae of the reactants and products. You should balance equations by changing only the large numbers. Never change the small numbers because this cahnges the formulae of the substance.

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