periodicity (chapter 10)

Question 1

why does atomic radius decrease across a period?

Answer 1

positive nuclear charge increases as atomic number increases, hence electrons in the outer shell are pulled closer to the nucleus.

Question 2

general trend in ionic radius from na+ to Si+

Answer 2

the ionic radius decreases due to increasing as the nuclear charge increases

Question 3

general trend in ionic radius from p-3 to Cl-

Answer 3

ionic radius decreases due to increasing nuclear charge on anion

Question 4

state and explain trend in electrical conductivity across the metals of period 3

Answer 4

• Conductivity increases across the metals from sodium to aluminum because they are metallic elements they have a positive ion lattice surrounded by a sea pf delocalised electrons. these electrons allow charge to flow across the metal when a voltage is applied.
• the higher the charge on the metal the greater the proportion of electrons hence higher the electrical conductivity

Question 5

state and explain the trend in melting points across the period 3 elements

Answer 5

• the melting points increase till silicon
• from na to al m.p increases as metals have increasingly strong metallic bonding and their melting points increase.
• silicon has a giant covalent structure so it has the highest mp
• elements after silicon are simple molecules, they have weak id-id forces between their molecules so low mps
• exception- sulfur has a melting point greater than that of phosphorous

Question 6

state and explain the electrical conductivity of non-metals of period 3

Answer 6

silicon has a lower electrical conductivity than any metals as it does not have any delocalised electrons within its structure. (classes as a semi-conductor)
-across the period the conductivity decreases further.

Question 7

trend of first I.E across a period

Answer 7

In general, the first ionisation energy increases across Period 3 as the positive nuclear charge increases, as electrons are in the same shell the shielding effect remains constant.

Question 8

sodium + oxygen

Answer 8

4Na + O2 -> 2Na2O
- bright yellow flame
- the sodium oxide formed is a white solid

Question 9

magnesium + oxygen

Answer 9

2Mg + O2 -> 2MgO
- White flame

Question 10

aluminum + oxygen

Answer 10

4Al + 3O2 -> 2Al203
- the metal is protected by a layer of aluminum oxide, powdered aluminum reacts better
- white flame

Question 11

silicon + oxygen

Answer 11

Si + O2 -> SiO2

Question 12

phosphorous + oxygen

Answer 12

4P + 5O2 -> P4O10
- yellow or white flame
- clouds of white phosphorous (V) oxide seen

Question 13

sulfur + oxygen

Answer 13

S + O2 -> SO2
- blue flame
- has to be ignited in order to burn
- toxic fumes of SO2 formed

Question 14

sodium + water

Answer 14

2Na(s) + Cl2(g) → 2NaCl(s)
- sodium reacts vigorously with cold water, melting into a ball of molten metal and leaves behind a strongly alkaline solution of NaOH pH 14

Question 15

magnesium + water

Answer 15

Mg(s) + 2H2O(l) Mg(OH)2(aq) + H2
(g)
- mg reacts very slowly with cold water, solution formed is weakly alkaline pH 11 as MgOH formed is slightly soluble

Question 16

why does a lower concentration of OH- ions enter the solution when Mg reacts with water compared to when Na reacts with water?

Answer 16

because sodium hydroxide that is formed is much more soluble in water than magnesium hydroxide.

Question 17

Mg + steam

Answer 17

Mg(s) + H2O(g) → MgO(s) + H2(g)
-reacts vigorously with steam to make magnesium oxide and hydrogen

Question 18

aluminum oxide + acid

Answer 18

Al2O3(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2O(l)
al2o3 acts as a base

Question 19

aluminum oxide + hot conc alkali

Answer 19

Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
al2o3 acts as an acid

Question 20

silicon dioxide + hot concentrated alkali

Answer 20

SiO2(s) + 2NaOH(aq) → Na2SiO3
(aq) + H2O(l)
- sio2 acts as an acid forming sodium silicate

Question 21

phosphorous(V) oxide + water

Answer 21

acidic solution of pH 2 is formed
P4O10(s) + 6H2O(l) → 4H3PO4

Question 22

sulfur oxides in water

Answer 22

SO2 +H2O -> H2SO3
SO3 +H2O -> H2SO4
- pH of solution = 1

Question 23

state the nature of period 3 oxides

Answer 23

Na2O - basic
MgO - basic
Al2O3 - amphoteric
SiO2 - acidic
P4O10 - acidic
SO2/SO3 - acidic

Question 24

state the period 3 oxide’s :-
(i) melting point
(ii) electrical conductivity in liquid state
(iii) bonding
(iv) structure

Answer 24

1. Na2O (i) high (ii) good (iii) ionic (iv) giant ionic
2. MgO (i) high (ii) good (iii) ionic (iv) giant ionic
3. Al2O3 (i) high (ii) good (iii) ionic (iv) giant ionic
4. SiO2 (i) very high (ii) none (iii) covalent (iv) giant covalent
5. P4O10 (i) low (ii) none (iii) covalent (iv) simple molecular
6. SO2 (i) low (ii) none (iii) covalent (iv) simple molecular