Electron Configuration and the Periodic Table

• The period number determines the value of n
• Groups 1A and 2A have the s−orbital filled
• Groups 3A − 8A have the p−orbital filled
• Groups 3B− 2B have the d−orbital filled

Examples

• Locate the element in the periodic table.
• Find the closest noble gas for the element (Atomic number ofthe inert gas should be less than that of the element in question.
• Subtract the atomic number (electrons) of the inert gas from that of the element in question.
• Arrange the remaining electrons in the following orbitalsusing the period number as an indication for that.

The closest noble gas for ₇₈Pt is ₅₄Xe.
The difference in the number of electrons is 78-54 = 24 electrons.
→ ₇₈Pt: [₅₄Xe], + 24


Our element is in period 6 (i.e. n=6) , so we fill the 6s with 2 electrons, (i.e. 6s²) leaving 22 electrons remaining.
→ ₇₈Pt: [₅₄Xe]6s2, + 22


Right after that, there is a gap to accommodate the 14 electrons of 4f orbitals, so we take 14 electrons of what we have for this reason (i.e. 4f¹⁴), leaving 8 electrons.
→ ₇₈Pt: [₅₄Xe] 6s2, + 4f¹⁴, +8


The remaining 8 electrons will fill the (n-1) d orbitals, (i.e. 5d⁸)
→ ₇₈Pt: [₅₄Xe] 6s2, + 4f¹⁴, 5d²
Which is the condensed electronic configuration of ₇₈Pt.

Exercises