Chapter 6 Chemical Bonding

contents You will learn about: Ionic bonds – electron transfer Covalent bonds – electron sharing Properties of ionic and covalent compounds

The noble gases (Group 0 elements) are also known as inert gases. They are gases.

Noble Gas Electronic Configuration When the outer shell of an element is completely filled with electrons, the element is very stable. Atoms of noble gases in Group 0 are very stable. Hence, they do not react with other elements. Noble gas configurations are very stable because they have fully filled outer/valence shells.

Noble gases (except for helium) have 8 electrons in their outer shells. Examples Arrangement of electrons in the outer shell He helium 2 Duplet structure Ne neon Octet structure 2.8 Ar argon Octet structure 2.8.8

The tendency for atoms to surround themselves with a stable ‘octet’ of electrons is called the octet rule. Helium has only 2 electrons in its outer shell , but is stable as it too has a fully filled outer shell of electrons. The maximum number of electrons that can be accommodated in the 1st shell is 2. This is called the duplet rule.

Other elements do not have the stable electronic configuration where the outer shell is fully filled with electrons forming either the duplet or octet structure. Thus, they react with one another by losing, gaining or sharing their valency electrons to achieve the stable noble gas electronic configuration during a chemical reaction. ( valency electrons are electrons in the outer shell used in chemical bonding)

Chemical bonding Covalent bonding Ionic bonding By losing or gaining of electrons (i.e transferring of electrons) Takes place between atoms of metals and non-metals By sharing of electrons between atoms of non-metals Covalent substances Ionic compounds

-  + Formation of Cations (positive ions) and Anions (negative ions) An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons .It can be either positively or negatively charged. atoms form ions by gaining or losing electrons to obtain the stable electronic structure of a noble gas - negative ion  add electrons more electrons than protons neutral atom electrons = protons + remove electrons positive ion more protons than electrons

Electronic configuration of Metals & Non-metals

Electronic configuration of Metals & Non-metals Metals tend to lose their valence electrons to achieve the stable noble gas electronic configuration as they have few valence electrons. Non-metals tend to gain electrons to achieve the stable noble gas electronic configuration as they have a lot of electrons in their valence shell Note : only the valence (outer) electrons are involved in bonding; those in the complete inner shells do not take part.

Li atom loses an electron readily and become a lithium ion, Li+ Formation of Cations Examples Li atom loses an electron readily and become a lithium ion, Li+

+ loses 1 electron 2,8 Na+ ion Na atom 2,8,1 11 protons 11 protons Na atom loses an electron and becomes a positive ion, sodium ion, Na+ which have the stable noble gas electronic structure as Neon (2,8). loses 1 electron + Na 2,8 Na+ ion Na atom 2,8,1 11 protons 10 electrons 12 neutrons 11 protons 11 electrons 12 neutrons Charged (+1) Neutral 11 protons (+11) and 10 electrons (-10)  overall charge = (+11 -10) = +1 [all +ve charges (protons) balanced by -ve charges (electrons)]

3+ loses 3 electrons 2,8 Al3+ ion Al atom 2,8,3 13 protons 13 protons Al atom loses 3 electrons and becomes a positive ion, aluminium ion, Al3+ which have the stable noble gas electronic structure as Neon (2,8). 3+ loses 3 electrons Al Al 2,8 Al3+ ion Al atom 2,8,3 13 protons 10 electrons 14 neutrons 13 protons 13 electrons 14 neutrons Charged (+3) Neutral 13 protons (+13) and 10 electrons (-10)  overall charge = (+13 -10) = +3 [all +ve charges (protons) balanced by -ve charges (electrons)]

 Now You Try! Draw diagrams to show the formation of (a) A potassium ion (b) A calcium ion

Formation of Anions ¯ gains 1 electron Cl atom gains 1 electron from a metal atom and becomes a negative ion, named as chloride ion, Cl- which has the same noble gas electronic structure as Argon (2,8,8). x ¯ Cl gains 1 electron Cl chlorine atom 2,8,7 chloride ion 2,8,8 17 protons 17 electrons 18 neutrons Charged (-1) 17 protons (+17) and 18 electrons (-18)  overall charge = (+17 -18) = -1 17 protons 18 electrons 18 neutrons Neutral

Now You Try ……… Draw diagrams to show the formation of a fluoride ion (b) an oxide ion (c) a nitride ion

Cations- more examples 12 10 19 18 Number of electrons 20 No of protons Mg Mg2+ K K+ Symbol Magnesium atom Magnesiumion Potassium atom Number of neutrons 24 39 Note: A positive ion still has the same number of protons and neutrons as its atom, but, it will have less electrons than protons.

Anions – more examples Sulphur atom Sulphide ion Oxygen atom Oxide ion Symbol S 32 S2- 32 O 16 O2- 16 16 16 8 8 8 16 No of protons Number of electrons 16 18 8 10 16 8 Number of neutrons Note: A negative ion still has the same number of protons and neutrons as its atom, but, it will have more electrons than protons.

Ionic bonding – involves the transfer of electrons from one atom to another so that each can achieve the noble gas electronic configuration. usually formed between atoms of metals and non-metals positive and negative ions are formed after the transfer of electrons The oppositely charged ions are attracted to each other by strong electrostatic force of attraction.

This strong electrostatic force of attraction between the oppositely charged ions is called ionic bond. Example

ionic bonds – electron transfer

Diagrammatic Representation of Ionic Bonding Example 1 Formation of sodium chloride through ionic bonding chlorine atom, Cl 2.8.7 electron transfer sodium atom, Na 2.8.1

- 2.8 2.8.8 An ionic compound, sodium chloride ( NaCl ) is formed. + - electron of Na - electron of Cl Na Cl sodium ion, Na+ 2.8 chlorine ion, Cl- 2.8.8 has the same electronic structure as the noble gas, neon has the same electronic structure as the noble gas, argon ‘DOT and CROSS’ Diagram

ionic bonds – electron transfer Example 2 Formation of magnesium chloride through ionic bonding electron transfer electron transfer Mg Cl Cl magnesium atom, Mg 2.8.2 chlorine atom, Cl 2.8.7 chlorine atom, Cl 2.8.7

ionic bonds – electron transfer has the same electronic structure as the noble gas, neon has the same electronic structure as the noble gas, argon magnesium ion, Mg2+ 2.8 2+ chloride ion, Cl- 2.8.8 - Cl Mg

Compounds with ionic bonds Ionic bonds are formed between atoms of metals and non-metals in compounds. Examples include: Compounds with ionic bonds Elements Name Formula Metals Non-metals sodium chloride NaCl sodium chlorine magnesium oxide MgO magnesium oxygen potassium iodide KI potassium iodine calcium bromide CaBr2 calcium bromine

Structures of Ionic Compounds Ionic compounds have giant ionic lattice structure.

Properties of ionic compounds I. Boiling points and Melting Points Have high melting points (above 250oC) and high boiling points (above 500oC) Reason: The ionic bonds (electrostatic force of attraction) between the ions are very strong . A very large amount of heat energy is needed to overcome these strong bonds. This also explains why all ionic compounds are solids at room temperature. + -

II. Solubility Ionic compounds are usually soluble in water but insoluble in organic solvents. Reason: water molecules can separate the positive ions from the negative ions, causing them to dissolve. Exceptions: silver chloride, barium sulphate are ionic compounds which are insoluble in water. Organic solvents eg petrol, alcohol and turpentine

properties of ionic compounds III Electrical conductivity Ionic compounds do not conduct electricity in the solid state because the ions are not free to move about. When the substance is in molten state (melted in liquid form)) or aqueous state ( when dissolved in water), it can conduct electricity. Reason : In these states, the ions are free to move. The moving ions conduct electricity.

What is covalent bond? Covalent bond is a bond formed by the sharing of electrons between atoms of non-metals. After bonding, each atom attains the stable noble gas electronic configuration. Why must atoms of non-metals share electrons while atoms of metals and non-metals form ions?

Why covalent bonds are formed between non-metal atoms? For elements with 4 valency electrons, gaining or losing 4 electrons to achieve a noble gas electronic configuration requires a large amount of energy. Thus, the non-metallic elements combined by sharing of electrons to form molecules. Valency electrons - are electrons in the outermost shell used to form bonds. Valency - is the number of electrons an atom uses to form bonds Valence electrons - are electrons in the outermost shell

The molecules formed can be Simple covalent molecules like H2, O2, H2O, CO2, NH3, CH4, HCl, N2, Cl2, etc or (ii)Giant covalent molecules (or macromolecules)( which is a three dimensional network of atoms bonded together by covalent bonds to form a giant molecule ) like diamond, graphite, silicon dioxide, etc

Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Formation of Covalent Bonds Covalent Molecule Chemical formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Structural diagram hydrogen gas H2 Other ways to represent : H H H Single covalent bond H + H H atom H atom H H x o H2 molecule

Other ways to rep : H or H O xx x O H H Covalent Molecule Formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Simple Way of Showing the Bonds water H2O O H H H O Other ways to rep : or H O xx x H H

C or H H H H x x x x H C Other ways to rep : Covalent Molecule Formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Simple Way of Showing the Bonds methane CH4 H C H H C H H Other ways to rep : or H x x H C x H x H

Other ways to rep : x xo ox x or x O xoC ox O x xx xx O C Covalent Molecule Formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Simple Way of Showing the Bonds carbon dioxide CO2 O C O C Other ways to rep : x xo ox x or x O xoC ox O x xx xx

Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Covalent Molecule Formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Structural diagram oxygen O2 O O O O Double bond (2 pairs of electrons) Other ways to rep : x x o o or xo O xo O

Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Covalent Molecule Formula Electronic Structure of Molecule (‘Dot and Cross’ Diagram) Structural diagram nitrogen N2 N N N N Triplebond (3 pairs of electrons) Other ways to rep : x x o o or xo N xo N xo

Structure of Covalent Substances (a) Simple Molecular Structure Example 1 :Iodine is a simple diatomic covalent molecule. It has a simple molecular structure. Reason : There exists weak intermolecular forces of attraction, between the iodine molecules, also known as weak Van der Waals’ forces. These weak forces of attraction requires only a small amount of heat enerrgy to overcome.

Example 2 Methane is also a simple covalent molecule. Four covalent bonds (C-H) are held together by strong forces of attraction. However, weak Van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.

Properties of simple covalent compounds volatility Covalent molecules have strong covalent bonds between atoms, but the Van der Waals’ forces which exist between separate molecules are weak. During melting or boiling, the molecules do not break up into atoms, but merely move further apart. Thus, simple covalent molecules are volatile, i.e., they have low melting points and boiling points, as not much heat energy is needed to overcome the Van der Waals’ forces. Molecules do not break up into atoms but merely move further apart during melting and boiling

Properties of simple covalent compounds solubility Most covalent molecules are insoluble in water but are usually soluble in organic solvents. Exceptions: Alcohol and sugar are covalent compounds which are soluble in water. Some covalent molecules dissolve in water because of chemical reactions. E.g., chlorine. electrical conductivity Covalent substances (elements or compounds) do not conduct electricity whether in the solid or molten state. This is because they do not contain ions or free electrons. Exceptions: carbon, in the form of graphite, conducts electricity. Covalent compounds such as hydrogen chloride and hydrogen sulphide also conduct electricity when dissolved in water.

Differences between Ionic and Covalent Bonding Ionic Bond Covalent Bond Formed between metals and non-metals. Formed by transferring of electrons. Formed between non-metals and non-metals. Formed by sharing of electrons.

properties of ionic and covalent compounds summary of main differences in properties between ionic compounds and covalent compounds Property Ionic Compounds Covalent Compounds (with simple Molecular Structure) Boiling point high low Electrical conductivity do not conduct electricity when solid; conduct when molten or dissolved in water do not conduct electricity in any form Solubility in Water usually soluble usually insoluble Solubility in organic solvents insoluble soluble

properties of ionic and covalent compounds