Atoms, Molecules And Ions

hemistry
C2 revision booklet

Atoms, Molecules and Ions.
AN ATOM is the smallest particle of an element. They cannot be split into smaller particles in chemical reactions. Iron is made of iron atoms (Fe). Sulphur is made of sulphur atoms (S)
A MOLECULE is a small group of atoms joined together. The atoms may be the same (e.g. O2) or different (e.g. H2O). The chemical formula shows the number and type of atoms present. Non-metal compounds are made of molecules: Carbon dioxide contains CO2 molecules Methane (natural gas) contains CH4 molecules

AN ION is an atom or group of atoms with an electrical charge (+ or -). Metal compounds such as sodium chloride or copper sulphate contain ions. Sodium chloride is made of Na+ and Cl- ions Copper Sulphate is made of Cu2+ and SO42- ions Note that metals form positive ions while non-metals form negative ions.
Ions will have a full outer shell of electrons( like a noble gas)

A solid is represented by (s). e.g. H2O(s) is ice. A liquid is represented by (l) e.g. Fe(l) is molten iron. A gas is represented by (g) e.g. H2O(g) is steam. A solution in water is represented by (aq). Salt dissolved in water is NaCl(aq).

You should remember that the common gases are diatomic (have 2 atoms in each molecule). These are Oxygen O2; Hydrogen H2; Nitrogen N2; and Chlorine Cl2.

Elementary Particles
Atoms are made up of smaller particles called protons, neutrons and electrons.
The protons and neutrons cluster together in a small nucleus at the centre of the atom while the electrons orbit the nucleus.
The main properties of the particles are:
|Particle |Mass |Charge |
|PROTON |1 |+1 |
|NEUTRON |1 |0 |
|ELECTRON |0 |-1 |

Every element has an atomic number, which is the number of protons( or electrons) in the nucleus.

Atoms are neutral, so that the number of electrons is the same as the number of protons.
The mass of an atom is almost entirely made up of protons and neutrons, which have the same mass as each other (the mass of electrons can be ignored).

These relations can be summarised as: ATOMIC NUMBER = NUMBER OF PROTONS NUMBER OF PROTONS = NUMBER OF ELECTRONS MASS OF ATOM = PROTONS + NEUTRONS

| Element |Atomic Number |Protons |Electrons |Neutrons |Mass Number |
|Na |11 |11 |11 |12 |23 |
|C |6 |6 |6 |6 |12 |
|U |92 |92 |92 |146 |238 |

Electronic Structure
The electrons orbit the nucleus in 'shells'. These can hold the following numbers of electrons:

the innermost shell can contain up to 2 electrons the next shell can contain up to 8 electrons the next shell can contain up to 8 electrons (although this can be expanded up to 18)

Fluorine has atomic number 9. Its electronic structure is 2.7
Sodium has atomic number 11. Its electronic structure is 2.8.1
Calcium has atomic number 20. Its electronic structure is 2.8.8.2

The number of electrons in the out shell, tells what group the element is. It is the

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