Topic 3.2: Physical properties

3.2.1 Define the terms first ionization energy and electronegativity

First Ionization Energy is the minimum energy required to remove an electron from a neutral gaseous atom in its ground state.

Key words:

Minimum Energy, Neutral Gaseous Atom and Ground State

Electro-negativity defined as the relative attraction that an atom has for the shared pair of electrons in a covalent

Key Words:

Relative Attraction, Shared Pair and Covalent


3.2.2 Describe and explain the trends in atomic radii, ionic radii, first ionization energies, electro-negativities and melting points for alkali metals (Li to Cs) and the halogens (F to I)

Atomic Radii

Since the outer electrons are difficult to locate, in practise, the atomic radius is measured as half the distance between two bonded atoms. For this reason noble gases are given no value as they do not bond with other atoms.

On descending a group, the atomic radius increase. This is because the outer electrons are getting further from the nucleus. This applies for both alkali metals and halogens.

Ionic Radii

It is defined by the distance between nucleus and outer most electrons of a positive metal cation or negative non-metal anion.

• Both metal cation and non-metal anion increase in size on descending the group
• Metal cations tend to be smaller than their atom as they have lost their outer most electrons
• Non-metal anions tend to be larger than their atom as they have gained electrons into their outer energy levels

Ionization Energy

It is defined by the minimum energy required to remove an electron from a neutral gaseous atom in its ground state.

• Ionization energy decreases on descending a group
• The outer most electrons are easier to remove as they are a greater distance from the nucleus shielding from the positive nucleus charge by the core electrons

Electronegativity

Electronegativity is defined as the relative attraction that an atom has for the shared pair of electrons in a covalent bond.

• On descending a group, electronegativity decreases. While the atomic radius and the nuclear charge increases, the level of shielding increases and the effective nuclear charge decrease.
• The most electronegative elements are in the top right of the periodic table. (F, O and N). Which are involved in hydrogen bonding.

Melting points

• The variation in melting points is related to the strength of the inter-molecular forces holding the atoms together
• Melting point of metals decreases on descending the group. As the atoms become larger, the metallic bonds holding them together gets weaker. The valence electrons are further from the nucleus.
• Melting point of non-metals increases on descending the group. This is due to the increase in the strength of the Van der Waal's forces between the molecules. The strength of these forces is determined by the total number of electrons in the atom.
• Across a period, the metals in groups 1, 2 and 3 display an increase in melting point. This is due to an increase in strength of the metallic bonds as the number of valence electrons increases
• The Group 4 element will have the highest melting point as it will be a covalently bonded macro-molecule. This contains only strong covalent bonds in the solid phase.
• The non-metal elements in Groups 5, 6 and 7 will have much lower melting point. Theses are also covalently bonded but are simple molecular structures with weaker Van der Waal's forces holding them together.
• The Group 0 noble gases will have the lowest melting point due to it being monoatomic. It will therefore have the weakest of Van der Waal's forces.

3.2.3 Describe and explain the trends in atomic radii, ionic radii, first ionization energies and electro-negativities for elements across period 3

Atomic radii

Across a period, the atomic radius decreases. This is because of the increase in effective nuclear charge and no increase shielding, therefore the electrons get pulled closer towards the nucleus

Ionic radii

Across a period, the metal cation and the non-metal anion get smaller. This is because of an increase in protons (Effective Nuclear Charge) whilst all the ions have the same electronic configuration

First Ionization Energies

Ionization energy generally increases across a period. This is due to the increase effective nuclear charge as the number of protons increases with no increase in shielding.

Electronegativity

Across a period, electronegativity increases. This is the result of an increased number of protons and thus an increased effective nuclear charge with no greater level of shielding


3.2.4 Compare the relative electro-negativity values of two or more elements based on their positions in the periodic table

By looking at the difference in electro-negativity values

We can tell whether a molecule has covalent or ionic bonds