CHAPTER 3: COVALENT SUBSTANCES

Question 1

what are covalent bonds

Answer 1

covalent bonds occur when electrons are shared between atoms according to the octet rule between non-metals

Question 2

single covalent bond

Answer 2

when atoms share 2 electrons

Question 3

valence structure

Answer 3

the line is used to represent the 2 electrons in a covalent bond

Question 4

double covalent bond

Answer 4

two pairs of electrons are shared between atoms

Question 5

triple covalent bond

Answer 5

three electron pairs are shared between the atoms

Question 6

VSEPR theory

Answer 6

• valence shell electron pair repulsion theory predicts molecular shape
• negatively charged electron pairs (covalent bonds or non-bonding pairs of electrons) in an atom repel each other
• as a result, these electron groups are arranged as far away from each other as possible
• the non-bonding pairs of electrons (lone pairs) influence the shape of a molecule but are not a part of it
• double and triple bonds are treated the same as single bonds

Question 7

shapes of molecules

Answer 7

refer to table

Question 8

electronegativity and polarity

Answer 8

• electronegativity determines how electrons are distributed within molecules
• Due to differences in electronegativity, you can have UNEVEN distribution of electrons in a covalent bond
• As the difference in electronegativity of two atoms increases, a covalent bond increases in polarity

Question 9

non-polar molecules

Answer 9

• non-polar molecules: “have a balance of
  electron distribution”
• In a covalent bond, the electrons are
  shared somewhat equally

Question 10

polar molecules

Answer 10

• polar molecules: “Have an imbalance in the
  electron distribution”
• In a covalent bond, the electrons will
  stay closer to the more electronegative
  atom as it has a stronger pull on the
  electrons in the bond

Question 11

non-polar diatomic molecules

Answer 11

Electronegativity difference = Higher electronegativity value –
Lower electronegativity value

• If two atoms in a covalent bond are the same (have
  the same electronegativity) → the electrons are shared equally → molecule is non-polar

Question 12

polar diatomic molecules

Answer 12

• if the bond is between atoms of 2 different elements, the electrons will stay closer to the most electronegative atom
  
  • it has a stronger pull on the electrons in the bond

Question 13

dipoles

Answer 13

• in hydrogen fluoride, electrons are pulled toward the fluorine over hydrogen
  
  • the fluorine atom has a partial negative charge → delta negative
  • hydrogen has a partial positive charge → delta positive charge
• the separation of the positive (δ+) and negative (δ-) charges is known as a DIPOLE
  
  • a dipole is when there are oppositely charged poles at each end of the molecule.
  • → when representing a dipole, we draw an arrow facing the direction where electrons are going

Question 14

polarity of polyatomic molecules (more than 2 atoms)

Answer 14

• when looking at polarity of polyatomic molecule we consider 2 things
  
  • the polarity of the bonds
  • the shape of the molecule
    
    • symmetrical polar molecules are non-polar (dipoles cancel each other out)
    • asymmetrical polar molecules are polar

Question 15

symmetry of polyatomic atoms

Answer 15

• symmetrical individual dipoles cancel each other perfectly
  
  • non-polar molecule
• in asymmetrical molecules, the individual dipoles do not cancel each other out
  
  • there is a net dipole created so it is polar

Question 16

electronegativity determining the type of bond

Answer 16

• electronegativity difference = 0
  
  • electrons are shared equally
  • bond: non-polar covalent
• electronegativity difference = less than 1.7
  
  • electrons attracted to more electronegative atom
  • polar covalent bond
• electronegativity difference =1.7
  
  • have both polar covalent and ionic bonding characteristics. E.g. AlCl3
• electronegativity difference = more than 1.7
  
  • electrons transferred to the more electronegative atom
  • ionic bond

Question 17

intramolecular bonds

Answer 17

• bonds WITHIN molecules
  
  • eg. covalent, ionic, metallic

Question 18

intermolecular bonds

Answer 18

• bonds BETWEEN molecules
• 3 types
  
  • dispersion dorces
  • dipole-dipole forces
  • hydrogen bonding

Question 19

dispersion forces

Answer 19

• dispersion forces occur in all molecules (polar and non-polar)
• result of attraction between temporary (instantaneous) dipoles that form in molecules
• temporary dipoles are due to random fluctuations in the distribution of electrons in molecules because electrons are constantly in motion within atoms
• these temporary dipoles can induce dipoles in the neighbouring molecules
• weakest intermolecular force

Question 20

strength of dispersion forces

Answer 20

• dispersion forces are stronger between larger molecules because it is easier to create temporary dipoles in molecules with a larger number of electrons.
• long molecules have more contact area to interact with other molecules compared with more compact molecules → form stronger dispersion bonds

Question 21

dipole-dipole forces

Answer 21

• occurs ONLY between Polar molecules (that have permanent dipoles)
• stronger than dispersion forces
• delta positive and negative charges of molecules attract each other
• the more polar a substance is, the stronger the dipole-dipole attraction
• molecules with larger difference in electronegativities of the atoms and larger asymmetry in the molecule shape are more polar
• the stronger the dipole-dipole forces are, the higher the melting and boiling points will be because you need more energy to break the bonds

Question 22

hydrogen bonding

Answer 22

• When hydrogen is bonded to a VERY small and highly electronegative element (N, O, F), it becomes REALLY partially positive and the other element becomes REALLY partially negative.
• STRONGEST form of dipole-dipole, so it’s classified on its own
  
  • it is 10 times stronger than any other dipole-dipole interaction
  • 1/10 the strength of an ionic or covalent bond
• Positive hydrogen nucleus (proton) attracted to the non-bonding pair of electrons on N, O or F
• Small size of hydrogen atom allows it to get close to the N,O,F
• Results in strong attractive force

Question 23

requirements for hydrogen bonding

Answer 23

• Hydrogen atom covalently bonded to N, O or F
• Non-bonding pair of electrons on N, O or F of neighboring molecules

Question 24

type of intermolecular bonding

Answer 24

• symmetrical and non-polar
  
  • only dispersion forces
• asymmetrical and polar
  
  • dispersion and dipole-dipole forces
• asymmetrical and polar (has hydrogen and N,O,F)
  
  • dispersion, dipole-dipole, and hydrogen bonding

Question 25

strength of intermolecular bonds

Answer 25

• The stronger the bonds, the greater the melting and boiling points of covalent molecular substances.
• Hydrogen bonds are the strongest of all three types of intermolecular forces (hydrogen, dipole-dipole and dispersion forces).
• Even though dispersion forces between molecules are the weakest bonds, they can be stronger than dipole-dipole attraction and hydrogen bonding in substances with large molecular masses

Question 26

ball and stick model vs space-filling model

Answer 26

ball and stick model
- displaying the molecule shape
- shows the shape but not the relative sizes of the atoms

space-filling model
- showing the relative size and position of the atoms in the molecules
- does not show bond angles or type of bonds

Question 27

properties of non-metal molcules

Answer 27

• very low melting temperatures and boiling temperatures
• is unable to conduct electricity in any phase