Concise Summary on Electron Configuration and the Periodic Table

Electron Configuration: At the end of each period, the pattern of electron filling repeats; e.g.,

• F: [He] 2s²2p⁵

• Cl: [Ne] 3s²3p⁵

• Br: [Ar] 4s²4p⁵.

Core and Valence Shells:

• Valence electrons (in the shell with the highest n) are involved in chemical reactions.

• Core electrons (lower n values) are chemically unreactive.

Development of the Periodic Table:

• 1864: John Newlands observed properties repeating every eight elements (Law of Octaves).

• 1869: Mendeleev and Meyer proposed a periodic arrangement of elements based on properties: termed periodicity.

• The modern table is arranged by atomic number; similar properties are grouped vertically.

Effective Nuclear Charge:

• Nuclear charge (Z) = number of protons.

• Effective nuclear charge (Z_eff) is experienced by shielded electrons, taking into account core electron shielding.

• Z_eff increases across a period due to increasing protons but constant core electrons.

• Formula: ( Z_{eff} = Z - S ), where S = shielding constant.

Periodic Trends:

• Atomic Radius: Decreases across a period (more nuclear attraction) and increases down a group (higher n).

• Ionization Energy: Energy required to remove an electron; increases across a period and generally decreases down a group.

  • Formula: ( IE = E{final} - E{initial} )

• Electron Affinity: Energy change when an atom accepts an electron; tends to increase across a period.

  • Formula: ( EA = E{final} - E{initial} )

• Metallic Character: Tends to increase down a group and decrease across a period.

Classification of Elements:

• Main Group: Groups 1, 2, and 13-18.

• Transition Metals: Groups 3-12 with incompletely filled d subshells.

• Noble Gases: Group 18, very stable due to filled outer electron shells.

Chemical Properties:

• Group trends influenced by valence electron configurations and underlying atomic structure (metallic vs non-metallic character).

• Different trends observed for Group 1 (alkali metals) and Group 2 (alkaline earth metals) due to varying reactivity and charge behavior in ions.