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Electron Arrangement in Atoms. Section 5.2. Electron Configuration. The arrangement of electrons into specific energy levels and orbitals around the nucleus. What role does energy and stability play in the way electrons are arranged in an atom?.
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Electron Arrangement in Atoms Section 5.2
Electron Configuration The arrangement of electrons into specificenergy levelsandorbitalsaround the nucleus.
What role does energy and stability play in the way electrons are arranged in an atom? • In an atom, the electrons and the nucleus interact to make themost stablearrangement possible. • How? Electrons occupy the lowest possible energy orbitals first. • Change always proceeds toward the lowest possible energy state. • High energy systems areunstable(i.e.unfilledorincompleteenergy levels or orbitals)
Aufbau Principle • Rule #1 - Electrons enter orbitals of lowest energy first. • Lowest energy state is called theground state.Highest energy state is called theexcited state. • Energy levels can overlap adjacent energy levels. • Aufbau meansbuilding up.
Orbital Diagram Electron Configuration Aufbau Principle H = 1s 1s1
Aufbau Principle energy Electrons filllow energy orbitals (closer to the nucleus) before they fill higher energy ones www.colorado.edu/physics/2000/applets/a2.html
Writing Electron Configurations Aufbau Principal: Fill subshells in order by counting across periods, from hydrogen up to the element of interest
Pauli Exclusion Principle Rule #2 - An atomic orbital may hold a maximum of two electrons. • A vertical arrow is used to indicate an electron. • In order for the two electrons (with negative charges) to occupy the same orbital they must have opposite spins,clockwise (up arrow)andcounterclockwise (down arrow).
Pauli Exclusion Principle For example: 1s 2s Be = 1s 2s 2 2 The firstelectron in an orbital is represented with an“up” arrowor clockwise arrow.
Hund’s Rule Rule #3 - When electrons fill orbitals of equal energy, one electron will enter each orbital until all the orbitals contain one electron (all “up” or clockwise spins). • Then, electrons with opposite spin fill each orbital(all “down” or counterclockwise). In other words… for p, d and f orbitals:First fill all orbitals “up”, then fill “down”
Unpaired electrons Hund’s Rule 2s 2p 1s C = 2 2 2 1s 2s 2p
Unpaired electron Hund’s Rule 2s 2p 1s F = 2 2 5 1s 2s 2p
valenceelectrons 4 valenceelectrons 7 Valence electrons An atom’s valence electrons are the electrons in the highestenergylevel of an atom. C = 1s2 2s2 2p2 F = 1s2 2s2 2p5
valenceelectrons 2 Valence electrons Ti = 1s2 2s2 2p6 3s2 3p6 4s2 3d2 Why aren’t the 3d 2 electrons considered valence electrons? An atom’svalence electrons are the electrons in the highestenergylevelonly of an atom.
