Download Electron configuration of atoms

Electron configuration of
atoms
Shells, sub-shells and orbitals
Syllabus point: 2.2.1 a,b,c,d
HSW 1 & 7
A level SYLLABUS
Bohr Model of the Atom
• Recall that Hydrogen’s atomic emission spectrum is
discontinuous, or made up of only certain
frequencies of light.
• Why?
• Niels Bohr proposed a quantum model for Hydrogen
that seemed to answer this question.
Bohr’s Model Cont.
•Bohr built on Planck’s and
Einstein's concepts of quantized
energy (only certain values are
allowed).
•Bohr proposed that the Hydrogen
atom only has certain allowable
energy states.
•The lowest allowable state was
termed: ground state.
Bohr’s Model Cont.
•When an atom gains energy, it is said
to be in an excited state.
•Even though Hydrogen contains only
one electron, it can have many
excited states.
•Bohr continued to expand his model
by stating that electrons found closer
to the nucleus had less energy than
electrons found at greater distances
from the nucleus.
Bohr’s Model Cont.
Bohr assigned a
quantum number,
n, to each
shell/energy level:
• n1
• n2
• n3
• Etc.
http://www.biologydaily.com/biology/upload/thumb/d/de/307px-Bohratommodel.png
Bohr’s Explanation of Hydrogen’s
Spectrum
• When a Hydrogen atom is in the
ground state, n=1, it does not
radiate any energy.
• When energy is added from an
external source, the electron
moves to a higher energy orbit.
• The atom is now in an excited
state.
• The electron then drops to a
lower state and emits a photon
corresponding to the difference
between the energy levels of the
two orbits.
How can we apply this theory to
explain atomic spectra?
We know that white light can be split into a spectrum
by passing it through a prism.
After the sunlight had been broken down into its
components by one prism, if a narrow ray of the light
from the first prism was passed through another prism
there would be no further breakdown.
Joseph von Fraunhofer is best known for his discovery of the dark
absorption lines known as Fraunhofer lines in the Sun's spectrum, and for
designing achromatic telescope objectives.
In 1814, he developed a spectroscope to study the
spectrum of the light given off by the sun. He was
amazed to discover that in the midst of the rainbow of
colors was a series of black lines.
Joseph von Fraunhofer
(March 6, 1787 – June 7,
1826)
These dark lines were later determined to be the
result of the absorption of selected frequencies of the
electromagnetic radiation by an atom or molecule.
Bunsen and Kirchhoff further developed the spectroscope by incorporating
the Bunsen burner as a source to heat the elements. In 1861, experiments
by Kirchhoff and Bunsen demonstrated that each element, when heated to
incandescence, gave off a characteristic color of light. When the light was
separated into its constituent wavelengths by a prism, each element
displayed a unique pattern or emission spectrum.
Emission Spectra Complement Absorption Spectra
The emission
spectrum seemed
to be the
complement to the
mysterious dark
lines (Fraunhofer
lines) in the sun's
spectrum.
This meant that it was now possible to identify the chemical composition of
distant objects like the sun and other stars.
They concluded that the Fraunhofer lines in the solar spectrum were due to
the absorption of light by the atoms of various elements in the sun's
atmosphere.
Each element has an
unique spectrum……..
Sample
injected
into flame
Light of
specific
wavelengths
emitted
The light
emitted is
analysed by
splitting it
into its
component
colours
Atomic Emission Spectra
•When electricity is passed
through a tube of gas, the
atoms in the tube absorb
energy and become excited.
•The atoms release the energy
absorbed in the form of light.
•Each atom has specific
frequencies it will release in
the light form.
Examples of emission spectra
 Hydrogen
 Oxygen
 Sodium
 Potassium
Flame Tests
 Flame Test: A test used in the identification of certain elements.
 It is based on the observation that light emitted by any element
gives a unique spectrum when passed through a spectroscope.
Flame spectrum for lithium.
(Notice the faint bands of color in the spectra.)
Analysis and
Detection
Using Atomic Emission
Spectroscopy
Development
Aided by the rapid progression
in technologies such as
electronics and computing.
Modern machines will do the
analysis and interpret the results.
Atomic Emission Spectroscopy
Advantages:
 Very quick
 Automated process
 Only requires a small sample,
 the brightness of the spectrum will
determine how much sample there is
(concentration)
Atomic Emission Spectroscopy
Disadvantages:
 Destructive – the sample being tested is
burned!
 Only identifies the presence of elements –
does not identify compounds.
 Usually need spectra of known compounds to
compare to unknown results
 Analytical machines can be expensive and only
suitable for particular tasks
Applications
quality control in chemical manufacture
e.g. potassium compounds often have
sodium contamination – atomic emission
spectroscopy could determine if any sodium
was present and how much.
Working out the composition of distant stars
Applications
Detection of alcohol, drugs or metal
ions in the blood
Detect impurities in products that
need to be pure, such as drug
molecules and alloys
Detect pollutants in water, soil and air
However………..
•It was eventually
found that Bohr was
incorrect.
•Electrons do not
travel in circular
orbits around the
nucleus.
•Poor Bohr!