Chapter 5: Electrons and Bonding

Question 1

1. What are electron shells? Explain.
2. What is the principal quantum number n?
3. What is the maximum number of electrons in the first four shells?
4. How is this calculated?

Answer 1

1. Electron shells are regarded as energy levels. The energy increases as the shell number increases.
2. The shell number (or energy level number) is called the prinicipal quantum number n.
3. n=1 2 electrons
   n=2 8 electrons
   n=3 18 electrons
   n=4 32 electrons
4. The maximum number of electrons in a shell is given by 2n².

Question 2

1. What is an atomic orbital?
2. Describe an s-orbital.
3. Describe a p-orbital.
4. Describe d- and f-orbitals.
5. How are these orbitals grouped together?

Answer 2

1. An atomic orbital is a region around the nucleus that can hold up to two electrons, with opposite spins. Models visualise an atomic orbital as a region in space where there is a high probability of finding an electron.
2. In an s-orbital, the electron cloud is the shape of a sphere. Each shell (from n=1) containes 1 s-orbital.
3. In a p-orbital, the electron cloud is in the shape of a dumbell. Each shell from n=2 contains 3 p-orbitals. These are drawn at right angles to one another. They are referred to as p_x, p_y and p_z.
4. Each shell from n=3 contains 5 d-orbitals. Each shell from n=4 contains 7 f-orbitals.
5. Within a shell, orbitals of the same shell are grouped together as sub-shells.

Question 3

1. What is the pattern in the filling of orbitals?
2. What is the order of filling?
3. Explain why the 4s sub-shell is filled before but also emptied before the 3d sub-shell.

Answer 3

1. Orbitals fill in order of increasing energy. Within each shell, the new type of sub-shell added has a higher energy.
2. The order of filling for each shell is 1s2s2p3s3p4s3d4p4d4f.
3. For atoms of d-block elements, the 4s sub-shell is at a lower energy than the 3d sub-shell, so is filled first. The energies of the 4s and 3d sub-shells are very close together and, once filled, the 3d energy level falls below the 4s energy level, and hence empties before the 3d sub-shell.

Question 4

How are electrons filled within orbitals? Consider the repelling nature of like charges and the properties of spin.

Answer 4

• Electrons are negatively charged and repel one another. To help counteract this repulsion, the two electrons in an orbital must have opposite spins (represented by an up arrow and a down arrow).
• Within a sub-shell, the orbitals have the same energy. One electron occupies each orbital before pairing starts. This prevents any repulsion between paired electrons untul there is no further orbital available at the same energy level.

Question 5

What is the electron configuration of Krypton? [Z=36]

Answer 5

1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶

Question 6

1. What is shorthand electron configuration?

Answer 6

1. Shorthand electron coniguration refers to expressing electron configurations more simply in terms of the previous noble gas in the periodic table plus the outer electron sub-shells.

Question 7

The periodic table can be divided into blocks corresponding to their highest energy sub-shell.

1. What is the s-block?
2. What is the p-block?
3. What is the d-block?

Answer 7

1. The s-block refers to elements with the highest energy electrons in the s-sub-shell (left block of two groups).
2. The p-block refers to elements with the highest energy electrons in the s-sub-shell (right block of six groups).
3. The d-block refers to elements with the highest energy electrons in the d-sub-shell (centre block of ten groups).

Question 8

When forming ions, specifically which electrons are lost or gained.

Answer 8

When forming ions, the highest energy sub-shells lose or gain electrons.

Question 9

1. What is ionic bonding?
2. Give examples of common cations.
3. Give examples of common anions.

Answer 9

1. Ionic bonding is the electrostatic attraction between positive and negative ions. It holds together cations (positive ions) and anions (negative ions) in ionic compounds.
2. Common cations include metal ions (e.g. Na⁺, Ca²⁺, Al³⁺) and ammonium ions (NH₄⁺).
3. Common anions include non-metal ions (e.g. Cl^–, O2^–) and polyatomic ions (e.g. NO^3–, SO₄^2–)

Question 10

Show the dot-and-cross diagram for the formation of magnesium chloride from its constituent elements.

Answer 10

Question 11

Describe the general structure of ionic compounds.

Answer 11

Ionic compounds are arranged in a giant ionic lattice, where each ion is surrounded by oppositely charged ions.

Question 12

Explain these statements.

1. Almost all ionic compounds are solids at room temperature.
2. Many ionic compounds dissolve in solvents such as water.
3. In the solid state, an ionic compound does not conduct electricity. But once melted or dissolved in water, the ionic compound does conduct electricity.

Answer 12

1. At room temperature, there is insufficient energy to overcome the strong electrostatic forces of attraction between oppositely charged ions in the giant ionic lattice. High temperatures are needed to provide the large quantity of energy needed to overcome these strong electrostatic forces of attraction. Therefore most ionic compounds have high melting and boiling points.
2. This is because water is a polar solvent. Polar water molecules break down the lattice and surround each ion in solution.
3. In the solid state, the ions are in a fixed position in the giant ionic lattice so there are no mobile charges. Once melted or dissolved in water, the solid ionic lattice breaks down so the ions are now free to move as mobile charge carriers.

Question 13

Explain the general trend in melting and boiling point for ionic compounds.

Answer 13

The melting and boiling points are higher for lattices containing ions with greater ionic charges, as there is a stronger attraction between ions. Ionic attraction also depends on the size of the ions (i.e. smaller ions have a higher melting point as smaller ions are closer together and, hence, have more stable bonds).

Question 14

1. What are the two main steps in a compound dissolving in water (a polar solvent)?
2. Under what circumstances is an ionic compound not soluble?
3. Hence, what factors does the solubility of an ionic compound in water depend on?

Answer 14

1. The ionic lattice is broken down and the water molecules attract and surround the ions.
2. An ionic compound may not be souluble if the compound is made of ions with large charges, because this means the ionic attraction may be too strong for water to be able to break down the lattice structure.
3. The solubility of an ionic compound in water depends on the relative strengths of the attractions within the giant ionic lattice and attractions between ions and water molecules. For some compounds, the solubility decreases as the ionic charge increases.

Question 15

Give a summary of properties for ionic compounds.

Answer 15

Most ionic compounds:

• have high melting and boiling points
• are soluble in water
• conduct electricity only in the liquid state or in aqueous solution.

Question 16

1. What is covelent bonding?
2. What types of atoms does covalent bonding occur between?

Answer 16

1. Covalent bonding is the strong electrostatic attraction between a shared pair of electrons and nuclei of the bonded atoms.
2. Covalent bonding occurs between atoms in non-metallic elements (e.g. H₂, O₂), compounds of non-metallic elements (e.g. H₂O, CO₂) and polyatomic ions (e.g. NH₄⁺).

Question 17

Describe a covalent bond in terms of atomic orbitals, electrostatic attraction, and the reason for forming a covalent bond.

Answer 17

A covalent bond is the overlap of atomic orbitals, each containing one electron, to give a shared pair of electrons. The shared pair of electrons is attracted to the nuclei of both the bonding atoms. The bonded atoms often have outer shells with the same electron structure as the nearest noble gas.

Question 18

1. An ion attracts oppositely charged ions in all directions, forming a giant ionic lattice. How are covalent bonds different?
2. What is a molecule?

Answer 18

1. In a covalent bond, the attraction is localised, acting solely between a shared pair of electrons and the nuclei of the two bonded atoms.
2. A molecule is the smallest part of a covalent compound that can exist whilst retaining the chemical properties of the compound.

Question 19

1. In terms of electrons, what is the main difference between ionic and covalent bonding?

Answer 19

1. In ionic bonding, electrons are transferred. In covalent bonding, electrons are shared.

Question 20

Give the dot-and-cross diagram and displayed formula for:

1. hydrogen, H₂
2. water, H₂O
3. ammonia, NH₃
4. methane, CH₄

Answer 20

Question 21

1. What is displayed formula?
2. What are lone pairs?

Answer 21

1. A displayed formula shows the relative positioning of atoms and the bonds between them as lines. Lone pairs are drawn as two dots around the atom.
2. Paired electrons that are not shared are called lone pairs.

Question 22

What are the number of covalent bonds usually formed by carbon, hydrogen, oxygen and nitrogen atoms?

Answer 22

• Carbon forms 4 bonds.
• Hydrogen forms 1 bond.
• Oxygen forms 2 bonds.
• Nitrogen forms 3 bonds.

Question 23

Boron, in Period 2, has the electron configuration 1s²2s²2p¹.

Explain how the substance boron trifluoride shows that predictions for bonding cannot be based solely on the noble gas structure.

Answer 23

In boron trifluoride, boron’s three outer-shell electrons are paired with electrons from three fluorine atoms so that boron has six electrons in its outer shell.

This is not a noble gas structure, and hence shows that covalent bonding does not always form covalent bonds to match the electronic arrangement of a noble gas.

Question 24

Give all possible formulae for the fluorides of the non-metal phosphorus, sulfur and chlorine, which are all in Period 3 of the periodic table.

Answer 24

Question 25

Show how different numbers of unpaired electrons lead to different possibilities for covalent compounds of sulfur.

Answer 25

Question 26

What is an expanded octet? Give an example and explain.

Answer 26

Sulfur, phosphorus, silicon and chlorine are common examples of elements that form an expanded octet. This is where molecules deviate from the octet rule by having more than 8 electrons in the outer shell of the central atom. This is only possible from the n=3 shell, when a d-sub-shell becomes available for the expansion.

Question 27

1. What is a double covalent bond?
2. Give the dot-and-cross diagram and displayed formula for O₂ and CO₂.

Answer 27

1. In a double covalent bond, the electrostatic attraction is between two shared pairs of electrons and the nuclei of the bonding atoms.
2. See attached image.

Question 28

1. What are triple covalent bonds?
2. Show the dot-and-cross diagrams and displayed formulae for nitrogen and hydrogen cyanide.

Answer 28

1. In a triple bond, the electrostatic attraction is between three shared pairs of electrons and the nuclei of the bonding atoms.
2. See attached image.

Question 29

1. What is a dative covalent bond?
2. Specifically, where do the electrons come from?

Answer 29

1. A dative covalent or coordinate bond is a covalent bond in which the shared pair of electrons has been supplied by one of the bonding atoms only.
2. In a dative covalent bond, the shared pair of electrons was intially a lone pair of electrons on one of the bonded atoms.

Question 30

1. Show the dot-and-cross diagram for the formation of NH₄⁺ from an ammonia molecule and an H⁺ ion.
2. Show the displayed formula for NH₄⁺.

Answer 30

Question 31

How is the strength of covalent bonds measured?

Answer 31

Average bond enthalpy (kJ mol^-1) serves as a measurement of covalent bond strength. The larger the value fo the average bond enthalpy, the stronger the covalent bond.