Periodicity Flashcards
Describe the trend of the atomic radii of the Group 3 elements across the period.
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.
Why are the size of anions bigger than cations?
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.
Why can Phosphorus and Sulfur exhibit multiple oxidation numbers?
They have the presence of vacant and energetically accessible d orbitals which can be used for bonding through the expansion of the octet structure.
Why does MgO have a higher melting point than Na2O?
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
Why does MgO have a higher melting point than Al2O3 even though Al2O3 has a higher charge density?
Al3+ has a higher charge density and hence higher polarising power and hence Al2O3 has some covalent character, weakening the ionic bond in Al2O3.
Why does NaCl have a higher meltinh point that MgCl?
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.
Why does AlCl3 have a simple molecular structure?
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.
Why does PCl5 have a higher melting point than SiCl4?
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.
Why does Na2O and MgO dissolve in water to give an alkaline solution?
Due to the presence of the basic O2- anion, which reacts with H2O by accepting a H+ to give OH-.
Why does MgO dissolve in water partially to give a weakly alkaline solution?
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+
Why doesn’t Al2O3 and SiO2 dissolve in water?
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.
Why do P4O10 and SO3 form an acidic solution when dissolve in water?
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+.
What happens when Al2O3 is reacted with NaOH?
Al2O3 dissolves in excess NaOH to form a colourless complex Al(OH)4-(aq)
Al2O3(aq) + 2NaOH(aq) + 3H2O(l) -> NaAl(OH)4
When does SiO2 react with NaOH?
SiO2 reacts with hot concentrated NaOh to form Na2SiO3
SiO2(s) + 2NaOH(conc) -> Na2SiO3(aq) + H2O(l)
Define hydration.
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)
Define hydrolysis
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.
How do metal ions undergo hydration?
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.
What is the reaction between water and [Mg(H2O)g]2+?
[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)
What is the reaction between water and [Al(H2O)g]3+?
[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)
