Study Portal 12 – Chapter 12

Structure of Molecules

 

Q1.      Fill in the blanks with appropriate words.

  1. i) A chemical bond is the Electrostatic force which binds atoms to constitute molecules.
  2. ii) The bond forms when attractive forces Stranger the repulsive forces.
  • The water molecules have covalent type of bonding.
  1. The metals have Free
  2. The water has Polar bonding, which makes it liquid although it is covalent.
  3. The ionic compounds always have Solid physical state.
  • Difference in physical states among H2O and H2S is because of Hydrogen Binding.
  • The most electro negative element is Fluorine.
  1. In the Coordinate Covalent type of bond, both shared electrons comes from the same atom.
  2. Same types of atoms have covalent type of bond.

Q2.      Choose the correct answer.

  1. i) The KCl has the type of bonding:

(a) ionic                             (b) covalent

(c) co-ordinate covalent     (d) metallic

  1. ii) A covalent bond forms by:

(a) Transfer of electron                 (b) sharing of electron

(c) both by transfer and sharing (d) no transfer and sharing

  • The metals have character:

(a) solid     (b) lustrous (c) conductor (d) all of above

  1. Generally the covalent compounds are physically:

(a)  liquid   (b) gas             (c) solid           (d) plasma

  1. The metals conduct electricity because of

(a)  polarity                        (b) free electron

(c) both by polarity and free electron

(d) neither polarity nor electron are involved

Q3.      Answer the following questions in short.

  1. i) What is bond? Name different types of bonds formed between atoms.

Ans.  A chemical bond is defined as a force of attraction between atoms that holds them together in a molecule. The valence electrons, which are involved in chemical bonding, are termed as bonding electrons. There are four types of chemical bonds. (a) Ionic bond (b) covalent bond (c) coordinate covalent bond (d) metallic bond.

  1. How you can differentiate between two compounds having ionic and covalent bonding?

Ans.  Chemical bond which is formed due to complete transfer of electron from one atom to other atom is called ionic bond. While chemical bond which is formed due to mutual sharing of electrons is called covalent bond. Compounds having ionic bond have high melting and boiling points.

iii)     What type of bond will be formed between following (a) Na and Br2 (b) Ca and Cl2 (c) C and O2 (d) N2 and H2.

(a) Na and Br2          Covalent Bond

(b) Ca and Cl2        Ionic Bond

(c) C and O2          Covalent Bond

(d) N2 and H2         Non-Polar Covalent Bond

  1. iv) What do you know about Lewis structure of a compound explains with example?

Ans.  The covalent bond which is formed by the sharing of single electron pair is called single covalent bond. For case a single line is written between the two bonded atoms. It cans also be denoted by the electron cross and dot structure. Such cross dot structure of compounds are called Lewis Structure, e.g. H2, CH4.

  1. v) What different types of covalent bonds are there?

Ans.  The covalent bond can be classified into three different types depending upon the numbers of electron pairs donated.

(a)  Single covalent bond e.g. H2, F2, HCl, CH4

(b) Double Covalent bond e.g. O2, CO2, C2H4

(c)  Triple covalent bond e.g. C2H2

  1. vi) What is the difference between simple covalent bond and coordinate covalent bond?

Ans.  When one electron is contributed by each bonded atom, one bond pair is formed and it forms a single covalent bond. Coordinate covalent bonding is a type of covalent bonding in which the bond pair of electrons is donated by one bonded atom only. The atom which donates the electron pair is called donor and the atom which accepts the electron pair is called accepter.

vii)          What is meant by polarity of a covalent bond?

Ans.  The bond formed by the equal sharing of valence electrons is called covalent. In covalent bond electrons are formed equally near the two nuclei. In certain cases the electrons though shared by both the nuclei tend to be near to one nucleus than the other. This is particularly true in case of heterogeneous nuclear molecules.

viii)   Correlate bonding and special feature of chemical compound.

Ans.  The covalent compounds have sharing of electrons and thus there does not exist strong interacting among the constituting particles like that of ionic compounds, hence they have melting and boiling points and mostly the covalent compounds are gases. The covalent compounds do not conduct electricity unless they are ionized by some factor.

  1. ix) Heating water would cause breakage of inter molecular forces or interamolecular forces first.

Ans.  The forces that hold atoms in a compound or chemical bond. In addition to these strong forces, relatively weak forces also exist in between the molecules which are called inter molecular force. By heating water, inter molecular forces break first.

  1. x) What types of inter and intra molecular forces are present in H2

Ans.  In water molecules hydrogen is bonded to a high electro negative element ‘O2’. Result in a highly polar molecule formed. In these molecules the dipole-dipole interaction between the ‘H’ of one molecule and ‘O2’ of 2nd molecule is relatively much stronger. Which result in high melting and boiling points of the compounds having such type of interaction force which has boiling point 1000 C

Q4.      Answer the following questions with reasoning.

  1. i) Can you explain why similar atoms form covalent bond rather than ionic bond?

Ans.  Similar atoms have following factors which favor the formation of covalent bond.

(a)     The similar atoms having higher ionization energy which can not lose electron easily. Therefore, they can not form ionic bond; instead they acquire stability by mutual sharing of electrons.

(b)     They have equal electron affinities tend to complete their outer shell by mutual sharing of electrons

(c)     Equal electro negativity does not permit to transfer of electrons instead the atom share the electron and form covalent bond.

  1. ii) Is hydrogen bonding a true type of intra molecular attraction? Explain with reasons.

Ans.  Hydrogen bonding is a special type of inter molecular forces present in the permanently polar molecules. This bonding can be considered unique dipole-dipole attraction. This force of attraction develops between molecules that have a hydrogen atom bonded to a small highly electro negative atom with lone pairs of electrons such as nitrogen, oxygen and fluorine. The covalent bond between hydrogen atom and other atoms becomes polar enough to create a partial positive charge on hydrogen atom and a partial negative charge on the other atom. Due to this boiling point of the compounds are affected greatly.

iii)     Explain why water pipes get ruptured during winter in cold regions?

Ans.  This is the hydrogen bonding due to which water expends when freezing. By expending of water; water pipes get ruptured during winter in cold regions. In the liquid state water molecules move randomly, however when water freezes, the molecules arrange themselves in an ordered form. That gives them open structure. This process expends the molecules.

  1. iv) Why H2O has boiling point 1000C while H2S has B.P = -600C?

Ans.  The molecules where hydrogen bonded to a highly electro negative element. Result in a highly polar molecule in such molecules the dipole-dipole interaction between the hydrogen of one molecule and of 2nd molecule is relatively much stronger which result in higher melting and boiling point of the compounds having such type of interaction so water has boiling point 1000C while H2S has P.B= – 600C

  1. v) Why NaCl is soluble in cold water but not in boiling oil?

Ans.  However ionic and polar compounds are dissolving easily in polar solvents like water. So NaCl is soluble in cold water but not in boiling oil which is a non polar solvent.

Q5.   What is the difference between the inter-molecular force and intra- molecular force?

Ans.  Inter-molecular forces are the forces of attraction present between the molecules to bring them closer and to bind molecules. These are quite different from intra-molecular forces which are present between the atoms to form a molecule. Dipole-dipole forces and hydrogen bonding are inter-molecular forces.

Q6.   Discuss the importance of bonding in establishing the characters of a chemical compounds?

Ans.  A chemical bond is defined as a force of attraction between atoms that holds them together in a molecule. In other words during bond formation there is some forces which holds the atoms together.

When two approaching atoms come closer they attractive as well as repulsive forces become operative. The formation of a chemical bond is a result of net attractive force which dominates. The energy of that system is lowered and molecule is formed, otherwise if repulsive forces become dominant no chemical bond will be formed. In that case there will be increase in the energy of the system due to creation of repulsive forces.

Q7.   What is meant by polarity of a covalent bond? Explain with examples.

Ans.  A covalent bond may be called a polar covalent bond if there exists a reasonable difference of electro negativities between the bonded atoms. There will be unequal attraction for the bond pair of electrons between such atoms. It will result in the formation of polar covalent bond. The difference between electro negativities of hydrogen and chlorine is more it attracts the shared pair of electron towards itself with a greater force. A partial negative charge is there fore created on chlorine and in turn a partial positive charge on hydrogen due to electro negativity difference. It creates polarity in the bond and is called polar bond.

Q8.   Write a note on types of bonds found in different compounds?

Ans.     Depending upon their nature there are following four types of chemical bonds.

  1. The ionic bond
  2. The covalent bond
  3. The co-ordinate covalent bond / dative bond
  4. Metallic bond
  5. Ionic Bond:- This type of bond is formed when one or more electrons are completely transferred from atom to the other as a result of this transfer ions are formed. The examples of compounds having ionic bond are, NaCl, CaCl2, NaF, NH4Cl etc.
  6. Covalent Bond:- This type of bond is formed when none of atoms are able to donate the electrons completely so they share the electrons e.g. in case of ‘H2’ molecules each ‘H’ atom shares an electron with the other hydrogen atom, and have each ‘ H’ achieves noble gas configuration of ‘H’, and makes a covalent bond. Covalent compounds are O2, CO2, H2O, CCl4 the Covalent bond can be further classified into 3 different types; they are (a) simple or single covalent bond (b) double covalent bond (c) triple covalent bond.
  7. Co-ordinate covalent bond:- This is a type of covalent bond in which both electrons of bond comes from the same atom e.g. formation of BF3 – NH3. there are two types of co-ordinate covalent bond; (a) non-polar covalent bond e.g. N2, H2, O2. (b) polar covalent bond e.g. H2O, HCl.
  8. Metallic Bond:- Metals have a special types of bonding called metallic bond, in the metals around the charge positive center on electron sea is present, these electrons are free to move and are called free electrons.

Q9.      Why the atoms joint together to form a molecule?

Ans.  It is the basic tendency of matter to be in the lowest energy state, that is why the atoms react to form molecules or form bonds between them. In the periodic table the most stable electronic structure is of the noble gasses, so all the other atoms react to attain the electronic structure of these noble gasses. For example the Na atom has the electronic configuration, 1S2, 2S2, 2P6, 3S1. the nearest noble gas of this configuration is Neon with electronic configuration  1S2, 2S2, 2P6. so the Na tries to attain this configuration when it reacts with the other elements e.g. in reaction with Cl to form NaCl, the Na losses 1 electron and attains the electronic configuration of Ne, in the same way Cl accepts 1 electron and attains the electronic configuration of his nearest noble gas Ar, during process of formation of NaCl the energy liberated and NaCl molecule is formed.

Q10.    What are the different types of covalent bonds?

Ans.  The covalent bond is formed by mutual sharing of electrons between two atoms. The electron that pair up to form a chemical bond are called ‘bond pair’ electron depending upon the number of bond pairs, covalent bond is classified into following three types.

  1. Single Covalent Bond:- When one electron is contributed by each bonded atom, one bond pair is formed and it forms a single covalent bond e.g. H2, Cl2, HCl etc.
  2. Double Covalent Bond:- When each bonded atom contributes two electrons, two bond pairs are shared and a double covalent bond is formed e.g. O2, C2H
  • Triple Covalent Bond:- When each bonded atoms contributes three electrons, three bond pairs are involved in bond formation. This type is called triple covalent bond e.g. N2 and C2H2.

Q11.    Write a note on characteristics of ionic compounds?

Ans.     The ionic compounds have following properties.

  1. Ionic compounds are mostly crystalline solids.
  2. Ionic compounds in solid state have negligible electrical conductance but they are good conductors in solution and in the molten form. It is due to presence of free ions in them.
  • Ionic compounds have high melting and boiling points. For example, sodium chloride has melting point 8000C and a boiling point 14130 as strong electrostatic forces of attraction between oppositely charged ions. So, a great amount of energy is required to break these forces.
  1. They dissolve easily in polar solvents like water. Water has high dielectric constant that weakens the attraction between ions.

Activity 4.1:- Write the electron cross and dot structure for the CO2 and NH3, N2 and Ethane molecules.

CO2                                                                                               

NH3                                                                                          

N2                                                                                          

C2H6

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