Chapter 2 Matter is Made of
Atoms
2.2 Electrons in Atoms
Objective
Objectives: Relate the electron to
modern atomic theory, Compare
electron energy levels in an atom,
Illustrate valence electrons by Lewis
electron dot structures
Electrons in motion
Electrons are in motion in the outer part
of an atom
 When two atoms come near, these
electrons interact
 Electrons and their arrangement in the
atom greatly affect an element’s
properties and behavior

Electrons in motion
Niels Bohr (early 20th century)- Proposed e-‘s
must have enough energy to keep them in
constant motion around the nucleus (enables
e-‘s to overcome the attraction of the positive
nucleus)
 Bohr’s model (1913)- Planetary model
The well-defined orbits are not correct.
Electron motion within an atom is irregular and
an electron may be close to the nucleus at
times.
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Electromagnetic spectrum
Electromagnetic radiation (radiant energy)
travels in the form of waves- have both
electrical and magnetic properties
 Electromagnetic waves can travel through
empty space
Ex: radiant energy from the sun travels
to Earth

Electromagnetic spectrum

Electromagnetic waves travel through
space at the speed of light, approx. 300
million meters/sec
Electromagnetic spectrum

Properties of waves: Frequency and wavelength
Frequency-number of vibrations/ second (Hertz-Hz)
Wavelength- distance between two corresponding
points on two consecutive waves
Wavelength is determined by frequency
Low frequency = long wavelength (low energy)
High frequency = short wavelength (high energy)

Electromagnetic spectrum
Electromagnetic spectrum

All the forms of radiant energy are parts
of a whole range of electromagnetic
radiation

The electromagnetic spectrum includes
gamma rays, X rays, ultraviolet (UV),
Visible, Infrared (IR), microwaves, radio
waves
Electromagnetic spectrum
Electromagnetic spectrum
Gamma Rays- highest frequency, shortest
wavelength, most energetic waves, can
pass through most substances
 X rays- lower frequency than gamma, but
still high energy waves, can pass through
soft body tissue, but stopped by bone
 Ultraviolet-slightly more energetic than
visible, can cause sunburns, mostly
absorbed by ozone

Electromagnetic spectrum

Visible- part of the spectrum to which
our eyes are sensitive, Our eyes and
brains interpret different frequencies as
different colors
ROY G BIV - Newton
Red- low energy
Violet- high energy
Electromagnetic spectrum

Infrared- less energy than visible, given off
by most “warm objects”

Microwaves/ Radio waves- lowest
frequency
Electrons and Light

EMISSION SPECTRUM: Spectrum of light released from
excited atoms of an element

Electrons can have only certain amounts of energy

Electrons can move around the nucleus only at distances that
correspond to those amounts of energy

When e- absorb energy, they move to a higher energy state

When they fall back to a lower energy state- they emit
energy (Light)
Electrons and Light
ENERGY LEVEL: Region of space in which
e-‘s can move about the nucleus of an atom
ELECTRON CLOUD: Space around the
nucleus of an atom where the atom’s e-‘s are
found
Each energy level can hold a limited number of
electrons
http://www.youtube.com/watch?v=QI50GBUJ4
8s&safety_mode=true&persist_safety_mode=1
&safe=active
# of electrons in each level

Lowest energy level is the smallest and
closest to the nucleus, Holds two
electrons max

Second energy level is larger because it is
farther away, Holds eight electrons max

Max # of e-‘s: 2, 8, 8,18, 32, 50, 78, 91
# of electrons in each level

Bohr Model Examples:
H-1 (1 p, 1 e, 0 n)
Li-7 (3 p, 3 e, 4 n)
0-16 (8 p, 8 n, 8 e)
VALENCE ELECTRONS

The electrons in the outermost energy level

Valence electrons are important because
they interact when atoms come near each
other

Chemical and physical properties of an
element are directly related to the number
and arrangement of valence electrons (v.e.)
VALENCE ELECTRONS

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Atoms in group 1 have one valence electron
Atoms in group 2 have two valence electrons
Atoms in group 13 have three valence electrons
Atoms in group 14 have four valence electrons
Atoms in group 15 have five valence electrons
Atoms in group 16 have six valence electrons
Atoms in group 17 have seven valence electrons
Atoms in group 18 have eight valence electrons
LEWIS DOT DIAGRAM

Illustrates valence electrons as dots around
the chemical symbol of an element.

Each dot represents one valence electron
(max # of v.e. is 8)

The symbol represents the core of the atom.
Ex:
.
.
Li 
Be 
B
LEWIS DOT DIAGRAM
Li has 2 e- in the first energy level and 1 in
the second,
 Be has 2 e- in the first energy level and 2
in the second,
 B has 2 e- in the first energy level and 3 in
the second

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More examples: C, N, O, F, Ne