Valence Electrons and the Second Law of Thermodynamics

Valence Electrons Atoms have: – “inner” electrons – “outer” electrons = VALENCE ELECTRONS These electrons are found in the highest energy level orbitals. Examples: – Na: 1s 2 2s 2 2p 6 3s 1 Na has 1 valence electron – S: 1s 2 2s 2 2p 6 3s 2 3p 4 S has 6 valence electrons Note: d orbital electrons do not count as valence electrons The maximum # of valence electrons an atom can have is 8 (2 “s” and 6 “p” electrons)

Wow... that’s super exciting!!! Ok, maybe not. But here’s why you should care: Physical & chemical properties of an element depend largely on its valence electron configuration! Why families have similar properties –all have the same # of valence electrons.

For example: All noble gases have eight valence electrons; all are clear, non-reactive gases. All alkali metals have one valence electron – all are soft, silver, highly reactive metals. Alkali metals Noble Gases

For Example: The “transition metals” all have the same number of valence electrons – they only differ in their numbers of d orbital electrons, which are in the inner shell. These d electron differences cause small changes in the properties of the metals but overall the transition metals can be thought of as one big family. Transition metals:

Why are the valence electrons so important? One reason is that when two atoms meet, only their outermost electrons interact – the inner electrons never meet! Therefore two atoms that have the same outer electron configuration tend to interact with other atoms the same way. So in a sense atoms are very superficial – only external appearances matter, kind of like in Hollywood. Johnny Depp and Skeet Ulrich

Valence Electrons and the 2 nd Law: Outer appearance is not the major factor in determining properties Valence electrons affect an atom’s properties because of the 2 nd Law of Thermodynamics. 2 nd Law: all systems naturally move towards their lowest energy, most stable, least organized state. – Ex – things fall down, not up. – Ex – you slump in your desk. – Ex – things fall apart as they age – Ex – your room gets messier over time. The only way to reverse this trend is to do work (expend energy) – Ex: pick something up, sit up straight, clean your room, etc.

Take-Home Message The lowest energy, most stable state for an atom is to have a full set of 8 valence electrons. This is why the noble gases are totally chemically nonreactive – they are already in their most stable state!