Inter and Intra molecular forces (VSEPR MODEL)

Question 1

What is a VSEPR model? (Pronounced Vesper)

Answer 1

A VESPER model is a better form of an electron dot diagram, which predicts the structures of nearly any molecule or polyatomic ion.

Question 2

What are the main types of VSEPR models?

Answer 2

Linear
Bent
Trigonal Pyramidal
Trigonal Planar
T-Shaped

Question 3

What is a VSEPR designation?

Answer 3

In the VSEPR model, the molecule or polyatomic ion is given an AXmEn designation, where:
A is the central atom,
X is the bonded atom,
E is a nonbonding valence electron group, (lone pairs)
and the ‘m’ or ‘n’ are integers

Question 4

List the AXmEn notation for each of the main VSEPR models

Answer 4

AX2 –> Linear (180 degrees)
AX2E –> Bent (v-shaped) <180 degrees
AX3 –> Trigonal Planar (120 degrees)
AX3E –> Trigonal Pyramidal (<120 degrees)

Next 4:
Square Planar
Tetrahedral
Trigonal Bipyramidal
Octahedral

Question 5

How can you find the AXmEn notation based on the electron dot diagram/electron Lewis Structure?

Answer 5

Draw the electron dot diagram of a compound first. Then, see how much valence electrons the CENTRAL ATOM CONTRIBUTES as well as the other atoms. The bonds are then classified as ‘groups’ based on how many ‘sides’ the bonds are in. For example, 2 double bonds or 2 single bonds are both classified as ‘two groups’

Hence, the two groups symbolise AX2 in the AXmEn Notation, which is why it will be classified as LINEAR

For example, in BCl3:

The central atom, Boron contributes 3 valence electrons and each chlorine atom contributes seven valence electrons. By drawing a lewis dot diagram, we can see that there are three electron groups around the central atom. Thus, the AXmEn Notation would be AX3.

AX3 is classified as a trigonal planar at every group being 120 degrees apart

Question 6

What does the E mean in AXmEn notation?

Answer 6

‘E’ refers to the NONBONDING VALENCE ELECTRON GROUP OR ALSO KNOWN AS LONE PAIR OF ELECTRONS (NO BONDS, JUST SEPARATE)

E.g. In SO2, there are three electron groups around the central atom - two double bonds and one lone pair. Here, the structure would be AX2E since a LONE PAIR is involved. (Note that a lone pair is not shared by two nuclei)

Question 7

What are inter and intra-molecular forces?

Answer 7

Intramolecular forces are the forces that hold atoms together within a molecule

Intermolecular forces are forces that exist between molecules.

Question 8

What is a non-polar covalent bond?

What about polar covalent bonds?

Answer 8

A non-polar covalent bond is formed between the same atoms or atoms with very similar electronegativities. The difference in electronegativity between bonded atoms is less than 0.5 (Symmetrical molecular are known as non-polar)

A polar-covalent bond is formed when atoms of slightly different electronegativities share electrons. The difference can go up to 1 and below 2. (Asymmetrical molecules are polar. Not symmetrical and irregular)

In short, a non-polar bond is a covalent bond between atoms that is equally charged while a polar bond is a covalent bond that is oppositely charged. Examples of non-polar bonds include Carbon dioxide which has a linear shape while polar bonds include water (H2O) which has a bent assortment.

Question 9

What is the rule of thumb between polar and non-polar covalent bonds?

Answer 9

An electronegativity difference of 1.7 is often used as the cut-off point between polar covalent bonding and ionic bonding.

Question 10

What is the classification and scale of bonds based on the electronegativity difference?

Answer 10

A non-polar bond ranges in the 0
Polar bonds range from 0 to 1.7 (<1.7)
Ionic bonds range from 1.7 to 4.0 (>1.7)

Question 11

In terms of the difference of electronegativity, what value helps decide whether a compound is ionic or covalent? (Without looking at whether the compounds are metals or non-metals)

Answer 11

A compound is ionic if the electronegativity difference is larger than 1.7
(The electronegativity of different elements in the periodic table will be given to you)

Likewise, a compound is COVALENT, if the electronegativity is SMALLER THAN 1.7

Question 12

When does a polar bond occur? How do you determine whether a molecule is polar?

Answer 12

A polar bond occurs when a pair of electrons is shared unequally by the two atoms they came from

E.g. In Hydrogen Chloride, the chlorine atom is more electronegative so it attracts the shared electrons more strongly than the hydrogen atom. Therefore, hydrogen chloride has a polar bond.

A molecule is polar if:

• There is only one polar bond present
• There is more than one polar bond in the molecule and they are not of equal value
• There is more than one polar bond in the molecule and they are not arranged symmetrically so they cannot balance each other out

Question 13

What are the general valencies of iron, lead and copper? (Valency means it does not have to reference a charge)

Answer 13

Iron generally has a valency of 2 or 3
Lead is assumed to have a valency of 2
Copper is either 1 or 2

Question 14

How do you draw electron dot diagrams (or lewis structures) for ionic compounds?

Answer 14

Write the ionic formula for the compound and then add up all the electrons.
Put both elements in brackets when doing the electron dot diagram and also make the non-metal element become stable, by filling all eight valence electrons. Also, add a + or - sign at the corner of the brackets to show what type of ion it was. (A positive cation or a negative anion)

For example, in sodium chloride, there are supposed to be eight total electrons to be shared. Since Sodium is a metal and loses an electron, that means that the chloride will occupy and keep all the 8 electrons, which is why Na would just be in brackets with no crosses around it, only the charge at the top right corner outside the bracket. Likewise, Cl would be in brackets but will have 8 crosses around it indicating a full set of stable electron shells. It will have a minus charge in the top right corner outside the bracket since it is a non-metal.

Question 15

How do you draw polyatomic ions in Lewis dot diagrams?

Answer 15

For polyatomic ions, add the total amount of electrons to be shared by adding the valence electrons of each element IN ADDITION to the charge given in the top right corner of the polyatomic formula

For example, Carbonate also known as CO3 (2-) will have a total electron sharing of 4 + (6x3) + 2 = 24 VALENCE ELECTRONS TO BE SHARED

SINCE IT INCLUDES THE CHARGE OF THE POLYATOMIC ION

Question 16

What bond do molecular substances have? (Ionic, covalent or both)

Answer 16

Molecular substances are held together by COVALENT BONDS
(Think of diatomic molecules)

Question 17

What are a few examples of covalent lattices? (Also known as covalent networks)

Answer 17

Diamond and Graphite

Question 18

What makes an ionic compound a hard solid? What makes them brittle?

Answer 18

An ionic compound is a hard solid, due to it being a crystalline lattice structure, which is supported by strong electrostatic attraction.

An ionic compound is brittle (Tendency to easily break or fracture when force is applied) due to the very strong electrostatic attraction between positively charged and negatively charged ions. When too much force is exerted, however, this lattice formed by the oppositely charged ions, will shatter, therefore making it brittle.

Question 19

Which AXmEn Notation includes lone pairs?

Answer 19

Bent
Trigonal pyramidal
Square Planar

Question 20

What is a terminal atom?

Answer 20

The terminal atoms are the atoms that are outside and connected to the central atom.

For example, CCL4 would have 4 chlorine bonded around the centre atom of Carbon. These Chloride ions will be classified as terminal atoms.

Question 21

What makes a molecule asymmetrical? What properties?

Answer 21

A molecule is asymmetrical if the terminal atoms are NOT IDENTICAL. (There is at least one different terminal atom compared to the other terminal atoms)

A molecule is also asymmetrical if there are LONE PAIRS AROUND THE CENTRAL ATOM

Question 22

How can you tell if a molecule is polar or non-polar? (By looking at the lewis dot diagram or structural formula)

Answer 22

Non-polar compounds will be SYMMETRIC

Polar compounds are ASYMMETRIC (They contain lone pairs around the central atom or have different terminal atoms)

Question 23

What property allows metals to have a reflecting look? What about their electrical conductivity?

Answer 23

Metallic bonds have freely moving, delocalised electrons, also known as the ‘sea of electrons.’ These electrons tend to oscillate and give off photons of light, allowing them to have their reflective property.

Similarly, their delocalised state of electrons also allows metals to conduct heat and electricity as there is easy access to electrical flow

Question 24

The greater the INTERMOLECULAR FORCE, THE GREATER THE MELTING OR BOILING POINT. TRUE OR FALSE?

Answer 24

True

Question 25

What type of bond is the STRONGEST INTERMOLECULAR FORCE?

Answer 25

Hydrogen Bonds

Question 26

When do you count the lone pairs when predicting the shape of a molecule?

Answer 26

You only need to find the LONE PAIRS AROUND THE CENTRAL ATOM, along with the number of bond groups to determine the shape of a molecule. Any lone pairs outside the central atom do not need to be counted for the determination of its shape.

Question 27

What do Polar and Non-Polar mean?

Answer 27

Polar means oppositely charged while Non-Polar refers to being equally charged

Question 28

What must be present in order for a hydrogen bond to occur?

Answer 28

In order for a hydrogen bond to be present, there MUST ALWAYS be a hydrogen atom and ONE OR MORE OF THE THREE: OXYGEN, NITROGEN AND FLUORINE

Question 29

What are the three main types of intermolecular forces?

Answer 29

London Dispersion Force
Dipole-Dipole Force
Hydrogen Bonds

Question 30

What is a London dispersion force? How can you identify a London Dispersion Force?

Answer 30

London dispersion forces are the weakest type of intermolecular force. They are formed because of the uneven distribution of electrons around an atom which then causes temporary dipoles, which are molecules that have a charge.

Also, the more electrons a molecule has, the STRONGER THE DISPERSION FORCES ARE

A London Dispersion Force can occur in BOTH non-polar and polar compounds. However, NON POLAR COMPOUNDS can ONLY HAVE LONDON DISPERSION FORCES

Question 31

Are intermolecular forces weaker or stronger than intramolecular forces?
Why are intermolecular forces important? What does it determine?

Answer 31

Intermolecular forces are MUCH WEAKER than intramolecular ones.

Intermolecular forces are important because they help determine the physical properties of molecules like their boiling point, melting point, density and enthalpies of fusion.

Question 32

How can you give an example of London dispersion forces affecting the physical properties of elements and compounds?

Answer 32

For example, Bromine has more electrons than chlorine and thus a STRONGER LONDON DISPERSION FORCE than chlorine. This would result in a higher boiling point since more energy is required to break these forces and bonds.

Question 33

What are dipole-dipole interactions

Answer 33

A dipole-dipole force occurs when a partially positive part of a molecule interacts with the partially negative charge of the neighbouring molecule.

Thus, it occurs between partially oppositely charged ions

Question 34

What are hydrogen bonds?

Answer 34

Hydrogen Bonds are a special kind of dipole-dipole interaction that occurs ONLY WHEN A HYDROGEN ATOM IS PRESENT - AND THAT IT IS BONDED TO EITHER:

OXYGEN
NITROGEN
FLUORINE

The hydrogen atom would be partially positive and the element it attaches to will be negative.Hydrogen bonds are one of the strongest types of intermolecular forces and lots of energy is required to break them.

Question 35

What is an easy way of distinguishing what bonds a polar compound has?

Answer 35

A polar compound would have a LONDON DISPERSION FORCE and A DIPOLE-DIPOLE INTERACTION