Chemistry OCR Module 2: Section 2 - Electrons, Bonding & Structure

What are the different shells and sub-shells and how many electrons can go in them.

Shell:

1. 2 electrons

2. 8 electrons

3. 18 electrons

4. 32 electrons

Sub-shells:

s - 1 orbital - 2 electrons

p - 3 orbitals - 6 electrons

d - 5 orbitals - 10 electrons

f - 7 orbitals - 14 electrons

What is an orbital?

1. An orbital is a bit of space than an electron moves in.Orbitals within the same sub-shell have the same energy.

2. If there are two electrons in an orbital, they have to ‘spin’ in opposite directions - this is called spin-pairing.

What is the shape of an s and p orbital?

1. S orbitals are spherical

2. P orbitals have dumbbell shapes. There are three p orbitals and they are at right angles to one another.

What is electron configuration?

The number of electrons than an atom or ion has, and how they are arranged.

How are electrons filled into an atom?

1. Electrons fill up the lowest energy sub-shells first.

2. The 4s sub-shell has a lower energy level than the 3d sub-shell, even though its principal quantum number is bigger. This means the 4s sub-shell fills ip first.

3. Electrons fill orbitals with the same energy singly before they start sharing.

4. For the configuration of ions from the s and p blocks of the periodic table, just remove or add the electrons to of from the highest-energy occupied subshell.

What is Ionic bonding?

An ionic bond is an electrostatic attraction between two oppositely charged ions.

What is a giant ionic lattice?

A regular structure which is a lattice, but is giant because it is made up of the same basic unit repeated over and over again.

It forms because each ion is electrostatically attracted in all directions to ions of the opposite charge.

e.g. Sodium Chloride

What are the physical properties of ionic compounds?

1. Ionic compounds conduct electricity when they are molten or dissolved, but not when they are solid.

   1. The ions in a liquid are mobile and they carry a charge.

   2. In a solid they are fixed in position by the strong ionic bonds.

2. Ionic compounds have high melting and boiling points.

   1. The giant ionic lattices are held together by strong electrostatic forces. It takes loads of energy to overcome these forces, so their melting and boiling points are very high.

3. Ionic compounds tend to dissolve in water.

   1. Water molecules are polar, part of the molecule has a small negative charge and the other bits have small positive charges. The water molecules are attracted to the charged ions. They pull the ions away from the lattice and cause it to dissolve.

What are covalent bonds?

A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

What is a dative covalent bond?

Take the example of the ammonium ion. It forms when the nitrogen atom in an ammonia molecules donates a pair of electrons to a proton.

How is bond enthalpy used as a measure of the strength of covalent bond?

The average bond enthalpy suggests that the larger the average bond enthalpy the stronger the covalent bond.

Why do electron pairs repel each other?

1. Electrons are all negatively charged, so electron pairs will repel each other as much as they can.

2. The type of the electron pair affects how much it repels other electron pairs. Lone pairs repel more than bonding pairs.

3. This means the greatest angles are between lone pairs of electrons, and bond angles between bonding pairs are often reduced because they are pushed together by lone pair repulsion.

   1. Lone pair/ lone pair angles are the biggest.

   2. Lone pair/bonding pair angels are the second biggest

   3. Bonding pair/bonding pair bond angles are the smallest.

4. So the shape of the molecule depends on the type of electron pairs surrounding the central atom as well as the number.

What are the different bonding shapes? (7)

1. Linear - 2 electron pairs around central atom

2. Trigonal planar - 3 electron pairs around central atom

3. Tetrahedral - 4 electron pairs around central atom, all bonding pairs

4. Trigonal Pyramidal - 4 electron pairs around central atom, 3 bonding pairs and one lone pair

5. Non-linear or bend - 4 electron pairs around central atom, two bonding pairs and 2 lone pairs.

6. Trigonal bipyramidal - 5 electron pairs around central atom.

7. Octahedral - 6 electron pairs around central atom

What is electronegativity?

An atom’s ability to attract the electron pair in a covalent bond is called electronegativity.

What is the Pauling Scale?

Electronegativity is measured on the Pauling scale. The greater an elements’s Pauling value, the higher its electronegativity.

Why might covalent bonds be polarised by differences in electronegativity?

1. In a covalent bond between two atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom. This makes the bond polar.

2. In a polar bond, the difference in electronegativity between the two atoms causes a permanent dipole. A dipole is a difference in charge between two atoms caused by a shift in electron density in the bond. The greater the difference in electronegativity, the more polar the bond.

3. The covalent bonds in diatomic gases are non-polar because the atoms have equal elecronegativities.

4. Some elements like carbon and hydrogen have pretty similar electronegativites so bonds between them are essentially non-polar.

What are polar molecules and why might molecules be polar?

Polar bonds have permanent dipoles. The arrangement of polar bonds in a molecule determines whether or not the molecule will have an overall dipole.

1. If the polar bonds are arranged symmetrically, so that the dipoles cancel each other out, then the molecule has no overall dipole and is non-polar.

2. If the polar bonds are arranged so that they do not cancel each other out then charge is arranged unevenly across the whole molecule, and it will have an overall dipole. Molecules with an overall dipole are polar.

Why is there a gradual transition from ionic to covalent bonding?

1. Only bonds between atoms of a single element can be purely covalent as the electrons are arranged completely evenly between the bond.

2. Realistically, most compounds come somewhere in between the two extremes, meaning they have often got ionic and covalent properties.

What are the different types of intermolecular forces?

1. Induced dipole-dipole or London (dispersion) forces.

2. Permanent dipole-dipole interactions.

3. Hydrogen bonding

What are induced dipole-dipole forces?

Induced dipole-dipole forces cause all atoms and molecules to be attracted to each other even noble gas atoms.

1. Electrons in charge clouds are always moving really quickly. At any particular moment, the electrons in an atom are likely to be more to one side than the other. At this moment, the atom would have a temporary dipole.

2. This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.

3. The second dipole can cause yet another dipole in a third atom.

What are the properties of induced dipole-dipole forces?

1. Larger molecules have larger electron clouds, meaning stronger induced dipole-dipole forces.

2. Molecules with greater surface areas have stronger induced dipole-dipole forces because they have a bigger exposed electron cloud.

3. Liquids with stronger induces dipole-dipole forces will have higher boiling points.

4. Induces dipole-dipole forces are responsible for holding iodine molecules together in a lattice.

   1. Iodine atoms are held together in pairs by strong covalent bonds.

   2. But the molecules are then held together in a molecular lattice arrangement by weak induced dipole-dipole attractions.

What are permanent dipole-dipole interactions?

The δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules. These are called permanent dipole-dipole interactions.

They also happen in addition to induced dipole-dipole interactions

When can hydrogen bonding occur?

Hydrogen can only happen when hydrogen is covalently bonded to fluorine, nitrogen or oxygen. Hydrogen has a high charge density because it is so small, and fluorine, nitrogen and oxygen are very electronegative. The bond is so polarised that a weak bond forms between hydrogen of one molecule and a lone pair of electrons on the fluorine, nitrogen or oxygen in another molecule.

Molecules which have hydrogen bonding usually contain -OH or -NH groups or HF

How does hydrogen bonding effect the properties of a substance?

1. They are soluble in water and have higher boiling and freezing points than molecules of a similar size that are unable to form hydrogen bonds.

2. In ice, molecules of H₂O are held together in a lattice by hydrogen bonds. When ice melts, hydrogen bonds are broken, so ice has more hydrogen bonds than liquid water. Since hydrogen bonds are relatively long, this makes ice less dense than liquid in water.

Why do intermolecular forces explain the trends in boiling points?

1. The main factor that determines the boiling point of a substance will be the strength of the induced dipole-dipole forces unless the molecule can form hydrogen bonds.

2. This explains why the boiling points of the group 7 hydrides increase from HCl to HI, although the permanent dipole-dipole interaction are decreasing, the number of electrons in the molecule increases, so the strength of the induced dipole-dipole interactions also increases.

3. But if you have two molecules with a similar number of electrons, then the strength of their induced dipole-dipole interactions will be similar. So if one of the substances has molecules that are more polar than the other, it will have stronger permanent dipole-dipole interactions and so a higher boiling point.

Why do simple covalent compounds have low melting and boiling points?

The intermolecular forces that hold together the molecules in simple covalent compounds are weak so do not need much energy to break. So the melting and boiling points are normally low, they are often liquids or need much energy to break. So the melting and boiling points are normally low, they are often liquids or gases at room temperature. As intermolecular forces get stronger, melting and boiling points increase.

Why are polar molecules soluble in water?

Water is a polar molecule, so only tends to dissolve other polar substances. Compounds with hydrogen bonds such as ammonia or ethanoic acid can form hydrogen bonds with water molecules, so will be soluble. Molecules that only have induces dipole-dipole forces, such as methane, will be insoluble.

Why do do simple covalent compounds not conduct electricity?

Even though some covalent molecules have permanent dipoles, overall, covalent molecules are uncharged. This means the cannot conduct electricity.

Why do covalent bonds not break during melting or boilign?

To melt or boil a simple covalent compounds you only have to overcome the intermolecular forces that hold the molecules together.

You do not need to break the much stronger covalent bond that hold the atoms together in the molecules. That is why simple covalent compounds have relatively low melting and boiling points.