CHEMISTRY- C6 CHEMICAL BONDING

IONIC BONDING

Formation of ionic compounds

Ionic bond:

The electrostatic force of attraction that is strong enough to hold together two unlike charges, namely a cation and an anion is defined as the ionic bond.

In an ionic bond,

• A metal atom and a non-metal atom are involved.

• The metal atom loses electron/electrons, and a non-metal atom accepts the electron/electrons.

• A positively charged ion or cation is formed from a metal atom.

• A negatively charged ion or anion is formed from a non-metal atom due to the acceptance of electrons from metal atoms.

Formation of sodium chloride

In the formation of sodium chloride,

As can be seen, a sodium atom has one valence electron, which it needs to lose to attain the neon gas electronic configuration, chlorine has seven valence electrons, it needs one more electron to attain the argon configuration. Sodium atom transfers an electron to the chlorine atom, hence an ionic bond is formed.

The crystal lattice of NaCl reveals that the ions are arranged in a three dimensional pattern, the positive ion alternating with a negative ion.

Crystal lattice of NaCl

The sodium chloride crystal is made up of ions. One sodium ion is surrounded by six chloride ions. One chloride ion is surround by six sodium ions. The crystal is cubic in shape.

Formation of magnesium oxide

A magnesium atom has two valence electrons, which it donates to the oxygen atom. Both magnesium and oxygen attain the neon configuration.

Formation of magnesium nitride

The electronic configuration of magnesium is 2,8,2 and that of nitrogen is 2,5.

A magnesium atom needs to lose two electrons, whereas the nitrogen atom needs to gain three electrons to attain the octet.

The chemical formula for magnesium nitride is Mg₃N_2.

Ion formation takes place as follows:

Each magnesium atom loses two electrons.

Each nitrogen atom gains three electrons.

Two magnesium atoms donate two electrons to each nitrogen atom while one magnesium atom donates one electron to each of the nitrogen atom. Hence, an ionic bond is formed. Magnesium and nitrogen, thus, attain stability by gaining the octet state.

The conditions necessary for formation of ionic compounds are:

a) It depends primarily on the formation of the cation and anion from neutral atoms.

b) When the difference in electro-negativities of the metal atom and non-metal is large, transfer of electrons take place.

c) A lower ionisation value facilitates easy formation of the cation. For example, the ionisation potential of sodium is 5.1ev, whereas for chlorine it is 13.00ev. Hence, it is easy to remove electrons from a sodium atom.

d) If the electron affinity value is high, it facilitates formation of an anion.

For example, the higher value for chlorine is -3.80ev, so it forms an anion easily.

[Energy released = -3.80ev]

Characteristics of electrovalent compounds

1) Ionic compounds consist of ions and not molecules. For example, sodium chloride is an electrovalent compound consisting of equal number of sodium and chloride ions.

2) Ionic compounds are hard crystalline solids. The crystal of sodium chloride has a definite shape. There is a strong electrostatic force of attraction, which holds the ions together. The ions cannot be easily separated. The crystals are hard and brittle.

3) Ionic compounds have high melting and boiling points. For example, sodium chloride has a high melting point of 1472^oF and boiling point of 2575.4^oF. Lithium chloride has a melting point of 1122.8^oF and boiling point of 2462^oF. Potassium chloride has a melting point of 1526^oF and boiling point of 2732^oF.

As these compounds contain ions held together by strong electrostatic forces, very high amount of energy is required to overcome this force and break the crystal lattice. This explains the high melting point and boiling point of ionic compounds.

4) Electrovalent compounds or ionic compounds are usually soluble in water, but insoluble in organic solvents like ether, acetone, benzene, carbon disulphide and carbon tetrachloride. 'Like dissolves like': Ionic compounds dissolve in polar or ionic solvents.

Water has a high dielectric constant. Water easily breaks the strong electrostatic force of attraction. The water molecule is polar in nature. The positively charged hydrogen atoms surround the anion. The negatively charged OH^- (hydroxyl) surround the cation. Hence, the cation is separated from the anion, breaking the crystal lattice. The organic solvents contain non-polar molecules. Therefore, they are unable to break the electrostatic forces of attraction.

5) Ionic compounds conduct electricity when dissolved in water. They also conduct electricity when melted. They do not conduct electricity in the solid state. In the solid state, the ions are held together in fixed positions by the electrostatic forces of attraction. Therefore, the ions are not free. When dissolved in water, the ions are separated. The water molecules break the strong electrostatic force of attraction. This results in the formation of free mobile ions. To conduct electricity, free mobile ions are required. In the molten state too, the ions are separated from each other. The strong electrostatic force of attraction is broken while melting resulting in the formation of free ions.

6) Ionic compounds have a high density. They are generally heavier than water. The oppositely charged ions in an ionic compound are held very closely by strong electrostatic forces of attraction, resulting in the number of ions per unit volume increases. Consequently, increasing in mass per unit volume of the compound.

7) Ionic compounds are involved in ionic reactions. The ionic reactions occur very fast as ions react quickly in solution.

8) Size of the cation involved in bonding is smaller compared to the neutral atom. A cation loses electrons, so there is a reduction in the number of shells. Hence, the size of the ion becomes smaller. On the other hand, in an anion, there is an increase in the number of electrons in a particular shell. This leads to a decrease in the effective nuclear charge. The protons feel a greater attraction towards the electrons, therefore the size is reduced compared to the atom.

COVALENT BONDING

Can we imagine a world without oxygen, nitrogen, water or carbon dioxide? Without any of these there would be no life at all. All the above mentioned compounds are covalent compounds formed by sharing of electrons. Non-metal atoms share the valence electrons and form molecules. These shared pairs of electrons contributed by each of the atoms involved in bonding are located between the nuclei of atoms.

Covalent bonding may take place between atoms of the same element as in a hydrogen molecule or a chlorine molecule.

It may also take place between atoms of different elements such as:

(i) Hydrogen and chlorine combine to form hydrogen chloride (HCl).

(ii) Carbon and oxygen combine to form carbon dioxide (CO₂).

Covalent bonds are formed between non-metal atoms. Each of the atoms involved in bonding contribute one, two, three or more electrons to form the shared pair.

Formation of hydrogen molecule

- electron present in K shell of the hydrogen atom

Here the shared pair consists of two electrons, one electron being contributed by each of the hydrogen atoms. This is called a shared pair.

Depending on the number of electron pairs shared between atoms which participate in bonding, covalent bonds are classified as follows:

SINGLE COVALENT BONDING

Let us now understand each type of bond in detail.

A single bond consists of one pair of shared electrons. In other words, it is formed by sharing one pair of electrons or two electrons between the atoms, each atom contributing one electron to the bond.

Hydrogen molecule

Hydrogen atom has one electron in its electron shell. It needs one more electron to attain a duplet.

A molecule of hydrogen is formed by the sharing of one pair of electrons between two hydrogen atoms as shown below:

The duplet for each atom is complete

The single covalent bond is represented by a short line () between the two atoms.2)">

A chlorine molecule (Cl₂)

A chlorine atom is extremely reactive. It cannot exist freely as it does not have a stable electronic configuration. Chlorine does not exist as single atom but as a diatomic molecule. A single covalent bond is formed between two chlorine atoms.

This bonding can be represented thus,

Each chlorine atom has an electronic configuration of 2,8,7. It needs one more electron to complete the octet. Each atom gets this electron by sharing one valence electron with another chlorine atom. So, two chlorine atoms share one electron, each to form a chlorine molecule. A strong force of attraction holds these atoms together.

In a molecule of chlorine, each atom has attained the electronic configuration of 2,8,8 resembling argon.

Such a covalent bond involving two atoms of the same element is called homo nuclear covalent bond.

A methane molecule

Methane is a hydrocarbon. It consists of hydrogen and carbon atoms.

Carbon:

Electronic configuration: 2,4 - Carbon atom needs four more electrons to complete its octet.

Hydrogen:

Electronic configuration: 1 - Hydrogen atom needs one more electron to attain the duplet.

As four electrons are needed by the carbon atom, we can infer that there would be four hydrogen atoms involved in bonding wherein, each would share one electron with one electron each of the carbon atom.

Carbon on the other hand, with four electrons forms four shared pairs with four different hydrogen atoms.

This bonding can also be shown as

In a methane molecule, the carbon atom attains the octet configuration of neon - 2,8, while the hydrogen atom attains the duplet configuration of helium. In total there are four carbon-hydrogen single bonds. In other words, the molecule has four pairs of shared electrons.

DOUBLE COVALENT BONDING

A double covalent bond consists of two pairs of shared electrons. It is formed by the sharing of two pairs of electrons between two atoms. It is actually a combination of two single bonds. It is represented by putting two short lines between the two atoms [=].

Let us take few examples to understand the double covalent bond in the oxygen molecule.

The oxygen atom cannot exist on its own. It does not have a stable, inert gas electron arrangement. Oxygen gas does not consist of a single atom, but consists of two atoms, which join to form a molecule.

-

electronic configuration = 2,6

Atomic number of oxygen is 8, electronic configuration is 2,6. It needs two more electrons to attain the inert gas configuration. The oxygen atom shares two of its valence electrons with another oxygen atom to form a diatomic oxygen molecule. Each of the oxygen atoms in the covalent molecule consist of eight electrons in the valence shell.

A double covalent bond is stronger than a single covalent bond.

An ethylene molecule (Ethene)

A molecule of ethylene consists of only carbon and hydrogen atoms. It is a hydrocarbon. It has a chemical formula C₂H₄. What type of covalent bonding is present in this compound?

Carbon:

Electronic configuration is 2,4. It needs four more electrons to complete its octet.

Hydrogen:

Electronic configuration is 1 . It needs one more electron to complete its duplet.

The two carbon atoms share two pairs of electrons to form a double covalent bond. The hydrogen atoms are each linked to the carbon atom by a single bond. In total there are four C-H single bonds and one C=C double bond.

The carbon atom has achieved the electronic configuration of neon and hydrogen, that of helium atom.

Diagrammatically it can be represented as shown below:

Differences between non-polar and polar covalent compounds

Differences between polar and non-polar covalent bonds

CHARACTERISTIC OF COVALENT BOND

1) Covalent compounds consist of molecules and not ions. The molecules do not have any electric charge on them. The molecules are held together by weak forces called Van der Waal's forces.

2) Covalent compounds are either gases, volatile liquids or soft solids. As there are weak, Van der Waal's forces between the molecules, they are not held in rigid position. The state depends on the bond energy. If the bond energy is very low, they stay as gases, if it is appreciable they are volatile liquids. If very high, they exist as soft solids.

3) Covalent compounds generally have low melting and boiling points. As Van der Waal's forces are weak, a very small amount of energy is required to break the bond between the molecules corresponding to low melting point and boiling point.

4) Covalent compounds dissolve in organic solvents. As they do not contain ions, solvation does not take place when water is added to the compound. Hence they do not dissolve in water.

5) Covalent compounds are bad conductors of electricity. They do not contain ions in the fused state, nor do ions migrate on application of an electric potential. Hence, there is no conduction of current.

6) Covalent compounds are less dense when compared to water. Very weak Van der Waal's forces hold the molecules together, hence there are large inter molecular spaces. Consequently less number of molecules per unit volume, which means mass per unit volume is also less. Hence they have a low density.

MIXED BONDING

Apart from coordinate covalent compounds, there are some compounds, which contain both ionic and covalent compounds.

For example, sodium hydroxide

In sodium hydroxide, there is an ionic bond between sodium ion Na⁺ and hydroxide ion, OH^-. There is a single covalent bond between oxygen and hydrogen atoms in the hydroxyl ion.

The sodium atom gives away one electron to the hydroxyl ion to form sodium hydroxide.

In hydrogen cyanide HCN, there is an ionic bond between hydrogen ion H⁺ and cyanide ion CN^-. There is a triple covalent bond between carbon and nitrogen in cyanide ion.

P/S : chapter ni senang la ckit nak bawa.. boleh la..