Chemistry Of The Elements: Periodicity Flashcards
Who’s are elements sorted into groups in the periodic table?
According to the number of valence electrons.
How are elements arranged into periods in the periodic table?
According to principle quantum number
How are elements arranged into blocks in the periodic table?
According to angular momentum number of their outermost electron
What is effective nuclear charge?
An attempt to quantify the actual attractive interaction experienced by electrons in a particular orbital to the nucleus.
It helps to calculate orbital energy, E = -(Zeff^2 x Rh)/n^2.
Therefore the orbital energy is dominated by n, the value of which will compensate for any increase in Zeff.
High Zeff= lower energy, more compact orbitals.
What are the two main values for Zeff?
Clementi values- derived from orbital energies, more reliable.
Slater values- based on screening of electrons from the nucleus.
How does Zeff vary according to Clementi values?
INCREASES across a period and down a group.
How does Zeff vary according to Slaters Rules?
Descending a group, Zeff increases until periods 3/4 after which it remains constant.
Zeff = Z - S
Where S is a shielding term and is calculated in the following way:
Electrons placed into groups: 1s, 2s2p, 3s3p, 3d, 4s4p, 4d, 4f
1) If an electron is in the same group as that being considered, add 0.35 (0.30 for 1s electrons).
2) If the considered electron is in a ns or np orbital, each electron in the n-1 shell contributes 0.85, whilst those in the n-2 shell contribute 1.00.
3) If the considered electron is in a nd or nf orbital, all lower energy electrons contribute 1.00.
This means the shielding by a particular shell is greatest the closer to the nucleus it is.
Why is there such a significant increase in Zeff from nd1 to nd10?
d electrons don’t shield each other well.
This affects the character of p block elements
What is penetration?
Some orbitals have regions of probable electron density near the nucleus from which they experience a higher Zeff than expected.
Why is the 4s filled before the 3d?
4s orbital has a greater Zeff because of penetration (4s maxima prior to the 3d maximum in the radial distribution function).
Once 4s filled, the 3d orbital falls sharply in energy to become core-like because it is ineffectively shielded by the 4s (most of its density lies above the 3d).
Why does s-p mixing diminish as you move across a period?
There is an increase in s/p orbital energy separation across the period because of penetration by the s orbital, meaning they experience a greater Zeff than expected on the basis of Z and S.
Why is it that the energy of 6s orbitals are lower than the energy of 5s orbitals?
4f orbitals poor at shielding but also RELATIVISTIC EFFECTS:
The mass of an electron changes with its velocity, and is greater for higher energy orbitals. Orbital energy and Bohr radius depend on electron mass such that they both decrease with electron mass.
Hence:
1) s orbital electrons experience the greatest effects of nuclear charge so relativistic contraction affects them the most for heavier atoms (specifically the 6s orbital).
2) s orbitals shield most effectively so the closer they are to the nucleus (see above) the better the shielding they provide (so d and f orbitals increase in energy).
For which elements are relativistic effects most prominent?
Pt Au Hg
Why is the size of an atom difficult to define?
Electron clouds are infinite.
Depends on its chemical environment/bonding it is involved in.
What is covalent radius?
Radius of an atom in a homoatomic bond (A-A), being half the value of the bond length.
If no such molecule exists, estimate from a heterotrophic bond.
How does covalent radius vary with oxidation state?
The more positive the oxidation state, the higher Zeff so the smaller r
What is metallic radius?
Half the inter nuclear distance within a metallic lattice, depending on coordination number of the metal
(Higher coord no, bigger radius)
What is ionic radius?
Size of an ion in a crystal lattice.
Note this does not always correspond to observed interatomic distances, specifically in solids (e.g. in LiI, Li+ ions are not in contact but I- ions are, leading to gaps).
In the gaseous state however interatomic distances are observed to be 85% of the sum of crystal ionic radii (stronger bond) where the power of attraction is diluted among the six neighbouring atoms.
What does ionic radius depend on?
Coordination number (proportional)
Charge of ion (fewer electrons, smaller size)
What are van der waals radii?
Half the nearest contact between atoms in which dispersion forces only are at work, therefore usually quoted a a broad range of values.
How does atomic radius change across a period?
Decreases (though by around 0.3A more for n=4,5 because of poorly shielding d orbitals (d block contraction)
How does atomic radius change down a group?
Increases
By 0.2-0.5A
Why is Ga the same size as Al and why is there little increase in size from Ga to Tl?
Expected expansion in size is counteracted by d block contraction (and for Tl, f-block contraction and relativistic contraction also).
What is ionisation energy?
The enthalpy change when one mole of atoms or ions in the gaseous state is oxidised by one mole of electrons
