3 - Bonding & Structure

3.1 Ionic bonding

- An ionic bond is a strong electrostatic attraction between oppositely charged ions

-Ionic bonds are usually found in compounds that contain metals combined with non-metals

- When this type of bond is formed, electrons are transferred from the metal atoms to the non-metal atoms during the chemical reaction

- What happens when sodium and chlorine react together and combine to make sodium chloride is

sodium + chlorine -> sodium chloride

-Sodium has just 1 electron in its valence/outer shell (2,8,1). Chlorine has 7electrons in its valence/outer shell (2,8,7)

-When these 2 elements react, the outer/valence electron of each sodium atom is transferred to the valence/outer shell of a chlorine atom

- Both the atoms obtain full shells and become 'like'

the noble gas nearest to them in the Periodic Table

-In these diagrams, the electrons from one atom are shown as crosses & those of the other atom as dots.

-In the reaction the sodium atom has become a sodium ion, in a s=process known as ionisation.

-This sodium ion has an electronic configuration like the noble gas neon.

-Only the outer/valence electrons are important in bonding, so we can simplify the diagrams by missing out the inner shells.

-The charges on the sodium and chloride ions are equal but opposite.

-They balance each other and the resulting formula for sodium chloride is NaCl.

-The oppositely charged ions attract each other and are pulled, or bonded, to one another by strong electrostatic forces.

-This type of bonding is called ionic bonding

-Scientists, using X-ray diffraction have obtained photographs that indicate the way in which the ions are arranged,

-The study of crystals using X-ray diffraction was pioneered by Sir William Bragg and his son Sir Lawrence Bragg in 1912.

-X-rays are a form of electromagnetic radiation.

-They have a much shorter wavelength than light therefore it is possible to use them to investigate extremely small structures

-When X-rays are passed through a crystal of sodium chloride, for example, you get a pattern of spots called a diffraction pattern.

-This pattern can be recorded on photographic film and used to work out how the ions or atoms are arranged in the crystal.

-Crystals give particular diffraction patterns depending on their structure, and this makes X-ray diffraction a particularly powerful technique in the investigation of crystal structures

Ionic structures

-Ionic structures are solids at room temperature and have high melting and boiling points

-The ions are packed together in a regular arrangement called a lattice

-Within the lattice, oppositely charged ions attract one another strongly

-Example : Sodium Chloride

- Many millions of sodium ions and chloride ions would be arranged in this way in a crystal of sodium chloride to make up the giant ionic lattice structure

-Each sodium ion in the lattice is surrounded by six chlorine ions, and each chlorine ion is surrounded by six sodium ions

Let's delve further into ionic structures

-Not all ionic substances form the same structures

-Caesium chloride (CsCl), for example forms a different structure due to the larger size of the caesium ion compared t=with that of the sodium ion

-This gives rise to the structure which is called a body-centred cubic structure

-Each caesium ion is surrounded by 8 chloride ions and, in turn, each chloride ion is surrounded by 8 caesium ions

g

Properties of ionic compounds

- They are usually solids at room temperature with high melting and boiling points

-This is due to the strong electrostatic forces holding the crystal lattice together

-A lot of energy is therefore needed to separate the ions and melt the substance

- They are usually hard substances

-They mainly dissolve in water

-This is because water molecules are able to bond with both the positive and the negative ions, which breaks up the lattice and keeps the ions apart

- They usually conduct electricity when in the molten state or in aqueous solution

The forces of attraction between the ions are weakened and the ions are free to move to the appropriate electrode. This allows an electric current to be passed through the molten compound

- They usually cannot conduct electricity when solid

because the ions are not free to move

Formulae of ionic substances

-Ionic compounds contain positive & negative ions, whose charges balance.

For example, sodium chloride contains one Na+ ion for every Cl- ion, giving rise to the formula NaCl

-This formulae which show the ratio of the number of ions present in any ionic compound

-The formula of magnesium chloride is MgCl2

-This formula is arrived at by each Mg2+ ion combining with two Cl-ions, and once again the charges balance

-The use of oxidation numbers is useful when working out the formula of a compound

-An oxidation number shows how oxidised or reduced an ion is compared to its atom

-Na+ has an oxidation number of 1+ because it is formed by the loss of one electron from a sodium atom but Mg2+ has an oxidation number of +2 because it is formed when a magnesium atom loses two electrons.

-Na+ can bond (combine) with only one Cl- ion, whereas Ng2+ can bond with two Cl- ions.

-A chloride ion has an oxidation state of -1 because it is formed when a chlorine atom gains one extra electron

-Some elements, such as copper and iron, possess two ions with different oxidation numbers.

-Copper can form the Cu+ ion and the Cu2+ ion, with oxidation numbers of 1 and 2 respectively

-Therefore it can form two different compounds with chlorine, CuCl and CuCl2

-We can also distinguish the difference by using Roman numerals in their names: CuCl is copper(I) chloride and CuCl2 is copper(II) chloride

-Similarly, iron forms the Fe2+ and Fe3_ ions and so can also form two different compounds with, for example, chlorine; FeCl2 (iron (II) chlorine) and FeCl3 (iron (III) chloride).

-Refer to table 3.1 for groups of atoms which have net charges

-For example, the nitrate ion is a single unit composed of one nitrogen atom and three oxygen atoms, and has one single negative charge.

-The formula, therefore, of magnesium nitrate would be Mg(NO3)2

-You will notice that the NO3 has been placed in brackets with a2 outside the bracket

-This indicates that there are two nitrate ions present for every magnesium ion

The ratio of the atoms present is therefore:

Mg (N 03)2

1Mg : 2N : 6O

Oxidation number

-Each atom in an element or compound is assigned an oxidation number to show how much it is reduced or oxidised

-Roman numerals are used in writing the oxidation number of an element

-This number is placed after the element that it refers to

-The oxidation number of the free element is always 0

- In simple monatomic ions, the oxidation number is the same as the charge on the ion

- Compounds have no charge overall. Hence the oxidation numbers of all the individual elements in a compound must add up to 0

The oxidation numbers of elements in compounds can vary. It is possible to recognise which of the different oxidation numbers a metal element is in by the colour of its compounds

- An increase in the oxidation number is known as oxidation

-A reduction/ decrease in oxidation number is known as reduction

-During a redox reaction, the substance that brings about oxidation is called an oxidising agent and is itself reduced during the process

-A substance that brings about reduction is a reducing agent and is itself oxidised during the process

-An oxidising agent is a substance that oxidises another substance and is itself reduced

-An reducing agent is a substance that oxidises another substance and is itself reduced

-A reducing agent is a substance that reduces another substance and is itself oxidised

3.2 Covalent Bonding

-Another way in which atoms can form stable compounds is by sharing the electrons in their outer shells

This occurs between non-metal atoms, and the bond formed as called a covalent bond. During the bond formation, the atoms involved gain the stability of the noble (inert) gas electron configuration,

Ex. Hydrogen Gas molecule, H2

-A covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations

-Each hydrogen atom in the molecule has one electron

-In order to obtain a full outer shell and gain an electronic configuration that is the same as the noble gas helium, each of the hydrogen atoms must have two electrons.

-To do this, the outer shell of the two hydrogen atoms overlap

-A molecule of hydrogen is formed, with two hydrogen atoms sharing a pair of electrons

-This shared pair of electrons is known as a single covalent bond and is represented by a single line in hydrogen

-A similar example exists in the diatomic halogen molecule, chlorine, Cl2

-Another simple molecule is hydrogen chloride. This time two different elements share electrons to gain the electronic configuration of their nearest noble gas. In this case hydrogen, with only one electron in its outer shell, needs to share one more electron to gain the electronic configuration of helium. Chlorine has seven electrons in its outer shell so it needs to share one more electron, this time to gain the electronic configuration of the noble gas argon.

Other covalent compounds

-Methane (natural gas) is a gas whose molecules contain atoms of carbon and hydrogen. The electronic configurations are: 6C2,4 1H1

- The carbon atom needs four more electrons to attain the electronic configuration of the noble gas neon.

-Each hydrogen atom needs only one electron to form the electronic configuration of helium

-Ammonia is a gas containing the elements nitrogen and hydrogen. It is used in large amounts to make fertilizers

-The electronic configurations of the two elements are

7N2,5 1H1

-The nitrogen atom needs three more electrons to obtain the noble gas structure of neon

-Each hydrogen requires only 1 electron to form the noble gas structure of helium.

-The nitrogen and hydrogen atoms share electrons, forming three single covalent bonds

-Water is a liquid containing the elements hydrogen and oxygen. The electronic configurations of the two elements are:8O 2,6 1H 1

- The oxygen atom needs two electrons to gain the electronic configuration of neon.

-Each hydrogen requires one more electron to gain the electronic configuration of helium

-Again, the oxygen and hydrogen atoms share electrons, forming a water molecule with two single covalent bonds

- Carbon dioxide is a gas containing the elements carbon and oxygen

The electronic configurations of the two elements are: 6C 2,4 8O 2,6

-In this case each carbon atom needs to share four electrons to gain the electronic configuration of neon

- Each oxygen needs to share two electrons to gain the electronic configuration of neon

-This is achieved by forming two double covalent bonds in which two pairs of electrons are shared in each case. CO2 is a linear molecule

-Another molecule which contains a double covalent bond is that of oxygen gas, O2.

-Each of the oxygen atoms has six electrons in its outer/ valence shell

8O 2,6

-Each needs to share another two electrons to gain the electronic configuration of the noble gas neon.

-As with the carbon dioxide molecule, the double bond can be shown by a double line between the two O2 atoms, representing two shared pairs of electrons. O=0

-Nitrogen gas, N2, is molecule which contains a triple covalent bond.

-A triple covalent bond is formed when three pairs of electrons are shared

-Each nitrogen atom has the electronic configuration shown below:

7N 2,5

-Both nitrogen atoms in the molecule need to share a further three electrons to gain the electronic configuration of the noble gas neon

-Methanol, CH3 OH, is a molecule which contains three different types of atom.

-When this happens and you need to draw a dot-and-cross diagram, simply make sure that atoms which bond with one another do not have dots or crosses,

-In this molecule, the atoms have these configurations:

6C 2,4 1H 1 8O 2,6

-The carbon atom needs to share four further electrons which it can do with the three hydrogens; each needs to share one more electron, and the oxygen atom.

-The oxygen atom, which shares one electron with the carbon atom, shares another electron with the remaining hydrogen atom.

-By doing this, the hydrogen atoms gain the electronic configuration of helium, and the carbon and oxygen atoms gain the electronic configuration of neon.

Covalent Structures

-Compounds containing covalent bonds have molecules whose structures can be classified as either simple molecular or giant covalent.

-Simple molecular structures are simple, formed from only a few atoms. --They have strong covalent bonds between the atoms within a molecule (intramolecular bonds) but have weak bonds between the molecules (intermolecular forces)

-Some of the strongest of these weak intermolecular forces occur between water molecules

-One type of weak bond between molecules is known ass van der Waals bond (or force), and these forces increase steadily with the increasing size of the molecule

-Examples of simple molecules are iodine, methane, water and ethanol

-Giant covalent structures contain many hundreds of thousands of atoms joined by strong covalent bonds.

-Examples of substances with this type of structure are diamond, graphite and silicon(IV) oxide

Properties of covalent compounds

-As simple molecular compounds, they are usually gases, liquids or solids with low melting and boiling points.

-The melting points are low because of the weak intermolecular forces of attraction which exist between simple molecules.

-These are weaker compared to the strong covalent bonds

-Giant covalent substances have higher melting points, because the whole structure is held together by strong covalent bonds

-It should be noted that in ionic compounds, the interionic forces are much stronger than the intermolecular forces in simple covalent substances and so the melting and boiling points are generally higher.

-Generally, they do not conduct electricity when molten or dissolved in water

-This is because they do not contain ions.

-However, some molecules react with water to form ions. For example, hydrogen chloride gas produces aqueous hydrogen ions and chloride ions when it dissolves in water.

-The presence of the ions allows the solution to conduct electricity

-Generally, they do not dissolve in water. However water is an excellent solvent and can interact with and dissolve some covalent molecules better than others

-Covalent substances are generally soluble in organic solvents

-Quartz is a hard solid at room temperature. It has a melting point of 1610C and a boiling point of 2230C

Different forms of carbon

-Carbon is a non-metallic element which exists in more than one solid structural form

-These are graphite and diamond

-Each of the forms has a different structure and so they exhibit different physical properties.

-The different physical properties that they exhibit lead to graphite and diamond being used in different ways

Graphite

-This is a layer structure

-Within each layer, each carbon atom is bonded to three others by strong covalent bonds.

-Each layer is therefore like a giant molecule

-Between these layers, there are weak forces of attraction and so the layers will pass over each other easily

-With only three covalent bonds formed between carbon atoms within the layers, an unbonded electron is present on each carbon atom

-These 'spare' (or delocalised) electrons form electron clouds between the layers and it is because of these spare electrons that graphite conducts electricity

Graphitic compounds

-In recent years, a set of interesting compounds known as graphitic compounds have been developed.

-In these compounds, different atoms have been fitted in between the layers of carbon atoms to produce a substance with a greater electrical conductivity than pure graphite

-Graphite is also used as a component in certain sports equipment, such as tennis and squash rackets

Graphene

-Discovered in 2004. graphene is so-called super material made up of single layers of graphite.

-It is able to conduct electricity one million times better than copper metal and has enormous potential in electronics

Diamond

-Each of the carbon atoms in the giant structure is covalently bonded to four others

-They form a tetrahedral arrangement similar to that found in silicon (IV) oxide

-This bonding scheme gives rise to a very rigid 3-dimensional structure and accounts for the extreme hardness of the substances silicon (IV) oxide and diamond

- All the outer shell electrons of the carbon atoms are used to form covalent bonds, so there are no electrons available to enable diamond or silicon (IV) oxide to conduct electricity

-It is possible to manufacture the different allotropes of carbon

-Diamond is made by heating graphite to about 300C at very high pressures

-Diamond made by this method is known as industrial diamond

-Graphite can be made by heating a mixture of coke and sand at a very high temperature in an electric arc furnace for about 24 hours

Buckminsterfullerene- an unusual form of carbon

-In 1985, a new allotrope of carbon was obtained by Richard Smalley and Robert Curl of Rice University, Texas.

-It was formed by the action of a laser beam on a sample of graphite

-This spherical structure is composed of 60 carbon atoms covalently bonded together

-Further spherical forms of carbon, 'bucky balls', containing 70, 72 and 84 carbon atoms have been identified and the discovery has led to a whole new branch of inorganic carbon chemistry

It is thought that this type of molecule exists in chimney soot

-Chemists have suggested that, due to the large surface area of the bucky balls, they may have uses as catalysts

-Also they may have uses as superconductors

Glasses and ceramics

-Glasses are all around us

-Glasses are irregular giant molecular structures held together by strong covalent bonds.

-Glass can be made by heating silicon (IV) oxide with other substances until a thick viscous liquid is formed

-As this liquid cools, the atoms present cannot move freely enough to return to their arrangement within the pure silicon (IV) oxide structure

-Instead they are forced to form a disordered arrangement.

-Glass is called a supercooled liquid

Metallic bonding

-Another way in which atoms obtain a more stable electronic configuration is found in metals

-The electrons in the outer shell of the atom of a metal move freely throughout the structure

-They are delocalised, forming a mobile 'sea' of electrons

-When the metal atoms lose the electrons, they form a giant lattice of positive ions.

-Therefore, metals consist of positive ions embedded in a sea of moving electrons.

-The negatively charged electrons attract all the positive metal ions and bond them together with strong electrostatic forces of attraction as a single unit

-This is the metallic bond

Properties of metals

-They conduct electricity due to the mobile gas electrons within the metal structure

-When a metal is connected in a circuit, the electrons move towards the positive terminal while at the same time electrons are fed into the other end of the metal from the negative terminal

-They are malleable and ductile. Unlike the fixed bonds in diamond, metallic bonds are not rigid, but they are still strong.

-If a force is applied to a metal, rows of ions can slide over one another

-They reposition themselves and the strong bonds reform.

-"Malleable" means that metals can be bent or hammered into different shapes

-"Ductile" means that the metals can be pulled out into thin wires

-They usually have high melting and boiling points due to the strong attraction between the positive metal ions and the mobile 'sea' of electrons

-They have high densities because the atoms are very closely packed in a regular manner.

-Different metals have different types of packing of atoms and in doing so they produce the arrangement of ions.