chemsity revision

small molecules

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• A substance with small molecules has strong covalent bonds that hold the atoms together in its molecules. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their properties.
• Most substances with simple molecules have low melting pointsand boiling points. Many are in the liquid or gas state at room temperature.
• nergy must be transferred to a substance to make it melt or boil. This energy overcomes the attractive forces between the particles in the substance:
• some forces of attraction are overcome during melting, allowing molecules to move over each other
• more of the forces of attraction are overcome during boiling, allowing the molecules to move freely away from each other
• The more energy that is needed, the higher the melting point or boiling point.

metal and alloys

• Metals consist of giant structures of atoms arranged in a regular pattern.
• The electrons from the outer shells of the metal atoms are delocalised, and are free to move through the whole structure. This sharing of delocalised electrons results in strong metallic bonding.
• The structure and bonding of metals explains their properties:
• they are electrical conductors because their delocalised electrons carry electrical charge through the metal
• they are good conductors of thermal energy because their delocalised electrons transfer energy
• they have high melting points and boiling points, because the metallic bonding in the giant structure of a metal is very strong - large amounts of energy are needed to overcome the metallic bonds in melting and boiling
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giant covalent molecules

• Covalent bonding leads to the formation of substances with different types of structures, for example:
• small molecules, which contain a fixed number of atoms joined by covalent bonds
• giant covalent substances, which contain many atoms joined by covalent bonds
• An example - Silicon dioxide
• Silicon dioxide (often called silica) is the main compound found in sand. It is an example of a substance with a giant covalent structure. It contains many silicon and oxygen atoms. All the atoms in its structure are linked to each other by strong covalent bonds. The atoms are joined to each other in a regular arrangement, forming a giant covalent structure. There is no set number of atoms joined together in this type of structure.
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ionic compounds

• An ion is an atom or group of atoms with a positive or negative charge. Ions form when atoms lose or gain electrons to obtain a full outer shell:
• metal atoms lose electrons to form positively charged ions
• non-metal atoms gain electrons to form negatively charged ions
• Forming positive ions
• Metal atoms lose electrons from their outer shell when they form ions:
• the ions are positive, because they have more protons than electrons
• the ions formed have full outer shells
• the ions have the electronic structure of a noble gas (group 0 element), with a full outer shell
• For elements in groups 1, 2 and 3, the number of electrons lost is the same as the group number.
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ionic compounds

The three states of matter

• Solids, liquids and gases
• The three states of matter are solid, liquid and gas. The particle model represents particles by small, solid spheres. It describes the arrangement, movement and energy of particles in a substance. The model can be used to explain the physical properties of solids, liquids and gases.
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IONIC BONDING

• Ions are electrically charged particles formed when atoms lose or gain electrons. They have the same electronic structures as noble gases.
• Metal atoms form positive ions, while non-metal atoms form negative ions. The strong electrostatic forces of attraction between oppositely charged ions are called ionic bonds.
• Ions
• How ions form
• Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.
• Metal atoms and non-metal atoms go in opposite directions when they ionise:
• Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions
• Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions
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