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PERIODICITY
Periodic Table • consists of rows , or ................................
- columns , or ................................
- is split into several blocks
- in each block the elements are filling, or have just filled, particular types of orbital
Group(s)
s block .............................................. p block ..............................................
d block ..............................................
f block ..............................................
The outer electron configuration is a periodic function i.e. it repeats every so often
Because many physical and chemical properties are influenced by the outer shell configuration of an atom, it isn’t surprising that such properties also exhibit periodicity...
ionisation energy, electron affinity, atomic radius, ionic radius,
electronegativity, melting points and boiling points
Periods
Introduction • the first two periods in the periodic table are not typical
- the first contains only two elements (H, He)
- the second (Li - Ne) contains the top elements of each group; these have small sizes and relatively high ionisation energies
- Period 3 is best for studying periodic trends.
Periods 2 & 3
Elements As you move from left to right the elements go from highly electropositive metals through metalloids with giant structures to the simple molecular structure of non-metals.
Li Be B C N 2 O 2 F 2 Ne < - metals - > metalloid giant molecule < non metals (simple molecules) >
Na Mg Al Si P 4 S 8 Cl 2 Ar < - - - - metals - - - - > metalloid < - non metals (simple molecules) - >
Initially one is filling the 3s orbital then the 3p orbitals The nuclear charge increases by one each time giving an increased pull on the electrons.
Periodicity F321 1
d
f
s p
Atomic Radius A problem with measuring atomic radius is that one is not measuring the true radius of an atom. In metals one measures the metallic radius (half the distance between the inter-nuclear distance of what are effectively ions). Covalent radius is half the distance between the nuclei of atoms joined by a covalent bond. The values are measured by X-ray or electron diffraction.
UNITS:- nanometres
Decreases across a given period
due to ...
increased nuclear charge attracting the electrons (which are going into the same shell) more strongly.
Argon’s value cannot be measured as it doesn’t form compounds.
N3-^ O2-^ F¯ Ne Na+^ Mg2+^ Al3+ Protons 7 8 9 10 11 12 13 Electrons 10 10 10 10 10 10 10
Electronegativity
A measure of the attraction an atom has for the electron pair in a covalent bond. Do not confuse with electron affinity.
UNITS:- Pauling Scale
Groups Decreases down a group.
Periods Increases across a period
As the nuclear charge increases so does the attraction for the shared pair of electrons in a covalent bond.
2 F321 Periodicity
Na Mg Al Si P S Cl Ar
atoms ions
Na Mg Al Si P S Cl Ar
Q.1 Explain the variation in atomic and ionic size for the following isoelectronic species.
Melting Point & Boiling Point
Boiling and melting points are a measure of the energy required to separate the particles in a substance.
Bond type is significant.
Boiling points tend to be a better measure and show better trends because solids can be affected by the crystal structure as well as the type of bonding.
UNITS:- Kelvin
Periods A general increase then a decrease
Metals
Na-Al Melting point increases due to the increasing metallic bonding caused by ...
- larger number of electrons contributed to the “cloud”
- larger charge and smaller size of ions gives rise to a larger charge density.
Non-metals
Si Large increase in melting point as it has a giant molecular structure like diamond
A lot of energy is required to break the many covalent bonds holding the atoms together.
P, S, Cl Very much lower melting points as they are simple covalent molecules
The melting point depends on the weak intermolecular van der Waals forces. The larger the molecule the greater the van der Waals’ forces
P 4 S 8 Cl 2 relative mass 124 256 71
melting point 44°C 119°C -101°C
Shape of P 4 Shape of S 8
Ar Monatomic species with the lowest melting point
4 F321 Periodicity
Na Mg Al Si P S Cl Ar
0
500
1000
2000
2500
1500
3000 Boiling
Melting
A COMPARISON BETWEEN PERIOD 2 AND PERIOD 3
Electronic Configuration Electronic configurations change in a similar fashion across the group. However, the elements in Period 3 have more electrons, and hence, more shells.
Period 2 ...2s^1 2s^2 2s^2 2p^1 2s^2 2p^2 2s^2 2p^3 2s^2 2p^4 2s^2 2p^5 2s^2 2p^6
Period 3 ...3s^1 3s^2 3s^2 3p^1 3s^2 3p^2 3s^2 3p^3 3s^2 3p^4 3s^2 3p^5 3s^2 3p^6
Superimpose an equivalent graph for Period 2 elements and explain the difference in values
Atomic Radius Period 2 radii are smaller
due to ...
lower atomic number so fewer electrons are going into shells closer to the nucleus
units nanometres
Melting Point
units Kelvin
Boiling Point
units Kelvin
Periodicity F321 5
Na Mg Al Si P S Cl Ar
Na Mg Al Si P S Cl Ar
0
500
1000
2000
2500
1500
3000
Na Mg Al Si P S Cl Ar
0
500
1000
2000
2500
1500
3000
Period 3
Period 3
Period 3
