Bohr and
Introduction
To Electron
Structure
Part I
Big Picture
Let’s look at three consecutive elements
Chlorine – atomic # 17
Argon – atomic # 18
Potassium - atomic # 19
How are they similar and different?
Chlorine
– yellow-green gas at room temperature
– Highly reactive
Argon
– Gas at room temperature
– Gas used in light bulbs
– Extremely unreactive
Potassium
– Solid metal at room temperature
– Highly reactive
Essential questions
What accounts for the element’s
differences?
What role do electrons play in all of
this?
Let’s start
by looking
back at our
atomic
theory of
the atom
Rutherford’s Model
Described the nucleus, but…
It did not explain similarity and
differences in element’s chemical
properties
It did not explain why the how the
electrons are arranged around the
nucleus.
Nor why the negatively charged electrons
did not fall into the positively charged
nucleus and collapse the atom.
6
Emission Spectra
In the early 1900’s, scientists observed
that different elements emitted visible
light when heated.
Analysis of this emitted light reveals
that chemical behavior is related to
electron arrangement.
(flame test demo)
Emission Spectra
Atoms can absorb energy, but
they must eventually release it
When atoms emit energy, it is
released in the form of light
Atoms don’t absorb or emit all
colors
The spectrum of colors emitted
can identify the element
8
Emission Spectra
Lithium chloride
Red
Sodium chloride
Yellow
Potassium chloride
Violet
Calcium chloride
Red-orange
Strontium chloride
Bright red
Line Spectra = specific wavelengths are
emitted; characteristic of atoms
10
The Bohr Model of the Atom
Bohr postulated that electrons do not fall
into the nucleus because they can only
travel in certain allowable orbits or
energy levels.
Proposed a model of the atom that
explained the light given off by the
heated or excited atoms.
When atoms are excited, electrons jump
up energy levels and then emit light as
they fall down to their ground states.
11
Bohr Model of Atom: Electron Orbits
In the Bohr Model, electrons travel in orbits
or energy levels around the nucleus
The farther the electron is from the nucleus
the more energy it has.
12
The Bohr Model of the Atom:
Orbits and Energy
Each orbit (energy level) has a
specific amount of energy
Energy of each orbit is
symbolized by n, with values of
1, 2, 3 etc; the higher the value
the farther it is from the
nucleus and the more energy
an electron in that orbit has
Tro's Introductory Chemistry, Chapter 9
13
The Bohr Model of the Atom:
Energy Transitions
Electrons can move from
a lower to a higher
(farther from nucleus)
energy level by absorbing
energy
When the electron moves
from a higher to a lower
(closer to nucleus)
energy level, energy is
emitted from the atom as
a photon of light
Tro's Introductory Chemistry, Chapter 9
14
The Bohr Model of the Atom
Ground and Excited States
Ground state – atoms with their electrons in
the lowest energy level possible; this lowest
energy state is the most stable.
Excited state – a higher energy state;
electrons jump to higher energy levels by
absorbing energy
Atom is less stable in an excited state; it will
release the extra energy to return to the
ground state
15
The limits of Bohr’s model
The Bohr Model very accurately predicts the
spectrum of hydrogen with its one electron
It is inadequate when applied to atoms with
many electrons
It has did not explain the chemical behavior
of atoms.
Even though the model is incorrect, it laid
the groundwork for future atomic models.
Ever wonder how glow sticks work?
Similar to how we use fire to excite our
atoms earlier, glow sticks use a
chemical reaction to excite electrons
The chemical reaction causes the
electrons to rise to a higher energy
level
When the electrons return to their
ground state, they emit light in the form
of chemiluminescence.
Check for Understanding
Now work with a partner to complete:
Electron Energy and Light
Bohr Atomic Models Questions