Chemistry: Additional: Bonding and Structure

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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

How many charges? There is a quick way to work out what the charge on an ion should be:

The number of charges on an ion formed by a metal is equal to the group number of the metal

The number of charges on an ion formed by a non-metal is equal to the group number minus eight

Hydrogen forms H+ ions.

Ionic compounds Group 1 and Group 7 The elements in Group 1 of the Periodic Table are called the alkali metals. They form ionic compounds when they react with non-metals. Their ions have a single positive charge. For example, sodium forms sodium ions, Na+.

The elements in Group 7 of the Periodic Table are called the halogens. They form ionic compounds when they react with metals. Their ions have a single negative charge. For example, chlorine forms chloride ions, Cl–.

Sodium chloride Sodium chloride, NaCl, forms when sodium and chlorine react together. It contains oppositely charged ions held together by strong electrostatic forces of attraction – the ionic bonds. The ions form a regular lattice in which the ionic bonds act in all directions.

Covalent Bonding is on the Next Page

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Covalent Bonding

A covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram.

Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

You will need to understand what covalent bonding is, and to remember some of the properties of molecules that are formed in this way.

A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.

Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level - so it is stable. The hydrogen atom is now in contact with two electrons in its highest energy level - so the hydrogen is also stable.

How many bonds? Atoms may form multiple covalent bonds - that is, share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.

There is a quick way to work out how many covalent bonds an element will form. The number of covalent bonds is equal to eight minus the group number (you can brush up on group numbers by reading through the section in AQA GCSE Science on the Periodic Table.)

Representing covalent bonds Covalent bonds can be represented in several different ways.

Straight lines and models Straight lines are the most common way to represent covalent bonds, with each line representing a shared pair of electrons. 2D or 3D molecular models are especially useful for showing the relationship between atoms in multiple covalent bonds.

Double and triple bonds Note that molecules can have a double covalent bond - meaning they have two shared pairs of electrons - or a triple covalent bond - three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.

A molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond

A molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond

Structure, Properties and Uses are on the Next Page

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Structure, properties and uses

Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. However, the molecules are held together by weak forces so these substances have low melting and boiling points. They do not conduct electricity.

Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Simple molecules Covalent bonds form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types:

Simple molecules

Giant covalent structures

Simple molecules These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.

Properties of simple molecular substances

Low melting and boiling points - This is because the weak intermolecular forces break down easily.

Non-conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.

Macromolecules Macromolecules have giant covalent structures. They contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. Their atoms are arranged into giant lattices, which are strong structures because of the many bonds involved. Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600°C.

Diamond Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.

Graphite Graphite is a form of carbon in which the carbon atoms form layers. Each carbon atom in a layer is joined to only three other carbon atoms. The layers can slide over each other because there are no covalent bonds between them. This makes graphite much softer than diamond. It is used in pencils and as a lubricant. Graphite conducts electricity.

Silica Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point. However, it contains silicon and oxygen atoms instead of carbon atoms.

Polymers Polymers have properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon the catalyst and reaction condition used to make it.

Thermosoftening polymers Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again. Poly(ethene) is a thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material.

Thermosetting polymers Thermosetting polymers have different properties to thermosoftening polymers. Once moulded, they do not soften when heated and they cannot be reshaped. Vulcanised rubber is a thermoset used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.

Ionic compounds Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions, while non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.

Melting points and boiling points Ionic bonds are very strong so a lot of energy is needed to break them. Ionic compounds contain many of these strong bonds so they have high melting and boiling points.

Conduction of electricity Ionic compounds conduct electricity when they are dissolved in water or when they are melted. This is because their ions are free to move and carry the current. However, ionic compounds do not conduct electricity when they are solid. This is because their ions cannot move around in their lattice structure.

Metals Metals are malleable - they can be bent and shaped. This is because they consist of layers of atoms. These layers can slide over one another when the metal is bent, hammered or pressed.

Alloys An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals.

Layers Alloys contain atoms of different sizes. These different sizes distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:

Brass - used in electrical fittings - is 70 per cent copper and 30 per cent zinc

18-carat gold - used in jewellery - is 75 per cent gold and 25 per cent copper and other metals

Duralumin - used in aircraft manufacture - is 96 per cent aluminium and 4 per cent copper and other metals

Shape memory alloys Shape memory alloys can return to their original shape after being bent or twisted. Nitinol is a shape memory alloy made from nickel and titanium. It is used in dental braces and spectacle frames.

Nanoscience A nanometre, 1 nm, is one billionth of a metre (or a millionth of a millimetre). Nanoparticles range in size from about 100 nm down to about 1 nm. They are typically the size of small molecules, and far too small to see with a microscope.

Properties and uses of nanoparticles Nanoparticles have a very large surface area compared with their volume, so they are often able to react very quickly. This makes them useful as catalysts to speed up reactions. They can, for example, be used in self-cleaning ovens and windows.

Nanoparticles also have different properties to the same substance in normal-sized pieces. For example, titanium dioxide is a white solid used in house paint and certain sweet-coated chocolates. Titanium dioxide nanoparticles are so small that they do not reflect visible light, so cannot be seen. They are used in sun screens to block harmful ultraviolet light without appearing white on the skin.

In addition to new cosmetics such as sun screens and deodorants, nanoscience may lead to the development of:

New catalysts

New coatings

New computers

Stronger and lighter building materials

Sensors that detect individual substances in tiny amounts

If you are taking the Higher Tier Paper, That Information is on the next page

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Structure, properties and uses - Higher Tier

Metals Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free electrons and metal ions. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.

Metals are good conductors of electricity and heat. This is because the free electrons can move throughout the metal.

Graphite and fullerenes

Graphite

Graphite is soft and slippery because there are only weak intermolecular forces between its layers.

From left to right - graphite, diamond, silica

Graphite is a good conductor of heat and electricity. This is because, like metals, graphite contains delocalised electrons. These electrons are free to move through the structure of the graphite.

Fullerenes Structure of a buckminsterfullerene molecule - a large ball of 60 atoms

Carbon exists as graphite and diamond, but it can also form fullerenes. These are cages and tubes with different number of carbon atoms. Buckminsterfullerene is one type of fullerene. Its molecules are spherical and contain 60 carbon atoms.

Fullerenes may be used for drug delivery systems in the body, in lubricants and as catalysts.

The tube fullerenes are called nanotubes. These are very strong. They are useful in reinforcing structures where lightness and strength are needed - for example, in tennis racket frames.

Simple Molecules

Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.