Chemical Bonding

The maximum electrons which can be accommodated in first, second, third shells are 2,8,18 respectively. (The sum of numbers in electronic configuration must be equal to the atomic number). Filling starts from inner orbits. Accordingly, the configuration is given.

a) Element R is most stable.

Atoms with octet configuration are stable. The arrangement of 8 electrons in the outermost orbit of an atom is called octet configuration. These atoms are chemically stable. Atom R contains 8 electrons in its second shell (outermost), hence it is most stable.

b) Element S donates electrons.

Atoms get stability when they acquire octet structure. To acquire octet structure they either gain, lose or share electrons and try to get 8 electrons in the outermost shell.

Now in the case of S, it can easily lose 2 electrons and get stability ( 2^nd shell has octet configuration ) instead of gaining 6 more electrons to get octet structure in 3^rd shell. So, it prefers to donate its 2 electrons and be stable.

c) SP₂ {S is magnesium (atomic no – 12) and P is fluorine (atomic no – 9), so it is MgF_2, magnesium fluoride}.

As explained above, S loses 2 electrons to be stable. P tries to gain 1 electron to get octet structure in 2^nd shell. So, the 2 electrons donated by S are acquired by 2 P atoms to form bond. Thus, both acquire stability and form compound.

a) Mg → Mg²⁺ + 2e^-

Cl + 1e^-→ Cl^-

b) Magnesium ion – Mg²⁺ is cation and fluorine ion - F^- is anion.

c) Ionic bond

Explanation – Magnesium atom loses 2 electrons and acquires positive charge and thereby its called cation. Fluorine atom gains 1 electron, acquires negative charge and becomes anion. Both obtain octet configuration and become stable. The type of bond formed by the transfer of electrons is called ionic bond.

Note: the sum of oxidation numbers of all elements in a compound is zero.

Consider x is the oxidation number of atoms whose oxidation number is to be found. Oxidation number of oxygen is -2. Applying the above rule-

1. 2x + (-2) = 0 ; x = 1 ; oxidation number of hydrogen is 1.

2. 2(1) + x + 3(-2) = 0 ; x = 4 ; oxidation number of carbon is 4.

3. 1 + x + 3(-2) = 0 ; x = 5 ; oxidation number of nitrogen is 5.

4. 3(1) + x + 4(-2) = 0 ; x = 5 ; oxidation number of phosphorous is 5.

Electronegativity difference between the constituent elements of a

compound decide the nature of compound. If the difference is 1.7 or more

than 1.7, then it generally shows ionic character and if its less than 1.7 it shows covalent character. Subtracting the respective electronegativity values of constituents gives difference.

1. Electronegativity difference of SO₂ is-

Electronegativity of O – Electronegativity of S

= 3.5 – 2.58 = 0.92

0.92 < 1.7 ; hence its covalent

2. Electronegativity difference of H₂O is-

Electronegativity of O – Electronegativity of H

= 3.5 – 2.2 = 1.3

1.3 < 1.7 ; hence its covalent

3. Electronegativity difference of CaF₂ is-

Electronegativity of F – Electronegativity of Ca

= 3.98 – 1.0 = 2.98

2.98 > 1.7 ; hence its ionic

4. Electronegativity difference of CO₂ is-

Electronegativity of O – Electronegativity of C

= 3.5 – 2.5 = 1.0

1 < 1.7 ; hence its covalent

a) BaCl₂

b) ZnO

c) 2

Valency is the combining capacity of an element. It can be treated as the number of electrons lost, gained or shared by an atom in a chemical combination.

The following are the steps to write chemical formulae from valency.

•First write the element with lower electronegativity.

•Exchange the valency of each element and write as suffix.

•Divide the suffix with common factor.

•If the suffix is 1, need not be written.

a) Ba is written first since it has lower electronegativity. The valences are exchanged and put as suffix, so Ba gets chlorine valency 1 as suffix and Cl gets barium valency 2 as suffix. Division by common factor 1 yields same. the suffix one is not needed. so, its simply written as BaCl₂.

b) Zn is written first since it has lower electronegativity. The valences are

exchanged and put as suffix, so Zn gets oxygen valency 2 as suffix and O gets zinc valency 2 as suffix. Division by common factor 2 yields Zn₁O₁. The suffix one is not needed. so, its simply written as ZnO.

c) CaO

We know that the valency of O is 2, as its given. Let’s consider the valency of Ca is x. So, by following above rules, the chemical formula should be, Ca₂O_x. But the formula of calcium oxide is CaO which can be written as Ca₁O₁. So, the suffix are divided by 2 to get Ca₁O_x/2 . Comparing Ca₁O_x/2 with Ca₁O₁ , we get that

x/2 = 1

x = 2.

Hence valency of Ca is 2.

Methane

The atomic number of carbon is 6. Its configuration is 2,4. Its outermost orbit contains 4 electrons. The atomic number of hydrogen is 1. Its configuration is 1. Its outermost orbit contains 1 electron. Carbon and hydrogen thus share these electrons, so that carbon acquires octet structure and hydrogen acquires duplet structure and become stable. This is a covalent compound.

Ethane

The atomic number of carbon is 6. Its configuration is 2,4. Its outermost orbit contains 4 electrons. The atomic number of hydrogen is 1. Its configuration is 1. Its outermost orbit contains 1 electron. In ethane, 2 Carbon and 6 hydrogens thus share these electrons, so that carbons acquire octet structure and hydrogens acquire duplet structure and become stable. This is a covalent compound.

Salt solution such as sodium chloride (NaCl) conducts electric current because it has ions in it which freely move about in solution.

These ions are produced when sodium chloride dissolves in pure water to produce sodium ions (Na⁺) and chloride ions (Cl^–). Oxidation is loss of electrons. Reduction is gain of electrons. Both occur here.

When you insert the graphite electrodes of a conductivity tester in the salt solution, the positive sodium ions (cations) usually move to the negative electrode (cathode), while the negative chloride ions (anions) move to the positive electrode (anion). This movement of ions to opposite ends of the electrodes allow electric current to flow through the solution.

On the other hand, sugar solution does not conduct an electric current because sugar (C₁₂H₂₂O₁₁) dissolves in water to produce sugar molecules. These sugar molecules are usually neutral (not charged), and so are unable to move to the opposite ends of the electrodes like the ions. Sugar doesn’t dissociate into ions when it is mixed in water.

Galvanometer is a device which shows the direction of electric current in a circuit. So, in the case of salt solution, conduction of current occurs and galvanometer shows a deflection. In the case of sugar solution conduction doesn’t occur, hence galvanometer shows no deflection.

Salt is ionic in nature and sugar is covalent in nature. Generally, solutions of ionic compounds conduct electricity and solutions of covalent compounds don’t conduct electricity.

Electronegativity difference between the constituent elements of a compound decide the nature of compound. If the difference is 1.7 or more

than 1.7, then it generally shows ionic character and if its less than 1.7 it shows covalent character. Subtracting the respective electronegativity values of constituents gives difference.

1. P and R

Electronegativity difference = 3.44 – 1.31 = 2.13

Ionic compound

2. P and S

Electronegativity difference = 3.44 – 2.5 = 0.94

Covalent compound

3. Q and R

Electronegativity difference = 3.16 – 1.31 = 1.85

Ionic compound

The electronic configuration of elements and valency decide the chemical formula of the compound. Electronic configuration of –

P is 2,6 ( oxygen )

Q is 2,8,7 ( chlorine )

R is 2,8,2 ( magnesium )

S is 2,8,6 ( sulphur )

P tries to gain 2 electrons and obtain octet.

Q tries to gain 1 electron and obtain octet.

R tries to lose 2 electrons and get stability.

S tries to gain 2 electrons and obtain octet.

1. P and R form the compound PR ( MgO ) by transfer of electrons. It’s a ionic compound.

2. P and S form the compound SP₂ ( SO₂) by sharing of electrons. Its covalent compound.

3. Q and R form the compound RQ₂ ( MgCl₂) by transfer of electrons which is a ionic compound.

The 2 electrons lost by magnesium are gained by 2 chlorine atoms.

Thus, MgCl₂ is formed.