Section 2.2 Electrons in Atoms
Objective
Relate the electron to modern atomic
theory.
Compare electron energy levels in an
atom.
Illustrate valence electrons by Lewis
electron dot structures.

Electron Motion and Energy

Opposites attract…

So, why are the negative electrons not
pulled into the positive nucleus of an
atom?
Niels Bohr (1885-1962)



A Danish scientist who worked the
Rutherford solved this very question.
He proposed the the electrons have
enough energy to keep them in
constant motion around the nucleus.
He compared the motion of the
electrons to the motion of the planets
around the sun.
Electromagnetic Spectrum

Electromagnetic spectrum – the whole
range of electromagnetic radiation.






Radio waves
Microwaves
Infrared rays
UV rays
X rays
Gamma rays
Electromagnetic Spectrum
Electrons and Light


When electrons of an atom are excited
they emit light.
Emission spectrum – the spectrum of
light released from excited atoms of an
element.
Emission Spectrum of
Hydrogen
Evidence for Energy Levels




When electrons absorb energy, the only absorb the
amount needed to move to a specific higher energy
state.
It is when the electrons fall back to the lower energy
state that they emit only certain amounts of energy
and therefore only certain colors of light.
Because of this Bohr reasoned that electrons move
around the nucleus only at distances that correspond
with the those amounts of energy.
These regions of space in which electrons can move
around the nucleus of an atom are called energy
levels.
Energy Levels of Hydrogen
The Electron Cloud Model


Because of on going research scientists
today realize that energy levels are not
neat, planet-like orbits around he
nucleus of an atom.
They are spherical regions of space
around the nucleus in which electrons
are most likely to be found.
Electron Cloud

Electron Cloud – The space around the
nucleus of an atom where the atom’s
electrons are found.
Electrons in Energy Levels





The lowest energy level is the smallest and
the closest to the nucleus.
The first energy level holds a maximum of
two electrons
The second energy level hold a maximum of
8 electrons
The third energy level holds 18 electron
You can use the formula 2n2 to determine
how many electrons each level holds
Valence Electrons


Valence Electrons – the electrons in the
outermost energy level
You can use the periodic table to
determine how many valence electrons
some elements have
Valence Electrons








Group
Group
Group
Group
Group
Group
Group
Group
1 – 1 valence electron
2 – 2 valence electrons
13 – 3 valence electrons
14 – 4 valence electrons
15 – 5 valence electrons
16 – 6 valence electrons
17 – 7 valence electrons
18 – 8 valence electrons
Valence Electrons

Why is it important to know how many
valence electrons an element has?


It is the electrons of an atom that interact.
Therefore, many of the chemical and
physical properties of an element are
directly related to the number and
arrangement of valence electrons.
Oxygen
Lewis Dot Diagrams

Lewis Dot Diagram – a diagram where
dots or other small symbols are placed
around the chemical symbol of an
element to illustrate the valence
electron.
Lewis Dot
Let’s Try It

Oxygen’s Lewis Dot

Carbon’s Lewis Dot

Sodium’s Lewis Dot