chapter 6/7 - covalent bonds and molecules

Question 1

do covalent bonds conduct electricity, why/why not

Answer 1

they dont as there are no free charged particles

Question 2

compare diatomic and polyatomic molecules

Answer 2

diatomic - 2 atoms (H2, O2 etc)

polyatomic - 2+ atoms - elements have different electrovalencies (H2O)

Question 3

how do the covalent bonds stay together

Answer 3

the protons repel one another but the electrostatic attraction between the protons and the shared electrons is stronger

Question 4

what is a fun fact about metals and non-metals

Answer 4

more metal elements but more non-metal compounds

Question 5

define a molecule

Answer 5

a discrete group of atoms that have been bonded together and have a known formula

Question 6

describe the types of electrons in a covalent bond

Answer 6

bonding electrons = shared with other atom
non-bonding = not shared
pair of non-bonding electrons = lone pair

Question 7

describe the different ways covalent bonds can be drawn - also describe the limitations of each

Answer 7

1) lewis diagram or electron dot diagram - electrons are shown by dots on one atom and crosses on another
- used to determine the formula of a molecule and see the types of bonds
- limitations include - doesnt show the relative size of atoms or the shape of the molecule
2) valence structures - dots are used to show lone pairs of electrons and then lines between the two atoms demonstrates a pair of shared electrons
- used for the same things as the lewis structure and simplifies it further
- limitations = same as lewis
3) ball and stick model
- used to show the shape of a molecule
- limitations = doesnt show the relative size of the molecules
4) space-filling model
- used to show the relative size and position of the atoms
- limitations = doesnt show the types of bonds or the angles of the bonds

Question 8

how do we know the shape of simple molecules

Answer 8

using teh valence shell electron pair repulsion (VSEPR) theory which predicts the shape of molecules based on teh types of covalent bonds are present
- based of the theory that valence electron pairs repel one another and want to be as fair away from one another as possible

Question 9

what properties does the shape of molecules mainly effect

Answer 9

melting point, solubility, boiling point, and hardness as it effects how the substance interacts with other molecules

Question 10

what are the different shapes that molecules and form and explain how each is formed

Answer 10

1) tetrahedral shape - when carbon bonds with other atoms to form 4 shared pairs of electrons -> each pair is 109.5 degrees from one another so that they are as far away from each other as possible (eg. methane)
2) pyrimidal shape - when an atom makes 3 single covalent bonds - lone pair takes up slightly more room then the bonded pairs so they are slightly less than 109.5 degrees away from one another (eg. NH3 - ammonia)
3) - V-shaped or bent shape - when an atom makes 2 single covalent bond - again lone pairs take up more room so less then 109.5 degrees apart (eg. water)
4) linear - either when an atom has a triple bond, or makes two double bonds or 1 single bonds - with teh double and triple bonds they are 180 degrees apart
5) triagonal planar - when an atom makes a double bond and two single bond - since there are no lone pairs each bonded pair are 120 degrees away from each other and the bond is in 2D instead of 3D
* WHEN THERE ARE ANY DOUBLE OR TRIPLE BONDS THE SHAPE OF THE MOLECULE IS 2D*

Question 11

what is surface tension and what causes it

Answer 11

it is the tension on the surface of a liquid which is caused by intermolecular bonds between molecules

Question 12

explain teh difference between inter and intra molecular bonds

Answer 12

intra is between the atoms in a molecule and inter is between molecules
intra is covalent bonds and there are three types of inter (dispersion, hydrogen-hydrogen and dipole-dipole)
intra is 100x stronger then inter

Question 13

why are covalent molecular substances soft

Answer 13

they are softer then both metallic and ionic compounds in general (except a diamond is teh strongest substance in the world and is a covalent substance) - they are soft because the intermolecular bonds are weak however tehy can still form crystals (eg. sugar)

Question 14

describe dipole-dipole forces

Answer 14

• only occur between polar molecules
• attraction is weak as the partial pos and neg charges are weak
• teh more polar a substance is the stronger the force is
• substances are more polar when there is a large difference in electronegativity or the compound is very asymmetrical
• strength of these forces is directly related to melting/boiling points (higher dipole-dipole forces = higher melting point -> polar moelcules = higher melting points then non-polar)

Question 15

describe hydrogen bonding

Answer 15

• it is the bonding between the lone pairs in nitrogen, oxygen or flourine and the partial pos charges on the hydrogen atoms
• special type of dipole-dipole that only occurs when hydrogens are bonded to nitrogen, oxygen or flourine
• 10x stronger then dipole-dipole but 1/10th of teh strength of covalent or ionic
• much higher melting/boiling points
• this occurs as nitrogen, flourine and oxygen are very electronegative and have small radii so they strongly attract teh partial pos charges of the hydrogen

Question 16

what is special about water and hydrogen bonding in water

Answer 16

water is weird as ice is less dense then water, wherever in pretty much every other compound the solid is denser then the liquid
- it is less dense as the hydrogen bonds between water molecules cause the water molecules to form a crystal lattice which spreads out the molecules

Question 17

describe dispersion forces

Answer 17

• present in all molecule types (non-polar and polar) and with noble gases
• other name is vaan der waals forces
• it is the weaked intermolecular force
• is present as electrons are attracted to nuclei of neighbouring atoms - this is called a temporary or instantaneous dipole
• they are formed as at some points in time the electron cloud is more at one side of the molecule then the other polarising teh molecule slightly - the negative side (more concentrated electron cloud) is attracted to neighbouring nuclei
• strength of these forces is dictated by the shape of molecule as this determines how close the molecules can get to one another - also dictated by the number of electrons in a molecule (the more electrons there are the stronger the dispersion force in general)
• stronger dispersion forces = higher melting/boiling points

Question 18

describe electronegativity and its effect on molecules

Answer 18

• electronegativity is the ability of an atom to attract electrons
• electronegativity determines teh level of polarity of a molecule (higher difference in electronegativity = higher polarity)
• electronegativity increases across a period and decreases down a group
• this trend can be simplified with the acronym PHONCALL CH (PH = flourine, O = ocygen, N = nitrogen, CALL = chlorine, C = carbon, H = hydrogen)

Question 19

how do you find out if a diatomic molecule is polar or not

Answer 19

• two atoms have identical electronegativities = non-polar
• two atoms have different electronegativities = polar (higher electronegativity means that there will be a high electron density around that atom then the other so that side has a partial neg charge)

Question 20

how do you find out if a polyatomic molecule is polar or not

Answer 20

polar = asymmetrical (triangular planar, pyrimidal and bent shaped molecules are polar) 
non-polar = symmetrical (linear and tetrahedral molecules)

Question 21

how do non-polar and polar substances react

Answer 21

polar and non-polar substances are immiscible (cant be mixed together)

Question 22

define a dipole

Answer 22

dipole or electric dipole = separation of pos and neg partial charges in a molecule
- if a molecule has a dipole charge then it is polar, if no net dipole charge then non-polar

Question 23

define electron density

Answer 23

it is teh measure of the probability of an electron being present in a particular spot
- if electron density is evenly distributed then nonpolar