Periodicity

Physical Properties of the Elements Every substance has a set of properties, unique traits or characteristics, that are used to identify it. Atomic radii, first ionization energy, electronegativities and melting points are examples of physical properties that experience periodicity (a repeating pattern or trend on the periodic table).

Electronegativity Electronegativity is a measure of the attraction an atom has for a shared pair of electrons when it is covalently bonded to another atom. ▫This largely determines how atoms bond together to form compounds. ▫In general, metals have few electrons in their outer shell and so tend to lose these electrons in reactions. ▫Non-metals have several outer-shell electrons and so tend to gain electrons in order to complete their outer shell.

Electronegativity Electronegativity increases from left to right across a period and from bottom to top in a group.

First Ionization Energy First ionization energy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state. ▫The outer-shell electron is the most easily removed, and so ionization energy is a measure of how tightly the out-shell electrons are held in an atom.

First Ionization Energy

Explaining the differences within a group Two main factors determine how tightly an out- shell electron is held. The force of electrostatic attraction between the positive protons in the nucleus and the outer-shell electron is directly related to the charges and inversely related to the distance between them.

Explaining the differences within a group As the size of the atoms increases, the attractive force on the outer-shell electron decreases. As the nuclear charge increases, the attractive force increases. BUT the outer-shell electrons are “shielded” from the full nuclear charge by the inner-shell electrons.

Explaining the differences within a group The concept of core charge is used to allow for this shielding. The effective nuclear charge felt by the outer-shell electrons, the core charge, may be found by subtracting the number of inner-shell electrons from the nuclear charge. Sodium: 11 protons and 10 inner-shell electrons = core charge of +1 Potassium: 19 protons and 18 inner-shell electrons = core charge of +1

Explaining the differences within a group The core charge remains constant within a group. This means that within a group the only factor affecting the electrostatic attraction between outer-shell electrons and the nucleus is the distance of the outer shell from the nucleus. As the atomic number increases within a group (going down the group), the attractive force between the nucleus and the outer-shell electrons decreases.

Trends Within the Alkali Metals and the Halogens Atomic radii and ionic radii increases going down groups 1 and 7 because there is an increase in the number of electron shells surrounding the nucleus as you go down these groups (indeed any group). The electron shells account for most of the volume of the atom, so an extra electron shell increases the atomic or ionic radius.

Trends Within the Alkali Metals and the Halogens Both first ionization energy and electronegativity decrease down groups 1 and 7 because of the decreasing electrostatic attraction between the outer-shell electrons and the nucleus. This is due to the increasing distance of the outer shell from the nucleus. The smaller the attraction of the outer shell to the nucleus, the easier it is to remove an electron from the outer shell (first ionization energy) and the harder it is to attract an electron to the outer shell (electronegativity).

Atomic Radii

Trends Within the Alkali Metals and the Halogens The melting points of groups 1 and 7 elements differ in the trends they exhibit. This can be attributed to the type of bonding exhibited by each group. ▫The stronger the bonding within a substance, the higher the melting point. ▫The members of group 1 are all metals and the melting point decreases down the group. ▫The group 7 elements are non-metals. As the atomic number of the halogen molecules increases, the strength of the van der Waals’ forces increases significantly and the melting point also increases.

Trends Within the Alkali Metals and the Halogens

Trends in properties within groups of the periodic table