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A Level Chemistry
Periodicity

The Periodic Table of Elements

Effective Nuclear Charge
• In any atom the nucleus exerts an attractive force on the
electrons
...
The effective charge increases with the nuclear
charge as there is no change in the number of inner
electrons
...

• It increases across a period but remains approximately the
same down a group
...


1) Across the period (from Na to Cl), nuclear charge of
atom increases due to increase in proton number
...

3) Thus, the effective nuclear charge felt by the valence
electrons increases significantly across the period
...
Thus, atomic radius decreases
...

From Si4+ to P3–, ionic radius increases
...

2) The valence electrons in the isoelectronic
series are now held more tightly to the
nucleus
...


Ionic radii
3) From Si4+ to P3–, there is an additional third quantum shell which
results in an increase in shielding effect
...


Ionisation Energy
The ionisation energy is the energy required to remove
one mole of electrons from one mole of atoms in the
gaseous state
...

The first ionisation energy of magnesium:
Mg (g)

Mg+ (g)

+

e-

744 kJmol-1

Ionization Energy
• Trend: Across the period (Na to Ar), there is a general
increase in the first ionization energies of atoms
...

2) Thus, the attraction between the electron
removed and the nucleus increases
...


Two exceptions in the general increase in first
ionization energy
1) Decrease in first ionization energy from Mg to Al

The 3p subshell being less penetrating, is further away from the nucleus
as compared to the 3s subshell
...


2) Decrease in first ionisation energy from P to S

Less energy is needed to remove the outermost electron from
S than from P since there is inter-electronic repulsion between
the paired 3p electrons in S
...


Melting Point
1) Na, Mg and Al form giant metallic lattice with metallic bonds
...

2) Si has the highest melting point as large amount of energy is
needed to break the strong, extensive covalent bonds between Si
atoms in the giant molecular structure
...
Thus, they have
very low melting points
...


Electrical Conductivity
1) Na, Mg and Al are good electrical conductors as their
metallic lattices contain delocalized electrons
...

2) Si is a semi-conductor as the electrons within the covalent
bonds are held tightly and not easily delocalized
...


Electronegativity
“The ability of an atom to attract the pair of electrons in a covalent bond to itself
...

Decreases down a group
because although the nuclear charge
is increasing, the effective nuclear
charge is less due to shielding of
filled inner shells and a greater
distance from the nucleus
...
5
3
...
5

2
...
5

1
...
5
Na

Mg

Al

Si

P

S

Cl

Ar

Electronegativity
Electronegativity is the measure of the tendency of an atom to
attract electrons in a covalent bond
...

SiO2 forms giant molecular structure
- large amount of energy is needed to
overcome the strong and extensive
covalent bonds between the Si and O
atoms and thus SiO2 has high melting
point
...


Molecular structures of P and S

Electrical Conductivity
• The ionic compounds (Na2O and MgO) conduct
electricity when molten (liquid) as the ions can move
through the liquid
...

• The oxides of the non-metals do not conduct
electricity
...

2) This is due to decreasing difference in electronegativity
between the element and oxygen
...
Hence, oxides of Na and Mg are ionic and basic
...
Hence, oxides of Si, P and S are covalent and acidic
...
However, it has significant covalent
character as electron cloud of O2– ions is distorted towards Al3+ due to its
high charge density
...


Reactions of oxides of third period
elements

Reactions of basic oxides

Reactions of amphoteric oxides

Reactions of acidic oxides

Periodic trends of the chlorides of elements
(Na to P)

Structure, bonding and melting points
1) Chlorides of Na and Mg form giant ionic lattice structures
- large amount of energy is needed to overcome the strong electrostatic forces of
attraction between the metal ions and Cl– ions and thus they have high melting
points
...


3) Gaseous AlCl3 is a covalent chloride
- electron cloud of Cl– ions is distorted towards Al3+ due to its high charge density
...


Reaction of NaCl & MgCl with water

Reaction of AlCl3 with water

Reaction of SiCl4 and PCl5 with water

Which diagram represents the change in ionic radius of the
elements across the third period (Na to Cl)?

• [AS Nov 2002 Paper I Q14]

Consecutive elements X, Y, Z are in period 3 of the Periodic
Table
...
What could be the identities of X, Y and Z ?
A
...
Magnesium, aluminium, silicon
C
...
Silicon, phosphorus, sulphur
• [AS Nov 2005 Paper I Q14]

Which pair of elements have bonds of the same type between
their atoms in the solid state?
A
...
Chlorine and argon
C
...
Sulphur and chlorine

• [AS June 2008 Paper I Q7]

The following graph shows the variation of a property of the first 20
elements in the Periodic Table with the atomic number of the
element
...
atomic radius
B
...
ionic radius
D
...
The two resulting solutions have the same effect on litmus
...
Sodium
B
...
Aluminium
D
...
Al2O3 and Al2Cl6
2
...
P4O10 and PCl3
• [AS June 2004 Paper I Q34]

Silicon tetrachloride, SiCl4, is a liquid of low boiling point
...
What types of bonding occur in SiCl4 (I)?
1
...
Covalent
3
...
What could be
the element Q?
A
...
Aluminium
C
...
Phosphorus
• [AS Nov 2002 Paper I Q13]

Learning Outcome
• describe and explain qualitatively (and indicate the
periodicity in) the variation in atomic radius, ionic radius,
melting point and electrical conductivity of the elements
• explain the variation in first ionization energy
• describe with equations, reactions of the elements with
oxygen
• describe the reactions, if any of the elements with
chlorine and water
...

• describe the reactions of the oxides and chlorides with
water
...




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