SCIENCE CT REVISION.

Revision: Atoms and Molecules

1) Atomic structure

• To determine the number of protons, neutrons and elections in an atom, use the Periodic Table to find the chemical symbol. Then check out the nucleon/mass number and the proton/atomic number.

Electronic Structure:

• First shell of atoms can only hold two electrons and the rest can hold a maximum of eight.
• The way the electrons are arranged in an atom is known as the electronic configuration.
• eg: Chlorine atom has an electronic configuration of 2,8,7.

Valence Electrons:

• The shell that is furthest away from the neucleus is called the outer shell or the valence shell.
• The Electons found in this shell are called valence electrons.
• The chemical properties of an element depends on the number of valence electrons.
• This is why elements with the same number of valence electrons( e.g. Gp 1 sodium and potassium) have similar chemical properties.

The Periodic Table:

• Group numbers show how many valence electrons an atom of each element has.
• Period numbers show how many electron shells an atom of each element has
• E.g. an atom of nitrogen found in Group 5, Period 2 will have 2 electron shells and 5 valence electrons.

2) Molecules of elements and compounds

• Atom: Smallest particle of an element
• Molecule: Two or more atoms chemically combined together.
• Molecules of Elements: Two or more atoms of the same element chemically combined together. E.g. Oxygen, ozone, nitrogen, hydrogen, phosporys.
• Molecules of Compunds: Two or more atoms of different elements chemically combined together. E.g. water, sulphur dioxide, carbon dioxide, ammonia, methane.

3)Bonding

• Noble Gases (e.g. Neon, Helium) exist as single atoms because they have a full valence shell.
• Noble gases are stable and unreactive.
• Most atoms undergo chemical bonding to achieve the noble gas configuration in thir outermost shell.
• They do so by losing/gaining or sharing their valence electrons.

Recall : How are atoms and ions different?

• Atom: Smallest particle of an element.
• Molecule: Two or more atoms chemically combined together.
• Ions: Atoms with an overall charge (after losing or gaining electrons)
• Formula unit: Show how ions in an ionic compound are electro-statically bonded together.

Ionic Bonding:

• Between metals and non-metals.
• Metal atom loses its valence electrons.
• These electrons are passed to the non-metal atom.
• The positive and negative charge on the two sets of ions electro-statically attract each other and form an ionic bond.
• E.g.: Sodium Chloride. Na loses 1 electron. Na now has 11 protons but 10 electrons. Overall charge of ion = (+). Metals form positively charged ions. The electron from Na is passed on to Cl. Cl now has 17 protons but 18 electrons. Overall charge on ion = (-). Non-metals form negatively charged ions.

Covalent Bonding:

• Covalent bonds are formed when non-metals react with one another
• Non-metals bond by sharing their valence electrons
• After bonding, each atom has an electronic configuration of a noble gas( duplet or octet stracture)
• A molecule is a group of two or more atoms held together by covalent bonds

1. Place positive (metal) ion in one bracket, negative (non-metal) ion in another.
2. Both ions should have complete valance shells.
3. The negative ion should possess an electron from the meal atom.
4. Show the charges on each ion.
5. Show the numbers of each ion if there are more than one.

To determine the chemical formula of ionic compounds: All positive charges must be equal to the negative charges.

• e.g. Ca2+ and Cl-
• There needs to be two chloride ions to balance out the two positive charges.
• Therefore the chemical formula for this ionic compound is CaCl2

How to determine Chemical formula of ionic compounds:

• Move the numerical value of the charge on each ion diagonally to the other ion
• In the chemical formula, numbers of ions within an ionic compounf are written as subscripts.
• Between non-metals.
• Valence electrons are shared between atoms.

In a dot and cross diagram for a molecule:

1. Each atom in the molecule should have a complete valence shell
2. Shared electrons within overlapping area between atoms.
3. Valency of atom is usually equal to the number of covalent bonds it can form ( e.g. Cl can form 1 bond, O can form 2)

4) Valency and Formula

• Valency is the combining power of an atom
• In ionic bonding valency of atoms is usually equal to the charges of the ions formed.
• The total combining power of the component elements of the compound must be the same.
• The chemical formula of a compund states the number and types of atoms in each compound.
• There are two ways to describe the number and types of atoms in a molecule of a compound. e.g. Water.

1. Using chemical formula: H2O
2. Using words: Each water molecule contains two atoms of hydrogen and one atom of oxygen combined together.

OKAY! THAT'S ALL!(: i'll add more later.