Chapter 5
Electrons in Atoms
Bohr model of the atom
Valence electrons are
the electrons in the
outer energy level of
an atom. No atom
has more than eight
electrons in its outer
level. Atoms with
eight valence
electrons are
exceptionally stable.
The Bohr model of the atom is not correct!!
There is substantial evidence that indicates
electrons do not move around the nucleus in
perfect circular orbits.
Heisenberg Uncertainty
Principle

Heisenberg concluded that it is impossible
to make any measurement on an object
without disturbing the object.

The Heisenberg Uncertainty Principle
states that it is fundamentally impossible to
know precisely both the velocity and
position of a particle at the same time.
Quantum Mechanical Model of
the Atom

Erwin Schrodinger (1887-1961) derived an
equation that treated the hydrogen atom’s
electron as a wave. This wave mechanical
model is also known as the Quantum
Mechanical Model of the atom, and is
what is used today.

The wave function predicts the three
dimensional region around a nucleus called
an atomic orbital which describes an
electron’s probable location.

Principal energy levels are the atoms
major energy levels. An atom’s lowest
principal energy level is assigned a principal
quantum number of 1.
Principal energy levels contain energy
sublevels. Principal energy level 1 contains
2 sublevel; principal energy level 2 contains
2 sublevels; principal energy level 3
contains 3 sublevels, and so on.
Do energy levels really have
energy?

The energy associated with an energy level
increases as the distance from the nucleus
increases.
 The lower the number of the principal
energy level, the closer the negatively
charged electron in it is to the positively
charged nucleus and the more difficult it is
to remove this electron from the atom.
Energy sublevels

Sublevels are labeled s, p, d, or f according
to the shapes of the atom’s orbitals. All ‘s’
sublevels are spherical and all ‘p’ sublevels
are dumbbell shaped.

Each principal energy level has one sublevel
containing one orbital, an s orbital, that can
contain a maximum of two electrons. Electrons in
this orbital are called s electrons and have the
lowest energy of any electrons in that principal
energy level. The first principal energy level
contains only an s sublevel; therefore, it can hold a
maximum of two electrons.

Each principal energy level above the first
contains one s orbital and three p orbitals. A
set of three p orbitals, called the p sublevel,
can hold a maximum of six electrons.
Therefore, the second level can contain a
maximum of eight electrons - that is, two in
the s orbital and 6 in the three p orbitals.

Each principal energy level above the
second contains, in addition to one s orbital
and three p orbitals, a set of five d orbitals,
called the d sublevel. The five d orbitals can
hold up to 10 electrons. Thus, the third level
holds a maximum of 18 electrons: 2 in the s
orbital, 6 in the three p orbitals, and 10 in
the five d orbitals.

The fourth and higher levels also have an f
sublevel, containing seven f orbitals, which
can hold a maximum of 14 electrons. Thus,
the fourth level can hold up to 32 electrons:
2 in the s orbital, 6 in the three p orbitals, 10
in the five d orbitals, and 14 in the seven f
orbitals.

One further, important note about orbital
shapes: These shapes do not represent the
path of an electron within the atom; rather,
they represent the region of space in which
an electron of that sublevel is most apt to be
found. Thus, a p electron is most apt to be
within a dumbbell-shaped space in the
atom, but we make no pretense of
describing its path.

Electron configuration is the arrangement
of electrons in an atom. Because lowenergy systems are more stable than highenergy systems, electrons in an atom tend to
assume the arrangement that gives the atom
the lowest possible energy. The most stable,
lowest-energy arrangement of the electrons
in atoms of each element is called the
element’s ground-state electron
configuration.
The Aufbau principle

Each electron occupies the lowest energy
orbital available. Following is an aufbau
diagram
 In order of increasing energy, the sequence
of energy sublevels within a principal
energy level is s, p, d, and f
Aufbau diagram
The Pauli exclusion principle

A maximum of two electrons may occupy a
single atomic orbital, but only if the
electrons have opposite spins.
 A toy top can spin only 2 directions, similar
to electrons in atomic orbitals, they can spin
only one direction.
Hund’s rule

Single electrons with the same spin must
occupy each equal-energy orbital before
additional electrons with opposite spins can
occupy the same orbitals.
Back to Valence Electrons…

Only certain electrons, called valence
electrons determine the chemical properties
of an element. Valence electrons are
defined as electrons in the atom’s outermost
orbitals, generally those orbitals associated
with the atom’s highest principal energy
level.
Electron Dot Structures

This consists of an elements symbols, which
represents the atomic nucleus and innerlevel electrons, surrounded by dots
representing the atom’s valence electrons.
 The American chemist G.N. Lewis (18751946) devised the method while teaching a
college chemistry class in 1902.
Example of Lewis dot
