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Electrons in Atoms
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Name: ______________________________
The chemical properties of atoms, ions, and molecules are related to the arrangement of electrons within them.
Over time, the atomic model has evolved regarding where electrons are located.
Stages of Atoms
 Democritus and Dalton: Atom is indivisible = no electrons
 Thomson’s model (Plum Pudding): described the atom as a ball of positive charge containing randomly placed
electrons.
 Rutherford’s model: Electrons are still randomly placed, but now there's a positive nucleus.
The Next Step:
 Max Planck's Quantum Energy Theory
o Energy is always absorbed or emitted in "packets" which he called quanta
 A ‘Photon” is a quantum of light
 Example:
 Bohr's Model (Planetary)
o Bohr used quantum energy theory and proposed that electrons move in particular paths with fixed
energy in "orbits" around the nucleus.
In Bohr's Model…
 Electrons cannot exist between energy levels, but can move from one level to another.
o Think of a ladder and the electrons can only jump from rung to rung.
o To move from one energy level to another one, an electron must gain or lose just the right amount of
energy.
o This amount of energy is called a quantum of energy.
o Bohr's model was only partially successful.
Two other contributions led to a new atomic theory.
 Louis de Broglie illustrated that electrons have a dual nature with properties of both particles and waves.
 Heisenberg's Principal of Uncertainty stated that it was impossible to know both the momentum and the
position of an electron.
Modern Atomic Theory
 Schrodinger's Quantum Mechanical Model is the atomic model in use today.
 It was able to explain many previously unexplainable phenomena.
 Restricts energy of electrons to certain values (same as Bohr).
 Does NOT define an exact path around the nucleus (unlike Bohr).
 Estimates the probability of finding an electron in a certain position (Heisenberg).
Quantum Numbers: Where is the electron??
 In Schrodinger's model, there are four "quantum" numbers that tell us where an electron is likely to be located.
o Principal (n), 1-7 gives the energy level.
o Subshell (l), s-p-d-f, gives the shape of the region of space.
o Orbital (m), gives us the orientation in space of the shapes.
o Spin (s): clockwise or counterclockwise
 No two electrons have the same four quantum numbers.
Principal Quantum Number (n)
 Designates the energy level.
 Tells how far away from the nucleus an electron is likely to be.
 Assigned values in order of increasing energy. (n = 1, 2, 3, 4, 5, 6, or 7)
 Corresponds to the rows/periods on the periodic table.
 Lower value of n means the electron is closer to nucleus and has less energy.
Subshell (l), s-p-d-f (Shapes) [0,1,2,3]
 Within each principal energy level there are a certain number of sublevels
 Energy level 1 has 1 sublevel; level 2 has 2; level 3 has 3 etc.
 These sublevels are called s, p, d, f.
 Each sublevel has a unique shape
 These shapes describe the region of space where an e- is likely to be located.
Orbitals (m)
 Within each sublevel there are a certain number of orbitals
 s sublevel: 1 orbital; p sublevel: 3 orbitals; d sublevel: 5 orbitals; f sublevel: 7 orbitals
 Each orbital can only hold 2 electrons.
 Orbitals within each sublevel have similar shapes, but different orientations in space
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s orbitals have a spherical shape.
o Hold a maximum of 2 electrons.
o First found in the 1st energy level
Draw:
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p orbitals have a pear or dumbbell shape
o 3 different orientations
o Each orientation holds a max of 2 electrons.
o The p sublevel as a whole holds a maximum of 6 electrons.
o First found in the 2nd energy level.
Draw:
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d-orbitals have five different orientations
o Each holds 2 electrons.
o The d sublevel as a whole holds a max of 10 electrons.
o First found in the 3rd energy level
Draw:
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f-orbitals do not appear until the 4th shell (energy level) and higher.
Maximum e-s Allowed
 Max # of e-s that can occupy a principal energy level is given by the formula 2n2.
 How many electrons are allowed in energy level 3? __________________
Let's Review
How many orbitals are in the following sublevels?
a) 3p ________
b) 2s ________ c) 4f ________
d) 4p ________
e) 3d ________
Electron Configuration: How are the electrons in any given element arranged?
 In an atom, electrons are positioned around the nucleus in the most stable arrangement possible.
 This is the arrangement that has the lowest energy.
 There are three important rules to help us find the configuration of electrons in atoms.
Aufbau’s Principal
 Electrons will occupy orbitals having the lowest energy first, and then in order of increasing energy.
 The “ground state” of an atom is when every e- is at its lowest energy.
Pauli’s Exclusion Principal
 An orbital can be empty, have 1 e-, or AT MOST have two electrons.
 Electrons occupying the same orbital must have opposite spins—one clockwise, one counterclockwise.
Hund’s Rule
 When e-s occupies orbitals of equal energy, 1 e- enters each orbital until all the orbitals contain 1 e- with parallel
spins.