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Electric charges

There are two types of electric charge in nature: a positive charge and a negative charge.

Electrical charges of the same type (with the same sign) repel each other.

Opposite electrical charges (with different signs) attract each other.

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The atom

All matter is made up of atoms.

The Ancient Greeks believed that the atom was the smallest particle of matter that could

exist.

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Particles inside the atom

Charge Mass

Electrons Negative (-) Negligible

Protons Positive (+) Similar to a hydrogen atom

Neutrons Neutral Similar to a hydrogen atom

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How an atom looks

Atoms are made up of electrons and the nucleus is made up of protons and

neutrons.

The number of electrons is the same as the number of protons in neutral atoms.

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Atomic models

Rutherford said that the atom is basically empty space

The planetary model (Bohr) says that electrons

move around the nucleus at very high speed.

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Atomic parameters

The atomic number Z of an atom is the number of protons it contains

in its nucleus.

The mass number A is the sum of protons and neutrons in the

nucleus of an atom.

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Isotopes

Atoms with the same number of protons but a different number of neutrons are

called isotopes.

The atomic mass of an element is the average of the mass numbers of its isotopes

according to their relative abundance.

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Ions

Atoms are not always neutral. They can gain or lose electrons in a process called

ionisation. When atoms are ionised they become ions.

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Organisation of atoms

Pure substances can be classified in:

- Simple substances

- Compound substances

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Simple substances

- Single atoms: This organisation is made up of single atoms with no bonds

between them.

- Large clusters: Atoms are all bonded to each other.

- Molecules: Two or more atoms of the same type bond together.

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Compound substances

- Molecules. Atoms of different types bonded together.

- Ionic crystals or compounds: Ions with different charges attract each other,

the ions bond together.

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Chemical formulas

- formulas for simple substances: N2 O2 O3 ...

- formulas in molecular compounds:H2O CCl4 ...

- formulas in ionic compounds: NaCl

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Chemical reaction

A physical change is a transformation that affects the form of a substance, but not its

composition.

A chemical change is a transformation that changes the composition of matter.

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Chemical equation

Reactants are the initial substances which, when mixed together, start a chemical

reaction.

Products are the new substances that form as a result of the chemical reaction between

the reactants.

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Lavoisier´s law

In a chemical reaction the total mass remains the same.

The total mass of the reactants is equal to the total mass of the products

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Energy in chemical reactions

A reaction is exothermic when thermal energy is produced or released when it occurs.

A reaction is endothermic when thermal energy is required to produce it.

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