Ions: Cations and Anions Formation

• Cations and anions form when one or more electrons are removed from or added to the highest occupied energy level.
• This process results in a stable outer configuration of electrons, typically with 8 valence electrons.

Formation of Cations

• Group 1A, 2A, and 3A elements achieve stability by losing electrons, forming positive ions termed cations.
• Group 1A elements have a valence of +1
• Group 2A elements exhibit a valence of +2
• Group 3A elements exhibit a valence of +3 as they lose one, two, and three electrons respectively, in the outer level.

Formation of Anions

• Group 5A to 7A elements attain stability when they gain electrons, forming negative ions called anions.
• Elements of the nitrogen group (5A) have a valence of -3
• Elements of the oxygen group (6A) have a valence of -2
• Group 7A halogens have a valence of -1 as their atoms gain 3, 2, and 1 electrons, respectively.
• Group 8A elements have eight valence electrons and zero valences, so they are unlikely to give up or take electrons.

Atomic Size

• Atomic size, or atomic radius, is the distance between the nucleus and its valence shell.
• Atomic size decreases from left to right across a period and increases down a group in the periodic table.
• Across a period, the increasing number of protons leads to a greater attraction between electrons and protons.
• The increasing nuclear charge pulls the electrons towards the nucleus, reducing the atomic size.
• Down a group, the number of energy levels increases, the atomic size increases.
• Inner-filled energy levels diminish electrostatic attraction between protons and electrons.
• This results in reduced effective nuclear charge and expansion of the atom.

Metallic Property

• Metallic property is the measure of an atom's ability to donate electrons.
• Metallic property decreases from left to right across a period and increases down a group which mirrors atomic size trends
• Smaller number of valence electrons and greater distance from the nucleus contribute to higher metallicity.

Ionic Size

• When a neutral atom gains or loses electrons, the nuclear charge remains constant, but electron-electron repulsion changes.
• In cation formation, the loss of electrons reduces electron-electron repulsions.
• In anion formation, the opposite occurs such that the electron-electron repulsions increase.
• Cations are smaller than their corresponding atoms while anions are bigger.

Ionization Energy

• Energy is required to remove electrons from an atom is termed ionization energy.
• Ionization energy increases from left to right across a period and decreases down a group.
• From top to bottom, the energy level and average distance of a valence electron from the nucleus increases, attraction weakens, and the electron is easier to remove.
• Across a period, the effective nuclear charge increases, strengthening the hold on electrons, and hence more energy is required to remove the electron.

Electron Affinity

• Electron affinity is the energy released when an electron is added to a neutral gaseous atom.
• The greater the electron affinity, the greater the attraction of an atom for an electron.
• Electron affinity increases from left to right within a period and decreases from top to bottom within a group.
• Elements on the left side of the periodic table exhibit a lower affinity for the incoming electrons whereas elements on the right side have a higher affinity.
• Group 7A elements possess the greatest electron affinity because they only need one electron to complete a stable configuration of 8 electrons.
• Noble gases have almost zero electron affinity because they are already stable and not expected to take electrons.