Trends in chemical properties (chemical periodicity)

Eg : elements in period 3

( Na to Ar )

Reaction with water

1. 2Na + 2H2O 2NaOH + H2

2. a. Mg +2H2O Mg(OH)2 +H2 (g)

b. Mg + H2O MgO + H2(g)

slow

cold

steam

fast

Reaction with oxygen ( Na to S )

1. 4Na + O2 2Na2O

Observation : burns with yellow flame to form white solid

2. 2Mg (s) + O2(g) 2MgO (s)

white

Observation : burns with intense white flame to form white solid

3. 4Al + 3O2 2Al2O3

4. Si + O2 SiO2

5. P4 + 5O2 P4O10 white solid at rtp

(or 4P + 5O2 P4O10 )

Observation: burns with white/yellow flame.

Another possible compound : P4O6

6. S + O2 SO2

Observation: burns with a blue flame/ choking or pungent gas produced

Further oxidation: 2SO2 + O2 2SO3

Reaction with chlorine ( Na to P )

1. 2Na + Cl2 2NaCl

white solid

Observation : burns with yellow flame /

forms white solid /colour of Cl2 disappears

2. Mg + Cl2 MgCl2

Observation : burns with intense white

flame / forms white solid

3. 2Al + 3Cl2 2AlCl3 Al2Cl6

covalent

simple molecular

ions formed in aq soln

Al3+(aq) or [Al(H2O)6]3+ and Cl-(aq)

Note : Al2O3 is ionic as difference in electronegativity between Al and O is larger

1800 C

Method :

Pass dry chlorine gas over heated aluminium foil in a tube.

4. Si + 2Cl2 SiCl4 (l)

5. P4 + 10Cl2 4PCl5 (s)

white solid

( or 4P + 10Cl2 4PCl5 )

Observation : burns with white flame / forms white solid

Another possible compound : PCl3 , colourless liquid

Oxides of elements in Period 3

I) Trend in acidity of oxides :

Na2O MgO Al2O3 SiO2 P4O10 SO3

Ionic oxide

Basic oxide

Mainly ionic

Oxide

Amphoteric oxide

Covalent oxide

Acidic oxide

Oxides becomes more acidic across the period

1. Na2O : basic oxide

a. dissolves in water strongly alkaline solution

Na2O + H2O 2NaOH (pH=13)

NaOH + HCl NaCl + H2O

(alkali)

b. able to neutralize acids

Na2O + 2HCl 2NaCl + H2O

2. MgO : basic oxide

a. dissolves slightly in water weakly alkaline solution

MgO + H2O Mg(OH)2 Mg2+ + 2OH-

pH = 9

Mg(OH)2 + 2HCl MgCl2 + 2H2O

(alkali)

b. able to neutralize acids :

MgO + 2HCl MgCl2 + H2O

3. Al2O3 :

a. stronger ionic bonds not broken by water

therefore insoluble in water

b. nature of oxide : amphoteric

i) Al2O3 + 6H+ 2Al3+ + 3H2O

acid

ii) Al2O3 + 2OH- + 3H2O 2Al(OH)4-

alkali aluminate ion

c. nature of hydroxide : amphoteric

i) Al(OH)3 + 3H+ Al3+ + 3H2O

ii) Al(OH)3 + OH- Al(OH)4-

4. SiO2 :

a. giant molecular structure with strong

Si – O bonds not broken by water ,

therefore insoluble in water

b. nature of oxide : acidic oxide

SiO2 + 2OH- SiO32- + H2O

hot , conc silicates

Notes :

(1) no reaction with aqueous alkali

(2) SiO2 + O2- SiO32-

acidic basic silicates

5. Oxides of P and S: P4O6 / P4O10*, SO2 /SO3

a. nature of oxide : acidic oxide

b. hydrolyses in water acidic solutions

i) P4O10 + 6H2O 4H3PO4 (phosphoric

acid)*

ii) P4O6 + 6H2O 4H3PO3 (phosphonic

acid)

Observation : white solid dissolves

iii) SO2 + H2O H2SO3 (sulfurous

acid)

iv) SO3 + H2O H2SO4

c. able to neutralize alkali :

i) P4O10 + 12OH- 4PO43- + 6H2O

phosphate

ii) SO2 + 2OH- SO32- + H2O

sulfite

iii) SO3 + 2OH- SO42- + H2O

II) Trends in physical properties of the oxides

Related to structure and bonding

Na2O MgO Al2O3 SiO2 P4O10 SO3

Ionic oxides

Giant ionic structure

Covalent oxide

Giant molecular

structure

Covalent oxide

Simple molecular

structure

1. Na2O , MgO , Al2O3 :

a. ionic with giant ionic structure

b. strong ionic bonds , therefore high

m.p

( m.p MgO Al2O3 Na2O )

c. presence of mobile ions,

good electrical conductor in aqueous

solution or molten state

2. SiO2 :

a. covalent with giant molecular structure

b. numerous strong covalent bonds between atoms , therefore relatively high m.p

( lower than m.p of MgO and Al2O3 ,

but higher than Na2O )

c. molecular , all electrons paired in covalent bonds

no free electrons or ions non conductor

3. P4O10 , SO3 :

a. covalent with simple molecular structure

b. exists as discrete molecules held by weak VDW forces low m.p

m.p of P4O10 (more electrons) SO3

c. molecular , all electrons paired in covalent bonds ,

no free electrons or ions ,

therefore non conductor

Atomic no

m.p of

oxides

Na Mg Al Si P S

Chlorides of elements in Period 3

I)Reaction with water

1. NaCl : dissolve in water without chemical reaction neutral solution

NaCl (s) Na+ (aq) + Cl- (aq)

2. MgCl2 :

a. ionic with a little covalent character

b. relatively high charge density on Mg2+ causes polarisation of water

Undergoes slight hydrolysis in water to form slightly acidic solution (pH=6.5)

MgCl2 + H2O Mg(OH)Cl(s) + HCl(aq)

However only very slight reaction – observation : dissolves without reaction

3. AlCl3 :

a.covalent with a little ionic character

b.high charge density on Al3+ polarises water

Undergoes substantial hydrolysis in water to form acidic solution ( pH=3 )

AlCl3 + 6H2O [Al(H2O)6]3+ + 3Cl-

covalent ionic

[Al(H2O)6]3+ + H2O [Al(H2O)5OH]2+

+ H3O+

( first of 3 equations , polarisation continues )

Overall equation :

AlCl3 + 3H2O Al(OH)3 + 3HCl (*)

H=exothermic

4. SiCl4 ,PCl3/PCl5

a. covalent chlorides

b. hydrolysed in water strongly acidic solution ( pH = 0 – 3 )

SiCl4 + 2H2O SiO2 + 4HCl(g)

PCl3 + 3H2O H3PO3 + 3HCl(g)

PCl5 + 4H2O H3PO4 + 5HCl(g)

HCl(g):

Observation

white fumes

SiO2 white

solid

II)Trends in physical properties of the chlorides

Related to structure and bonding

NaCl MgCl2 AlCl3 SiCl4 PCl5

Ionic

Giant ionic

structure

Covalent

Layered

structure

Covalent

Simple

molecular

structure

1. NaCl , MgCl2 :

a. ionic with giant ionic structure

b. strong ionic bonds therefore high m.p/b.p

( m.p NaCl MgCl2 )

c. presence of free ions : good conductor when molten or in aqueous solution

2. AlCl3 :

a. predominantly covalent with layered structure in the solid state

b. in each layer , all AlCl3 molecules are joined by dative bonds and forms a giant structure

Layers are held together by weak VDW forces

c. when it melts , the VDW forces and dative bonds have to be broken

relatively high m.p / b.p but lower than that of ionic chlorides

d. molecular , all electrons paired in covalent bonds

no free electrons or ions

therefore non conductor

3. SiCl4 , PCl5 / PCl3 :

a. covalent with simple molecular structure

b. i) exist as discrete molecules held by

weak VDW forces

therefore low m.p / b.p

ii) m.p of PCl5 > m.p of SiCl4

Reason :

no of electrons in PCl5 > SiCl4

stronger VDW forces , higher b.p/m.p

PCl5 ( white solid ) ,

SiCl4 ( colourless liquid )

c. molecular , all electrons paired in covalent bonds

no free electrons or ions

therefore non conductor

Atomic no

b.p of

chlorides

Na Mg Al Si P

Patterns in the formula of compounds (oxides & chlorides)

1. Oxidation no of the element = no of electrons used for bonding

Eg Compound Oxidation no

SO2 +4

Na2O +1

PCl3 +3

2. Many of the oxidation no corresponds to loss or gain of enough electrons to obtain a stable octet in its outermost shell.

Eg Na - 1e Na+

1s22s22p63s1 1s22s22p6

P + 3e P3-

[Ne]3s23p3 [Ne]3s23p6

3. The maximum oxidation no = group no

= no of valence electrons

Egs: oxides

max oxidation no (in SO3 )= +6

max oxidation no in P4O10 = + 5

a. S (Group 16) : forms SO2 and SO3

b. P (Group 15) : forms P4O6 and P4O10

Trend in maximum oxidation:

Increases across the period

Na Si Cl

+1 +4 +7