Ch#4 : Structure Of Molecules

Question 1

What are chemical bonds? Why do atoms react?

Answer 1

Atoms combine to form various substances. Fundamentally , some forces of attraction hold atoms together in substances. These forces are called chemical bonds. The forces of attraction which lead to chemical bonding are electrical in nature. The electronic structure of an atom helps us understand how atoms are held together to form substances. Elements other noble gases have a tendency to react with other elements. They are reactive because they tend to gain stability by losing or gaining electrons. When atoms lose or gain electrons , they acquire the electronic configuration of the nearest noble gas.

Question 2

Explain Ionic Bonds? Elaborate with an example (draw diagram for the example too).

Answer 2

Ionic Bonds : Ionic bonds are formed when a metal on the left hand side reacts with a non metal on the right hand side.They are formed between atoms when one loses electrons to form a cation and the other atom gains this electron to form an anion.

Anions and cations have opposite charges. They attract one another by electrostatic forces. “The forces of attraction which bind oppositely charged ions are called ionic bonds”. Compounds which consist of ions joined by electrostatic forces are called ionic compounds. Examples : NaCl , KCL and KBr. The total positive charge of the cations must be equal to the total nrgative charge of the anions. This is because ionic compounds are electrically neutral.

Example : NaCl ,
Na is metal and Cl is non-metal.

Metal atoms tend to lose electrons and non metals tend to gain electrons to acquire the nearest noble gas configuration.Since sodium has only 1 valence electron , it needs to lose 1 electron. It forms Na+1. Since the Cl atom has 7 electrons , it needs one more to aquire the nearest noble gas configuration. So it gains one electron to form Cl-. For every Na+ ion , you need one Cl- ion.

Question 3

Explain covalent bonds with example.

Answer 3

Covalent Bonds : A covalent bond is a bond formed by the mutual sharing of electrons between two atoms. They involve non metal atoms only.

Consider the formation of a covalent bond in a hydrogen molecule. A hydrogen atom has a single valence electron. Two hydrogen atoms share their valence electrons to form a diatomic molecule.

In the formation of this molecule , both atoms acquire the configuration of the nearest noble gas which is helium(1s²). An electron pair in the region between the two atoms is attracted to both hydrogen nuclei. This is a more stable situation than the one in seperate atoms so atoms form covalent bonds for this stability.

Pairs of valence electrons which are not shared between atoms are called lone pairs.

A covalent bond which is formed by sharing one electron pair is called a single covalent bond. Example : F²

A covalent bond formed by sharing two electron pairs is called a double covalent bond. Example : O²

A covalent bond involving three pairs of electrons are called triple covalent bonds. Example : N² and P².

Question 4

Differ between polar and non polar covalent bonds.

Answer 4

Non-polar : Polar
1. Formed when two indentical atoms share electron pairs : Formed when different atoms share electron pairs.

2. Example : F² , O² and N² : Example : H²O , COCl² and CO²
3. Both atoms have the same charge : The more electronegative atom has a slightly negative charge and the less electronegative atom has a slightly positive charge
4. A covalent bond where the two atoms exert the same force on the shared pairs : They exert different forces on the shared pair.

Question 5

Explain hydrogen bonding with diagram.

Answer 5

Dipole Dipole interactions : They are the intermolecular forces formed by the electrostatic force of attraction between the slightly positive end of a polar molecule and the slightly negative end of another polar molecule.

Consider molecules in which hydrogen is covalently bonded to to a very electronegative atom such as O , N or F is also weakly bonded to a lone pair of electrons on another electronegative ato. This other atom can be in the same molecule or a different molecule. This intermolecular force is called hydrogen bonding. O , N or F make hydrogen very electron deficient.
The interaction of a highly electron deficient hydrogen and a lone pair on a nearby atom such as N , O or F is called a hydrogen bond.