Chapter-4

Q: why do atoms react (combine) to form molecules?

Ans: It is a universal rule that “everything in universe tends to become more stable”. So atoms also tend to follow this rule by ;

By attaining electronic configuration of inert gases (ns 2, np6)

All noble gases have 2 or 8 atoms in their outer most shells. It means that they have completely fills valence shells. Their atoms have not any extra space to accommodate more electrons. In other words they are stable and non reactive. This is why they are also termed as inert gases.

All other atoms then try their best to have configuration like noble gases. In order to achieve this they lose or gain electrons. For this, in 1916 a chemist G.N Lewis used concept of octet rule, which is as follows

 OCTET RULE The tendency of an atom to have 8 electrons in their valence shell when bonding, is called octet rule. Remember all noble gases have 8 electrons in their valence shell except Helium

DUPLET RULE  The tendency of an atom to acquire two electron configuration in their valence shell, when bonding is called duplet rule.

Helium has two electrons in outer shell and also chemically inert. Some atoms near He (1s2) tends to have electronic configuration like this e.g. H, Li, Be etc.

SELF ASSESSMENT: Find the number of valence electrons in following atoms using periodic table

  • Carbon            (b) magnesium                        (c) Phosphorus

J as group number indicate the valence electrons in an atom, then according to periodic table

µ Carbon belong to group IV A, so it has four valence electrons.

µ Magnesium belongs to group IIA , so it has two valence electron

µ Phosphorus belongs to V A, so it has five electrons in its outer shell (valence shell)

L Find the valence shell configuration of following atoms by using periodic table

  • Silicon              (b) Sulphur      (c) Bromine     (d) Argon         (e)Potassium   (f) Nitrogen

Fill your answers

Q# Valence electrons Group # Q# Valence electrons Group # Q# Valence electrons Group #
a.     c.     e.    
b.     d.     f.    

SOME IMPORTANT TERMS:

?  Valence Electrons: the electrons in the outer most shell of an atom are called valence electrons.

? Bonding Electrons: the valence electrons which take part in forming chemical bonds are called bonding electrons.

Usually unpaired electrons in the valence shell of an atom are bonding electrons. Na, H, Cl, N, C have 1,1,1,2,3 and 4 bonding electrons respectively.

? Non Bonding Electrons:  “the valence electrons which do not take part in forming covalent bonds are called non bonding electrons”

** they are paired electrons in the valence shell of an atom

? Bonding Pair:  “ A pair of electrons shared between two atoms is called bonding pair”

?Lone pair:  “ pair of valence electrons not involved in bonding are called lone pairs”

In 1916 Walther Kossel put forth a number of proposals about valence, insisting on the “rule of eight.”

 Q: What is chemical bond ? write its various types?

ANS: Chemical Bond: The chemical force which keep the atoms together is called chemical bond.”

TYPES OF CHEMICAL BOND: I – Ionic or electrovslent bond     ii- Covalent bond   iii- Coordinate covalent bond

Q: Define ionic bond? Describe its formation with examples?

“The bond which is formed by complete transfer of one or more electrons from one atom to other atom.”                        Or

The electrostatic attraction between positive and negative ions is called ionic bond

BOND FORMATION:  i- the atom which loses its electrons become positively charged ion, is called cation.  The atom which gains these electrons becomes negatively charged ion is called anion.

ii- these two oppositely charged ions are held together by the electrostatic force of attraction to form an ionic bond.

J those compounds in which constituent ions are held together by ionic bonds are called ionic compounds

Example:  Sodium Chloride (NaCl), Potassium Chloride (KCl) , Potassium Sulphate (K2SO4)

FOTMATION OF SODIUM CHLORIDE:

Sodium chloride is formed as a result of reaction between sodium & chlorine.

2Na + Cl2 ————–ð  2 NaCl

Electronic configuration of Na & Cl

11Na = 1s2,   2s2 2p6    , 3s1         (2,8,1)

17Cl= 1s2,  2s2  2p6   ,  3s2 3p5     (2,8,7)

Above configuration show that sodium has 1 electron in its valence shell and chlorine has 7 electrons in it valence shell. Sodium metal has tendency to lose electron while chlorine being electronegative atom have tendency to gain electron

     Na ————————à Na+   +  e                                         11Na = 1s2,   2s2 2p6             (2,8,)

       Cl +e———————à Cl–                                                                         17Cl= 1s2, 2s2  2p6   ,  3s2 3p5          (2,8,8)

In this way both atoms attain noble gas configuration resulting in Na+ ion and Cl. These ions are held together by electrostatic force of attraction forming ionic bond.

Na+   + Cl———————à NaCl

Dot representation of ions

SELF ASSESSMENT: Describe formation of following cations

  1. Mg (atomic number :12)
  2.  Li (atomic number : 3)
  3. Al (atomic number : 13)
  4. K (atomic number :19)                       
  5. e.   Ca (atomic number: 20)

ANS: (NOTE: metals are usually electropositive so they tend to donate electrons and form cations)

Q# Electronic configuration Cation formation + dot structure formation
a. 12Mg= 1s2, 2s2, 2p6, 3s2 1s2, 2s2, 2p6, 3s2  à1s2, 2s2, 2p6 + 2e
b.    
c.    
d.    
e.    

SELF ASSESSMENT: Describe formation of following anions

  1. Oxygen (atomic number: 8)
  2. Fluorine (atomic number: 9)
  3. Sulphur (atomic number: 16)
  4. Bromine  (atomic number: 35)
  5. Phosphorus (atomic number: 15)

ANS: (NOTE: nonmetals are usually electronegative in nature so they tend to gain electron and form anions)

  Electronic configuration Anion formation + dot structure formation
a. 8 O= 1s2, 2s2, 2p4 1s2, 2s2, 2p4 + 2 e  à1s2, 2s2, 2p6          = O-2
b.    
c.    
d.    
e.    

SELF ASSESSMENT : For following pairs of atoms use electron dot structures to write the equation for the formation of ionic bond

  1. Na & Cl                        b. Mg & F

Solution: (a) Na is meta and Cl is non metal: metal atom tends to lose electron while non metal atom tends to gain electron to attain noble gas configuration of nearest noble gas.

Na (2,8,1) and Cl(2,8,7) : Na has 1 electron in valence shell while Cl has 7 electrons in valence shell, so Na will lose an electron and Cl will gain an electron to attain noble gas configuration.

After losing an electron Na will form Na+ and after gaining electron Cl will form Cl. For every Na+ ion, you need one Cl ion.

  • Mg is metal and F is non metal:

Describe yourself as descripted above in NaCl:

———————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————

SELF ASESSMENT Use electron dot for following pair of atoms, use electron dot & electron cross structures to write the equation for following compounds.

  • Mg & O                       b)    Al & Cl     c) MgO                        d) NaF             e)AlCl3
Sr. Electronic Config. Description
a. 12Mg= 1s2, 2s2, 2p6, 3s2   8O= 1s2, 2s2, 2p4 Mg will lose two electrons to obtain noble gas config. and form Mg2+ & by getting two electrons oxygen also have noble gas config. and form O2- Mg2+= 1s2, 2s2, 2p6                  O2-=1s2, 2s2, 2p6  
b.    
c.    
d.    
e.    

COVALENT BOND:

Def: A bond which is formed by mutual sharing of electrons is called covalent bond.

 In covalent bond the shared pair of electrons is counted towards the stability of both the atoms. These two atoms may be similar or dissimilar. In this way they acquire the stable configuration of nearest noble gas. The compounds formed due to covalent bonding are called covalent compounds.

TYPES OF COVALENT BOND

  1. Single Covalent bond:  “A covalent bond which id formed by sharing of two electrons is called single covalent bond.”

It is represented by a small line (—) between two atoms.

Example: A hydrogen atom has one electron in its valence shell. It requires one more electron to complete its nearest noble gas configuration of He (1s2). For this purpose two hydrogen atoms share their electron with each other resulting in formation of single covalent bond.

HŸ        +          xH                                HŸ   xH             HH

  • Double Covalent bond:  “A bond which is formed by mutual sharing of two electrons by each atom or two electron pairs is called double covalent bond.

It is represented by two lines (=) drawn horizontal together

Example:  sharing of two pairs of electrons (Double bond).Two atoms of oxygen share two pairs of electrons to form one molecule of oxygen

  1. Triple Covalent bond:  “A bond which is formed by mutual sharing of three electrons by each atom or three electron pairs is called triple covalent bond.

It is represented by three lines (—)

Example: sharing of three pairs of electrons (Triple bond) between two atoms of nitrogen share three pairs of electrons to form one molecule of nitrogen

 

SELF ASSESSMENT:

1.      Molecule of Methane (H = 1, C = 6)

Four atoms of hydrogen mutually share one pair of electrons each with a carbon atom to form a molecule of methane. As C has four electrons in its outer shell so it need more four electron to complete its octet & Hydrogen has one electron so it need 1 more to complete its duplet. So, C form four covalent bonds with H.

  • Structure of water molecule:

(Describe its formation by yourself)

——————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————-……………………………………………………

  • Structure of Ammonia (NH3)

(Describe its formation by yourself)

Three hydrogen atoms mutually share one pair of electrons each with a nitrogen atom to form a molecule of ammonia. Nitrogen has 5 electrons in its valence shell and need 3 more to complete its octet and H has one in its valence shell, need 1 more to complete its duplet. So N form three covalent bonds with H.

  • Carbon tetra chloride (CCl4)

Out of the four valence electrons in carbon, one is made available for each chlorine atom for sharing to form a molecule of carbon tetrachloride.

@———————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————C

  • Carbon Dioxide formation:

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  • Hydrogen Sulphide (H2S)

@————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————

  • Carbon Disulphide(CS2)
  • @————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————

INTER MOLECULAR FORCES

Q: What are non polar & polar covalent bonds?

NON POLAR COVALENT BONDS: “ when covalent bond is formed between two similar atoms resulting bonding will be non polar covalent bond.

 When covalent bond is formed between alike atoms which are equally electronegative, then the bond formed will be non polar.

Example: H2, Cl2, N2, F2 etc

POLAR COVALENT BONDS: “a covalent formed between two unlike atoms which differ in their electronegativity is said to be polar covalent bond

When a covalent bond is formed between two atoms of different electronegativity possessing elements then shared electron pair does not lie exactly between the two atoms.  It lies more towards the atoms which has more electronegativity.  Then atom with higher electronegativity develops slight negative charge. The other atom with higher electronegativity develops a slight positive charge.  Such a molecule is called polar molecule.

Example: H2O, HCl, HF betc

Q: what are inter molecular forces?

ANS: INTER MOLECULAR FORCES

Def: “the attractive forces between positive end of one polar molecule and negative end of other molecule are known as dipole dipole forces.

µ Dipole-dipole forces are present in Hydrochloric acid (HCl), acetone (CH3-C-CH3), Chloroform (CHCl3)

HYDROGEN BONDING: “the between the hydrogen atom of one molecule and a more electronegative atom of other molecule is called hydrogen bond.” A hydrogen bon is shown by dotted (………) line.

The molecules in which a hydrogen atom is bonded to a highly electronegative atom like fluorine(F), Oxygen (O) or nitrogen the shared electron pair lies more towards the electronegative element.This creates a polarity in bond. As a result slight positive charge get developed on H-atom.

µ Hydrogen bond is a weak bond. its bond energy ranges between 10 to 45 KJ/mol.

NATURE OF BONDING AND CHEMICAL PROPERTIES

Compounds that are consist of ions and are joined by electrostatic force of attraction between ions are called ionic compounds. At room temperature most of the ionic compounds are solids.

When we look at NaCl crystal lattice we find that each Na+ ion is surrounded by 6 Na-ions and each Cl is surrounded by 6 Na-ions. And this crystal as a whole is neutral. The large attractive forces result in a highly stable molecule. Ionic compounds also have high melting points. Sodium chloride has 801 C.

CsCl also form colourless cubice crystal like NaCl. Internal structure of NaCl is different from CsCl. In CsCl each Cl is surrounded by 8 Cs and each Cs is surrounded by 8 Cl. Thus in a crystal ions are strongly attracted by each other neighbor.

Mionic compounds condunct electricity in molten or aqueous state. If NaCl is used as an electrolyte in a cell. By applying voltage Na- cations will move toward negative electrode(cathode) while Cl- anions will move towards the positive electrode (anode). Flow of electric current can be measured by putting a current meter in path (as shown in diagram).

Same is the case when NaCl is used as aqueous solution. This is because when an ionic compound is dissolved in water electrons are free to move about in aqueous solution.