Periodicity

Question 1

Describe the trend of the atomic radii of the Group 3 elements across the period.

Answer 1

Across the period, atomic radii decrease as the number of protons in the nucleus of the atom increases, thus nuclear charge increases. There is also the same number of inner shell electrons hence shielding effect is approximately constant. Hence, there is an increase in effective nuclear charge and an increase in electrostatic forces of attraction between nucleus and valence electrons.

Question 2

Why are the size of anions bigger than cations?

Answer 2

Anions have one more filled principal quantum shell compared to cations, hence valence electrons are further from the nucleus than those in cations and have weaker electrostatic forces of attraction between the nucleus and valence electrons in anions than in cations.

Question 3

Why can Phosphorus and Sulfur exhibit multiple oxidation numbers?

Answer 3

They have the presence of vacant and energetically accessible d orbitals which can be used for bonding through the expansion of the octet structure.

Question 4

Why does MgO have a higher melting point than Na2O?

Answer 4

Because even though anionic charge and anionic radius remain the same, Mg2+ has a higher ionic charge and smaller ionic radius than Na+, hence the magnitude of lattice energy is larger than Na2O. Thus more energy is required to overcome the stronger ionic bonds in MgO

Question 5

Why does MgO have a higher melting point than Al2O3 even though Al2O3 has a higher charge density?

Answer 5

Al3+ has a higher charge density and hence higher polarising power and hence Al2O3 has some covalent character, weakening the ionic bond in Al2O3.

Question 6

Why does NaCl have a higher melting point that MgCl?

Answer 6

Mg2+ has a higher ionic charge and smaller ionic radius than Na+, hence Mg2+ has a higher charge density and hence higher polarising power than na+. MgCl2 hence has some covalent character, weakening the ionic bonds in MgCl2.

Question 7

Why does AlCl3 have a simple molecular structure?

Answer 7

Al3+ has a high charge density and hence high polarising power, hence Al-Cl bond has a significant covalent character and hence AlCl3 has a simple molecular structure.

At temperatures above 180 degrees, the chloride exists as a molecular dimer, Al2Cl6 held together by id-id attraction.

Question 8

Why does PCl5 have a higher melting point than SiCl4?

Answer 8

PCl5 has more electrons than SiCl4 hence it has an increase in polarisability of electron cloud in PCl5 and more energy is required to overcome the stronger id-id interactions for PCl5.

Question 9

Why does Na2O and MgO dissolve in water to give an alkaline solution?

Answer 9

Due to the presence of the basic O2- anion, which reacts with H2O by accepting a H+ to give OH-.

Question 10

Why does MgO dissolve in water partially to give a weakly alkaline solution?

Answer 10

MgO dissolves partially to give Mg(Oh)2 due to high lattice energy of MgO.
MgO(s) + H2O(l) (reverse sign) Mg(OH)2(s)
Mg(OH)2(s) (reverse sign) OH- + Mg2+

Question 11

Why doesn’t Al2O3 and SiO2 dissolve in water?

Answer 11

Al2O3 does not dissolve in water because of its extremely high lattice energy. Large amounts of energy is needed to break the strong ionic bonds.

SiO2 has a giant molecular structure and a large amount of energy is needed to break the strong and extensive covalent bonds between Si and O atoms.

Question 12

Why do P4O10 and SO3 form an acidic solution when dissolve in water?

Answer 12

Due to the presence of P-O and S-O double bond, which reacts with H2O to give P-O-H and S-O-H bonds, which can dissociate in water to give H+.

Question 13

What happens when Al2O3 is reacted with NaOH?

Answer 13

Al2O3 dissolves in excess NaOH to form a colourless complex Al(OH)4-(aq)
Al2O3(aq) + 2NaOH(aq) + 3H2O(l) -> NaAl(OH)4

Question 14

When does SiO2 react with NaOH?

Answer 14

SiO2 reacts with hot concentrated NaOh to form Na2SiO3
SiO2(s) + 2NaOH(conc) -> Na2SiO3(aq) + H2O(l)

Question 15

Define hydration.

Answer 15

Hydration is the physical attraction between cations and anions with the negative and positive dipole of the water molecules respectively. (ie. the formation of ion-dipole interactions)

Question 16

Define hydrolysis

Answer 16

The chemical reaction of the cation or covalent chlorides and water molecules such that the O-H bond In the water molecule is cleaved, giving H3O+ ions.

Question 17

How do metal ions undergo hydration?

Answer 17

The hydrated metal ions are complexes where water molecules around and form dative bonds with the cation.
The metal ion uses one empty 3s, three 3p and two 3d orbitals to form six hybrid orbitals that can accept the six lone pais from 6 H2O molecules.

Question 18

What is the reaction between water and [Mg(H2O)g]2+?

Answer 18

[Mg(H2O)g]2+ undergoes slight hydrolysis as Mg2+ relatively higher charge density polarises the electron cloud of the surrounding water molecules, weakening and breaking the O-H bond, resulting in the release of a proton.
[Mg(H2O)6]2+ (reverse sign) [Mg(H2O)5(OH)]+(aq) + H+(aq)

Question 19

What is the reaction between water and [Al(H2O)g]3+?

Answer 19

[Al(H2O)g]3+ undergoes extensive hydrolysis in excess water. It undergoes more extensive hydrolysis than [Mg(H2O)6]2+ as Al3+ has a higher charge density than Mg2+.
[Al(H2O)6]3+ (reverse sign) [Al(H2O)5(OH)]2+(aq) + H+(aq)

It also reacts vigorously with water and forms white solid with white fumes of HCl in limited amounts of water
AlCl3(s) + 3H2O(l) -> Al(OH)3(s) + 6HCl(g)
or
AlCl3(s) + 3H2O(l)-> Al2O3(s) + 6HCl(g)

Question 20

Why can Si and P undergo complete hydrolysis?

Answer 20

Si and P atoms have vacant and energetically accessible 3d orbitals.
Both of them reacts vigorously with water and forms white fumes of HCl.
SiCl4(l) + H2O (l) -> SiO2(s) + 4HCl(aq)
PCl5(s) + H2O(l) -> H3PO4(aq) + 5HCl(aq)

Question 21

Why do diagonal relationships occur?

Answer 21

The elements have similar electronegativity and their cations have similar charge density.

Question 22

What is the trend of chemical reactivity down Group 2?

Answer 22

Down the group, atomic radius increases, weaker electrostatic forces of attraction between the nucleus and valence electrons, metal atoms lose their valence electrons to form M2+ cations more easily. Greater tendency to be oxidised and chemical reactivity increases. The tendency fo M2+(aq) to be reduced to M decreases, tendency for Group 2 metals to be oxidised to M2+ increases, reducing power of Group 2 metals to be oxidised to M2+ increases and their reactivity increases.

Question 22

Why does the thermal stability of Group 2 carbonates increase down the group?

Answer 22

Down the group, the size of cations increases and the charge density of the cations decreases, electron cloud of CO3^2- is polarised to a lesser extent. C-O bond is weakened to a lesser extent hence thermal stability of group 2 carbonates increases down group.

This also works for all oxyanions such as nitrates, oxides, sulfates and hydroxides.

Question 23

How does the type of oxyanions affect thermal stability?

Answer 23

The atomic size of the oxyanions affects thermal stability, the larger the ionnthe more polarisable the electron cloud is and the more unstable the compound is.

Question 24

For Group 17 elements, what is the trend of their oxidising power?

Answer 24

Down the group, the size of atoms increases, this decreases the tendency for X2 to accept electrons and decreases the tendency for X2 to be reduced to X-. Hence, the oxidising power of halogens decreases down the group.