3.3 Chemical Properties
The Alkali Metals:
-
Li, Na, K, Rb, Cs
Soft, malleable metals
Low melting points because each atom can only contribute one electron to metallic bonding,
therefore the bond is weaker than for many other metals.
Low densities due to the atoms of the alkali metals being the largest in their periods.
Very chemically reactive and tarnish rapidly on exposure to air. This results from the fact that
there is just one electron in their valence shell which is easily lost.
Form ions with a single positive charge.
Going down the group, as atomic radius increases -> ionization energy decreases -> reactivity
increases.
Combine directly with reactive non-metals like oxygen, chlorine, bromine and iodine.
All react with water to form a metal hydroxide and hydrogen gas.
2M + 2H2O  2M+ + 2OH- + H2
When lithium reacts with water the reaction occurs slowly but steadily.
In comparison, with sodium the reaction is vigorous, producing large amounts of heat which
melt the sodium.
The reaction of potassium with water is violent, causing an explosion of purple flame.
The solution becomes strongly alkaline due to the formation of hydroxide ions.
Alkali Metals
Electronic Structure
Melting Point
Increasing atomic and ionic
radius
Decreasing ionization
energy
Decreasing electronegativity
Li
2,1
454
Na
2,8,1
371
K
2,8,8,1
336
Rb
2,8,8,18,1
312
For further reading:
http://www.rsc.org/chemsoc/visualelements/pages/data/intro_groupi_data.html
The Halogens:
-
F, Cl, Br, I
Very reactive non-metal (require just one electron to complete their valence shell)
Exists as diatomic molecules (e.g. F2)
Increases with molar mass when going down the group
Fluorine and Chlorine are gases, bromine a liquid and iodine a solid.
Electronegative elements (e.g. F-)
Reactivity decreases going down the group
Slightly soluble in water
Cs
2,8,8,18,18,1
302
Halogens
9F
Colour
Pale yellow
State at room T&P
Gas
Increasing atomic and ionic radius
Decreasing ionization energy
Decreasing electronegativity
17Cl
35Br
53I
Yellow green
Gas
Red brown
Liquid
Black/dark purple
Solid
Halide ion
Reagent
Aqueous Ag+
F-
Cl-
Br-
I-
No reaction
Cream
precipitate
Ag+ +Br-→ AgBr
Pale yellow
precipitate
Ag+ +I- →AgI
Chlorine
No reaction
White
precipitate (turns
black in sunlight)
Ag+ + Cl-→AgCl
No reaction
Solution goes
yellow then black
precipitate
Cl2 +2I- →I2 +2Cl-
Bromine
No reaction
No reaction
Solution turns
yellow then
brown
Cl2 +2Br-→
Br2+2ClNo reaction
Iodine
No reaction
No reaction
No reaction
Solution goes
yellow then black
precipitate
Br2+2I- →I2 +2BrNo reaction
For further reading:
http://www.chemicalelements.com/groups/halogens.html
3.3.2: Trends in the Oxides of the Period 3 Elements
-
LHS the elements have low ionization energies so they bond to other elements to form ionic
compounds.
Oxides of these elements are therefore ionic and contain the oxide ion.
This ion can form a bond to hydrogen ions and therefore act as a base dissolving in water.
In the middle of the periodic table the ionization energy increases and becomes too great for
cation formation.
Therefore the elements tend toward non-metallic behaviour and form covalent bonds.
RHS: Elements (excluding the noble gases) continue to be able to form covalent bonds but
gaining an additional electron also becomes energetically feasible.
These electrons also have the option of combining with metals to form ionic compounds where
they exist as anions.
Formula
Ratio of atoms
Bonding
Na2O
2:1
Ionic
MgO
2:2
Ionic
Acid-base
Basic
Basic
Al2O3
2:3
Highly polar
covalent
Amphoteric
SiO2
P2O5
2:4
2:5
Polar covalent Polar
covalent
Weakly acidic Acidic
SO3
2:6
Polar
covalent
Acidic
Cl2O7
2:7
Polar covalent
Acidic
character
Other Oxides
Na2O2
Period 3 Na
Mg
Al
elements
Electroni 2,8,1
2,8,2
2,8,3
c
structure
Boiling
1156
1380
2740
point / K
Metallic / Metalli Metalli Metalli
nonc
c
c
metallic
Decreasing atomic radius
Increasing ionisation energy
Increasing electronegativity
Decreasing metallic character
Increasing hydrolysis of chlorides
Increasing acidic oxides
P2O3
SO2
ClO2 & Cl2O
Si
P
S
Cl
Ar
2,8,4
2,8,5
2,8,6
2,8,7
2,8,
8
2528
553
718
238
87
Metalloi
d
Nonmetalli
c
Nonmetalli
c
Nonmetalli
c
Inert
gas
Homework Questions:
1. As the atomic radius of the alkali metals increases:
a) The ionization energy decreases & the reactivity decreases.
b) The ionization energy increases & the reactivity decreases.
c) The ionization energy decreases & the reactivity increases.
d) The ionization energy increases & the reactivity increases.
2. What are the main chemical properties of the alkali metals?
3.
The reactivity of the increases in the order
A. Br, I, Cl
B. F, Cl, Br
C. Br, Cl, F
D. Cl, Br, I
4. Give the colours of the following:
a. The colour this changes to when exposed to sunlight for a long time
b. iodine vapour
c. the precipitate initially formed when aqueous barium chloride reacts with aqueous silver
nitrate.
5. True or False
a. Mg is metallic
b. Si is metalloid
c. Ar is non- metallic
d. P is non-metallic