10. PHYSICAL PERIODICITY

Question 1

How is the covalent radius of an atom found

Answer 1

• the distance between the two nuclei of the same type of atom in a covalent bond can be determined
• this can be divided by two to arrive at the single covalent radius

Question 2

Why are mainly covalent radii used

Answer 2

• can be obtained for most elements

- so provide the best data for comparison purposes

Question 3

Why do the noble gases in Group 18 not have a covalent radius

Answer 3

because they do not form bonds with each other

Question 4

How are the van der Waals’ radii found

Answer 4

• the distance between the nuclei of two neighbouring touching atoms that are not chemically bonded together can be found
• this can be divided by two to arrive at the van der Waals’ radius

Question 5

Why is the value of the van der Waals’ radius higher than the single covalent radius of any element

Answer 5

because there is no overlap of electron clouds in van der Waals’ radii

Question 6

What property of SO2 enables it to act as a food preservative

Answer 6

• reducing agent

- kills bacteria

Question 7

What is the periodic pattern of atomic radii

Answer 7

atomic radii decrease across the period

Question 8

Why do atomic radii decrease across a period

Answer 8

• across the period, the nuclear charge increase as the number of protons increases
• the extra electron is added to the same principal quantum shell
• the shielding effect remains roughly constant
• the greater attractive force exerted by the increasing positive nuclear charge on the outer electrons pulls them in closer to the nucleus

Question 9

Explain the difference in size between the cations and their corresponding atoms

Answer 9

• cation contains fewer electrons than corresponding atom
• decreased shielding effect on outer electrons
• increased attraction by nucleus

Question 10

Variation in ionic radii from Na+ to Si4+

Answer 10

• ionic radii decrease
• the greater attractive force exerted by the increasing nuclear charge on the valence electrons pulls them in closer to the nucleus

Question 11

Variation in ionic radii from P3- to Cl-

Answer 11

• ionic radii decrease
• the greater attractive forces exerted by the increasing nuclear charge on the valence electrons pulls them in towards the nucleus

Question 12

Explain the difference in size between the anions and their corresponding atoms

Answer 12

• anion contains more electrons than corresponding atom
• increased repulsion between the electrons
• nuclear charge remains constant
• decreased nuclear attraction

Question 13

Why does melting point increase in the metals across a period

Answer 13

• the bonding is metallic
• the strength of the bonding increases/ the electrostatic forces of attraction between the ions increases
• as the number of delocalised electrons increases

Question 14

Why does electrical conductivity increase in the metals across a period

Answer 14

• the number of delocalised electrons increases

- more electrons are available to drift through the structure when a potential difference is applied

Question 15

Why does silicon have the highest melting point in the period

Answer 15

• silicon has a giant covalent structure
• the Si atoms are bonded to each other with strong covalent bonds
• the covalent bonds require a large amount of energy to break

Question 16

Why does silicon not conduct electricity

Answer 16

• it has a giant covalent structure
• Si atoms are bonded to each other with covalent bonds
• no free delocalised electrons to to move around and conduct charges

Question 17

Sulfur exists as

Answer 17

S8 molecules (solid)

Question 18

Phosphorus exists as

Answer 18

P4 molecules (solid)

Question 19

Explain the variation in melting points from P to Ar

Answer 19

• van der Waals’ forces decrease as size of molecule decreases
• sulfur has greatest melting point
• highest number of electrons in S8 molecule

Question 20

What is the general trend in first ionisation energies from Na to Ar

Answer 20

• the first ionisation energies increase
• the nuclear charge increases
• the distance between the nucleus and outer electrons remains reasonably constant
• shielding effect remains roughly constant
• increased attractive force by nucleus

Question 21

Why is there a rapid decrease in ionisation energies from the end of one period to the next

Answer 21

• the distance between the nucleus and other electrons increases
• the shielding by inner shells increases
• these factors outweigh the increased nuclear charge

Question 22

Why is there a decrease from s to p subshells

Answer 22

• electronic structure
• the 3p subshell is further away from the nucleus than the 3s subshell
• the shielding by inner shells increases
• these outweigh the increased nuclear charge