14 — Periodic table

Question 1

Groups

Answer 1

Vertical column of elements
18 groups
Elements in same group hv same number of valence electrons and thus hv similar chemical properties

Question 2

Metalloids

Answer 2

Contain properties of both metals and non-metals

Question 3

Explain why the metallic property of elements across the period decreases

Answer 3

Across the period, there is a decrease in metallic property and an increase in non-metallic property -> increasing tendency to gain e-

The proton number increases, number of protons in the nucleus and valence electrons in the valence electron shell increases while number of electron shells remain the same. The increase in nuclear charge results in a stronger forces of attraction between the positive nucleus and valence electrons. More energy is needed to lose electrons, resulting in lower tendency for the atom to lose electrons to form cation. Lesser energy is required to gain electrons, resulting in higher tendency for atom to gain electrons to form anions or share electrons. Thus, the metallic property decreases and non-metallic property increases.

Question 4

Why there is an increase in metallic properties and decrease in non-metallic properties down the group

Answer 4

Down the group, the size of the atom increases due to an increase in the number of electron shells. The valence electrons r further away from the nucleus. Hence, there is weaker forces of attraction between the positive nucleus and the valence electrons. Thus There is a higher tendency for the atom to lose its valence electrons to form a positive ion.

Question 5

Group 1 properties

Answer 5

Elements in group 1 — alkali metals. They react w water to form soluble metal hydroxides.
Physical properties of alkali metals:
1. Soft and can be cut easily
2. Good conductors of heat and electricity due to presence of delocalised mobile electrons
3. Low melting and boiling points
4. Low densities

Down the group, the melting and boiling points of alkali metals decreases. Size of atom down the group increases due to increase in number of electron shells, the valence electrons r further away from the positive nucleus, hence the forces of attraction between the nucleus and valence electrons r weaker. Metallic bonding is weaker and less energy is needed to overcome the forces of attraction.

Furthermore, the density of alkali metals generally increases.

Chemical properties:

1. They have 1 valence electron and react by losing 1 valence electron to achieve noble gas electronic configuration
2. Reducing power of alkali metals increases down the group (powerful reducing agents)
3. Reactivity increases down the group
   Size of atom increases due to increase in number of valence electron shells. The valence electrons are further away from the nucleus hence there are weaker forces of attraction betw the nucleus and the valence electrons. Lesser energy is needed to lose valence electrons. Thus, There is an increase in the ease of losing valence electrons.
4. They react with water to form alkali and hydrogen gas

Order of reactivity: Li<Na<K<Rb<Cs<Fr

Question 6

Group 1 elements and their reaction w water

Answer 6

1. Lithium -> reacts quickly. Lithium floats on water
   2Li(s) + 2H2O (l) -> 2LiOH (aq) + H2 (g)
2. Sodium -> reacts violently. Sodium darts around the water surface. Reaction may be explosive.
   2Na(s) + 2H2O (l) -> 2NaOH (aq) + H2 (g)
3. Potassium -> reacts very violently. Reaction is explosive.
   2K(s) + 2H2O (l) -> 2KOH (at) + H2 (g)

Question 7

Physical properties of halogen

Answer 7

1. Exist as diatomic molecules
2. Are non-metals
3. Have low melting and boiling points
4. Are coloured

Question 8

Appearance of halogen at rtp

Answer 8

Fluorine -> pale yellow gas
Chlorine -> yellow-green gas
Bromine -> reddish brown liquid
Iodine -> purple-black solid
Astatine -> black solid

Question 9

Trend in physical properties down group 17 and explanation

Answer 9

1. Down the group, melting and boiling point increases.
   Size of molecule increases, resulting in stronger intermolecular forces of attraction. Thus more energy is needed to overcome the stronger intermolecular forces of attraction.
   2.colour intensity increases

(Covalent bonds in diatomic molecule stronger as size increases)

Question 10

Chemical properties of halogens

Answer 10

1. Reactive non-metals -> have 7 valence electrons -> have a tendency to gain 1 e- to achieve noble gas electronic configuration
2. Oxidising power of halogens decrease down the group (Oxidising agents)
3. React w most metals to form salts called halides (eg fluoride bromide)
4. Reactivity of halogens decreases down the group as size of molecule increases due to increase in number of electron shells. Forces of attraction betw the positive nucleus and the negative valence electrons r weaker, making it harder for the nucleus to attract 1 more electron.

Order of reactivity: F2>Cl2>Br2>I2>At2>Ts2

Question 11

Displacement of halogens

Answer 11

The more reactive the halogen, the higher tendency it has to gain electrons to form negative ions compared to a less reactive halogen. Thus a more reactive halogen will displace a less reactive halogen from its halide solution.

Question 12

Displacement of halogens: aq Cl2 + aq NaBr

Answer 12

Cl2 (aq) + 2NaBr (Aq) -> 2NaCl (aq) + Br2(aq)
Light yellow—colourless—colourless—red-brown

Observation: solution changes from colourless to red-brown
Explanation:
Chlorine is more reactive than bromine hence chlorine displaces bromine rom aqueous sodium bromine

Products in halogen displacement r alw aq.
Halides (eg KBr, KI) (halogens in ions state) r colourless

Question 13

What is the similarity between displacement of metals and displacement of halogens

Answer 13

Both are always redox reactions
Both always involve colour change

Question 14

Physical properties of group 18

Answer 14

Noble gases:
1. Monoatomic non-metals
2. Colourless gases at rtp
3. Low melting and boiling points
4. Insoluble in water
5. Unreactive
Noble gases are unreactive as they have a fully filled valence electron shell. They do not lose, gain or share electrons and hence rarely react to form compounds.

Question 15

Noble gases and their uses

Answer 15

Helium: used in balloons
Argon/neon: used to fill tungsten bulbs to provide an inert atmosphere that prevents oxidation of the filament
Argon: used in manufacturing of steel
-> iron is the main component of steel but can oxidise thus argon provides an inert atmosphere to prevent iron from Oxidising (rusting)

Question 16

Transition elements

Answer 16

Metals found in groups 3 to 11
Group 12 does not belong in transition elements as they have no variable oxidation states.

Question 17

Physical properties of transition elements

Answer 17

1. High melting and boiling points
2. High densities
3. Has variable oxidation states (unique)
4. Forms coloured compounds (unique)
5. Transition metals and their compounds are good catalysts for many reactions, both in laboratory and in industry. (Unique)

Element and reaction w water
Iron: haber process for manufacture of ammonia
Nickel: manufacture of margarine from vegetable oil
Manganese (IV) oxide: decomposition of hydrogen peroxide

Question 18

Give three characteristic properties of transition metals

Answer 18

Has variable oxidation states
Forms coloured compounds
Transition metals and their compounds are good catalysts for many reactions

Question 19

Why elements in group 18 do not have oxidation states.

Answer 19

Elements in group 0 are inert gases as they have fully filled valence electron shell and noble gas electronic structure. Thus, they do not lose, gain or share electrons to form compounds so their oxidation state is 0, which is not present in the table.

Question 20

Describe the trend in melting point and reactivity down group 1 and group 7.

Answer 20

MP:
Down group 1, MP decreases.
Down group 7, MP increases.

R:
Down group 1, reactivity increases
Down group 7, reactivity decreases.

Question 21

Give 2 similarities betw H2 and elements in group 1

Answer 21

Both has 1 valence electron
Both tend to lose an electron when a compound is formed to form cations of charge +1, resulting in an oxidation state of +1

Question 22

Give 2 differences betw H2 and elements in group 1

Answer 22

H2 exists as diatomic molecules in gaseous state but group 1 metals exist as solid metals w low melting points at rtp

H is a good electrical insulator but group 1 elements are good electrical conductors

Question 23

When sodium is heated in hydrogen, the metal glows as an exothermic reaction takes place. How would the observation differ from when potassium is heated in hydrogen?

Answer 23

Potassium would glow more brightly and reaction will be hotter [1] Reaction would be more exothermic as potassium is more reactive than sodium. [1]

Question 24

Which 2 elements react tgt most vigorously?

Answer 24

1. Answ must be 1 cation and 1 anion!
2. Most reactive group 1 element + most reactive group 7 element
   Eg potassium + chlorine
   Instead of sodium + chlorine

Question 25

Explain in terms of electrons why fluorine is an Oxidising agent.

Answer 25

Fluorine has a high tendency to gain electrons to form anion F-, resulting in a loss of electrons in the substance it reacts w, thus Oxidising the substance.

Question 26

In practice, when x is reacted w y, the yield of xy is never 100%. Explain why.

Answer 26

X + y is a reversible reaction thus xy formed can decompose back to x and y, thus yield is never 100%.

Question 27

Desc the trend in atomic radii for group 1 and explain.

Answer 27

Atomic radii increases down group 1. Down the group, no. Of electron shells increases, thus size of atoms increases, resulting in an increase in atomic radii.

Question 28

Moving across period 2 from lithium to neon, behaviour of the elements when they form compounds change from metal to non-metal. Explain why. [3]

Answer 28

Across period 2, number of electron shells stay the same but nucleus gets increasingly positively charged. [1] Hence, Lithium, beryllium and boron that behave like metals will have a higher tendency to loser their valence electrons to form a positive ion to achieve stable electronic configuration as the attractive force between the nucleus and the valence electrons are weaker. [1]

Elements such as Carbon, Nitrogen, oxygen and fluorine, behave like non-metals. Higher nuclear charge will decrease the atomic radius of the atom, resulting in higher tendency to gain electrons to achieve noble gas electronic structure and thus forms negative anions. These non-metal elements can also share electrons with other non-metals, forming covalent molecules. [1]

Neon is a noble gas that does not form compounds as it has a stable electronic configuration with fully filled valence electron shell. Thus, it is inert.

Question 29

A jet of chlorine gas and a jet of iodine gas aimed at filter papers soaked in potassium bromide solution. State and explain observations.

Answer 29

Cl2:
Solution on filter paper turns brown. Chlorine is more reactive than bromine and thus displaces bromine from potassium bromide solution. Aq bromine solution is brown.

I2:
Colourless solution on filter paper remains colourless. Iodine is less reactive than bromine thus cannot displace bromine from potassium bromide solution. Some iodine gas dissolves in water to form aq iodine which appears to be brown in colour.

Question 30

Give 2 reasons why catalysts reduce costs in the long run

Answer 30

Catalysts provide an alternative pathway by lowering activation energy.

1. It speeds up rate of reaction thus reduces time required for a reaction to take place, which saves time as more products can be produced within a shorter time and therefore reduce costs.
2. It lowers activation energy, therefore lowering operating tempt such that lesser energy is required, hence lowering cost of industrial processes in the long run
3. Catalysts are used in small quantity as they remain chemically unchanged, regenerated at the end of the experiment and can be reused, therefore reducing cost of materials required without incurring additional cost.

Question 31

Suggest why He cannot be recovered once released into the atmosphere.

Answer 31

Helium has a very low density compared to other gases whose density are more than 1g/dm3 thus Helium gas will leave the atmosphere upon release. Helium is also an inert gas that has fully filled valence electron shell and thus has a noble gas electronic configuration. Thus, Helium does not lose, gain or share electrons to form compounds with other elements that can be easily retrieved from the atmosphere.

Question 32

Suggest why some people think that the use of He to fill party balloons should be discouraged.

Answer 32

He is a finite resource that is non-renewable. As He is essential for the function of technical equipment such as MRI scanners which is important in imaging purposes to identify medical treatments at hospitals, the less significant usage of He in party balloons result in COMPETING USE for the limited resources.

Question 33

Explain the electronegativity for nitrogen and fluorine. [2]

Answer 33

Electronegativity of an element increases across the period [1]
As the atomic radius decreases/nuclear charge increases [1]

Question 34

Account for the physical state of the halogens down the group [2[

Answer 34

Increase in boiling point is due to stronger intermolecular forces of attraction between halogen molecules down the group, due to an increase in molecular size. [2]