6: Shapes of Molecules and intermolecular forces

Question 1

What is electronegativity?

Answer 1

Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself

Question 2

How is electronegativity measured?

Answer 2

The Pauling scale is used to assign a value of electronegativity for each atom and is used to compare the electronegativity of the atoms.

Pauling electrongativity values depend on an element’s position in the periodic table

Question 3

What factors effect electronegativity

Answer 3

• Nuclear charge
• Atomic radius
• Amount of shielding of the nucleus

Question 4

How does nuclear charge effect electronegativity?

Answer 4

• Attraction exists between the positively charged protons in the nucleus and negatively charged electrons found in the energy levels of an atom
• An increase in the number of protons leads to an increase in nuclear attraction for the electrons in the outer shells
• Therefore, an increased nuclear charge results in an increased electronegativity

Question 5

How does atomic radius effect electronegativity?

Answer 5

• The atomic radius is the distance between the nucleus and electrons in the outermost shell
• Electrons closer to the nucleus are more strongly attracted towards its positive nucleus
• Those electrons further away from the nucleus are less strongly attracted towards the nucleus
• Therefore, an increased atomic radius results in a decreased electronegativity

Question 6

How does shielding effect electronegativity?

Answer 6

• Filled energy levels can shield (mask) the effect of the nuclear charge causing the outer electrons to be less attracted to the nucleus
• Thus, an increased number of inner shells and subshells will result in a decreased electronegativity

Question 7

Explain how the electronegativity of elements change going down the group?

Answer 7

• There is a decrease in electronegativity going down the group
• The nuclear charge increases as more protons are being added to the nucleus
• However, each element has an extra filled electron shell, which increases shielding
• The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii
• Overall, there is decrease in attraction between the nucleus and outer bonding electrons

Question 8

Explain how electronegativity changes going across a period

Answer 8

• Electronegativity increases across a period
• The nuclear charge increases with the addition of protons to the nucleus
• Shielding remains relatively constant across the period as no new shells are being added to the atoms
• The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table
• This results in smaller atomic radii

Question 9

When is a bond polar?

Answer 9

When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom

The greater the difference in electronegativity the more polar the bond becomes

Question 10

When will a bond be non-polar

Answer 10

• The bonded atoms are the same, or
• The bonded atoms have the same or similar elecctronegativity

Question 11

What is a pure covalent bond?

Answer 11

When molecules of elements in the covalent bond have the same electronegativty

Question 12

What is a dipole moment and how is it shown

Answer 12

• The dipole moment is a measure of how polar a bond is
• The direction of the dipole moment is shown by an arrow that points to the partially negatively charged end of the dipole. The arrow has a small vertical line to it near the beginning of the arrow

Question 13

How do you decide whether a molecule is polar?

Answer 13

You have to consider
* The polarity of each bond
* How the bonds are arranged in the molecule

Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such way that the individual dipole moments cancel each other out. For example, CO2 is non-polar

Question 14

What is a dipole?

Answer 14

The seperation of opposite charges is called a dipole.
A dipole in a polar covalent bond doesn’t change and so is a permenant dipole.

Question 15

What are intermolecular forces and the 3 types?

Answer 15

Intermolecular forces are weak interactions between the dipoles of different molecules. The three main types are:
* Induced dipole-dipole interactions (London forces)
* Permenant dipole-dipole interactions
* Hydrogen bonding

Question 16

What are London forces and explain the process in which they occur?

Answer 16

London forces are weak intermolecular forces that exist between all molecules, regardless of polarity.
They are only temporary as in any instant they might disappear.

Steps:
1. Movement of electrons produces a changing dipole in a molecule
2. At any instant, an instantaneous dipole will exist (the electrons will happen to be all on one side of the molecule)
3. The instantaneous dipole induces a dipole on a neighbouring molecule
4. The induced dipole induces further dipoles on neighboroughing molecules, which then attract another

Question 17

How does the number of electrons in each molecule effect the strength of induced dipole-dipole interactions?

Answer 17

The more electrons in each molecule:
* The larger the instantaneous and induced dipoles
* The greater the induced dipole-dipole interactions
* The stronger the attractive forces between molecules

Question 18

What does van der Waal’s forces describe?

Answer 18

Both induced dipole-dipole interactions and permenant dipole-dipole interactions?

Question 19

What are permenant dipoles and where do permenant dipole-dipole interactions occur?

Answer 19

• Permenant dipoles exist in molecules where there is a difference in electronegativity
• Some molecules are permenantly polar
• Permenant dipole-dipole interactions occur between permenant dipoles

Question 20

Compare the boiling points of F2 and HCl molecules with references to their intermolecular forces

Answer 20

• F2 molecules are non polar and so have only London forces acting between the molecules
• HCl molecules are polar and have permenant dipole-dipole interactions between the molecules AND they have London forces
• Extra energy is needed to overcome both type of attraction
• So, boiling point of HCl is higher than F2

Question 21

What are simple moleculear substances made up of?

Answer 21

A simple molecular substance is made up of simple molecules- small units containing a definite number of atoms with a definitie molecular formular
Eg, Neon- Ne, Hydrogen- H2, water- H2O, Carbon dioxide- CO2

Question 22

What is the structure of simple molecular substances?

Answer 22

In a solid state, simple molecules form a regular structure called a simple molecular lattice. In a simple molecular lattice:
* The molecules are held in place by weak intermolecular forces
* The atoms within each molecule are bonded together by strong covalent bonds

Question 23

Summarise the properties of simple molecular substance

Answer 23

• Low melting and boiling points
• Non-polar substances are soluble in non-polar solvents but insoluble in polar solvents
• The solubiloty of polar substance depends on the strength of the dipole so is hard to predict
• Simple molecular substances are non-conductors of electricity

Question 24

Explain the solubility of non-polar simple molecular substances in both polar and non-polar solvents

Answer 24

Non-polar solvent
1. When added to a non-polar substance, intermolecular forces form between molecules and solvent
2. The interactions weaken the intermolecular forces in a simple molecular lattice
3. So the intermolecular forces break and the compound dissolves
4. So, non-polar simple molecular substances tend to be soluble in non-polar solvents

Polar solvents
1. When added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent moleculs
2. As the intermolecular bonding within the polar solvent is too strong to be broken
3. So simple molecular substances tend to be insoluble in polar solvents

Question 25

Explain the solubility of polar simple molecular substances

Answer 25

* Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other * Solubility depends on the strength of the dipole

Question 26

Explain the electrical conductivity of simple molecular substances

Answer 26

* There are no mobile charged particles in simple molecular structures * With no charged particles that can move, there is nothing to complete an electrical circuit. * So, they are non-conductors of electricity

Question 27

Where does a hydrogen bond form

Answer 27

* A hydrogen bond forms between an electronegative atom that has lone pair(s) of electrons * And, a hydrogen atom connected to an electronegative atom, (F, O or N)

Question 28

What are the strongest type of intermolecular forces?

Answer 28

Hydrogen bonds are the strongest type of intermolecular forces

Question 29

What types of molecules give molecules significant hydrogen bonding and why?

Answer 29

* N-H * O-H * F-H These bonds all have: * Strong peremenant dipoles * A hydrogen atom * An atom with lone pairs of electrons

Question 30

What are the anomalous properites of water?

Answer 30

Hydrogen bonding in water results in unsual properties: * Higher than expected boiling point * High specific heat capacity * Ice is less dense than water

Question 31

Explain why ice is less dense than water and what occurs when ice turns into water

Answer 31

* Hydrogen bonds hold water molecules apart in an open lattice structure * The water molecules in ice are further apart than in water * The holes in the open tetrahedral lattice decrease the density of ice * Solid ice is less dense than liquid water and floats * When ice turns to water the lattice collapses on melting and molecules move closer together

Question 32

Explain why water has a relatively high melting and boiling point

Answer 32

* Hydrogen bonds are extra forces, over and above Lodon forces * A substancial amount of energy needs to break the hydrogen bonds * So water has much higher melting and boiling points * When the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken * When water boils, the hydrogen bonds break completely

Question 33

What is meant by the term hydrogen bond?

Answer 33

A strong dipole-dipole attraction between an electron-deficient hydrogen atom of –NH, –OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule.

Question 34

What is meant by the term induced dipole-dipole interaction?

Answer 34

Attractive forces between induced dipoles in different molecules – also called London Forces.

Question 35

What is meant by the term intermolecular force?

Answer 35

An attractive force between molecules. Intermolecular forces can be London forces, permanent dipole-dipole interactions or hydrogen bonding.

Question 36

What is meant by the term ion?

Answer 36

A positively or negatively charged atom or a (covalently bonded) group of atoms (a polyatomic ion), where the number of electrons is different from the number of protons.

Question 37

What is meant by the term ionic bonding?

Answer 37

The electrostatic attraction between positive and negative ions.

Question 38

What is meant by the term lone pair?

Answer 38

An outer shell pair of electrons that is not involved in chemical bonding