This document contains notes about Chapter 3 Periodicity for the IB HL Chemistry course. Some topics covered are: periods and groups, metals and nonmetals, effective nuclear charge, shielding constant and electron shielding, atomic radius, ionic radius, ionisation energy, electron affinity, electro...
Chemistry studies the trends and patterns in elements’ physical and chemical properties.
The first Periodic Table with 62 known elements was proposed by Dmitri Mendeleev.
3.1 The Periodic Table
The arrangement of electrons in the Periodic Table helps to predict their electron
configuration.
Periods and groups
The columns of the table are called groups and the rows are periods.
Groups are numbered according to the number of valance electrons. Periods are numbered
according to the number of energy levels.
Elements are placed in order of increasing atomic number (Z). Extending the PT is possible
by making elements artificially.
The position of the element in the PT is based on the sublevel of the highest-energy electron
in the ground-state atom:
- Elements with valence electrons occupy an s sublevel make up the s block
- Similarly for p block, d block, and f block.
n = the period number = the number of occupied main energy levels.
,e.g.: Na is [Ne]3s1 and is in Period 3 because it occupies 3 main energy levels, and Group 1
because there is 1 electron in the valence energy level.
Br [Ar]4s23d104p5 is in Period 4 because it occupies 4 main energy levels, and in Group
17 because it has 7 electrons in the valence energy level.
Metals and nonmetals
Halogens = a reactive group of nonmetals in group 17
Noble gases = unreactive group of nonmetals in group 18
Alkali metals = reactive group in group 1
Lanthanides (rare earth metals) and actinides make up the f block.
Metalloid elements: elements in blue diagonal staircase
- Physical properties (appearance) resembles metals
- Chemical properties resemble nonmetals.
- They are semiconductors.
Silicon (Si) is used to make computers.
Europium (Eu) is used in the security marking of banknotes
3.2 Periodic Trends
Elements show trends in their physical and chemical properties across periods and down
groups.
Physical properties
, The periodicity of the elements is reflected in their physical properties including atomic
radius, ionic radius, ionization energy, electron affinity, and electronegativity.
Effective nuclear charge
Do NOT use ENC to explain trends in the periodic table.
The effective nuclear charge is less than the full nuclear charge.
Nuclear charge: charge of nucleus = number of protons = atomic number. Increases
between successive elements.
Effective nuclear charge: positive net charge
Zeff = Z – S
S = shielding constant (σ) = the number of electrons between the nucleus and valence
electron. Electron shielding refers to valence electrons being shielded from the nucleus by
the repulsion of inner electrons.
E.g.: A sodium atom has Z=11. The outer electron is shielded from the nucleus by the 10
inner electrons.
Across a period, ENC increases because there is no change to the number of inner electrons,
but the valance electrons increase in number. A proton is added to the nucleus and one
electron is added to the valance energy level.
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