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Electrons in Atoms
Quantum Mechanical Model
Atomic Models
• John Dalton thought atoms were indivisible….turns out that they
are divisible as evidenced by subatomic particles.
– Subatomic particles disprove a rigid atom
• Joseph John Thomson (1856-1940) gets credit for the discovery of
the electron.
– “plum pudding atom”
– Didn’t account for protons and neutrons and other atomic properties
• Ernest Rutherford gets credit for the discovery of the nucleus.
– Didn’t account for protons and neutrons in the nucleus
• Niels Bohr – electrons are arranged in circular paths, or orbits,
around the nucleus.
– Does not account for unequal spacing of energy levels in an atom.
Quantum Mechanical Model
• Atom has no definite shape and electrons do
not have precise orbits.
– Energy level – region around the nucleus where
the electron is most likely to be moving.
– Quantum – amount of energy required to move
an electron from its present energy level to the
next higher level.
• The higher the energy level of an electron, the easier it
is for the electron to escape from the atom.
Quantum Mechanical Model
• Erwin Schrödinger (1887 – 1961) wrote and
solved a mathematical equation describing
the location and energy of an electron in a
hydrogen atom.
– Lead to the quantum mechanical model.
• Mathematical model, not a visible model.
Atomic Orbitals
• The quantum mechanical model designates
energy levels of electrons = principal quantum
numbers.
• Each principal quantum number refers to a
major or principal energy level
– n = 1, 2, 3, 4 (distance from the nucleus of the electron
increases with increasing n value)
– Within each n level there are energy sublevels
• Table 12.1 (pg.327)
Table 12.1: Energy levels, Sublevels,
and Orbitals
Principle Energy Level
Number of sublevels
Type of sublevel
N=1
1
1s (2 electrons)
N=2
2
2s, 2p, (8 electrons)
N=3
3
3s, 3p, 3d (18 electrons)
N=4
4
4s, 4p, 4d, 4f (32 electrons)
() = maximum number of electrons in that principal energy level.
Atomic Orbitals Defined
• = cloud shapes denoted by letters in the
quantum mechanical model. (fig. 12.2, pg.327)
– S = spherical
– P = dumbbell shaped, nodes.
– d and f orbitals are complex.
Electron Configurations
• = The ways in which electrons are arranged
around the nuclei of atoms.
1. Aufbau principle = electrons enter orbitals of
lowest energy first.
2. Pauli exclusion principle = an atomic orbital
may describe at most 2 electrons. (keep in mind
the number orbitals at each energy level, i.e. 2p has 3
orbitals)
3. Hund’s Rule = electrons enter orbitals of the same
energy level 1 orbital at a time before filling in the
second.
Pg. 328; 5 a-f
Pg. 332; 8-9
Unit Assignment Part 1
• Pg. 346; 24, 26-35 (due 3/14)