Physics 108 Lecture 27: Atomic Spectra

1
Physics 108 Lecture 27 Atomic Spectra
Emission and Absorption Spectra

• Each atom has a unique set of EM radiation
  (light) wavelengths it can produce.
• EM radiation is absorbed by atoms when electrons
  are shifted to higher level orbitals.
• EM radiation is emitted by atoms when electrons
  fall back into their normal lower level orbitals.
• Absorption and emission spectra are the sets of
  wavelengths absorbed or emitted by each type of
  atom. Many of these wavelengths are in the
  relatively high energy range of X-rays.
• Atoms with all electrons at the lowest level
  orbitals are said to be in the
• ground state.
• Atoms with some electrons shifted into higher
  level orbitals than normal are said to be in an
  excited state.

2
Visible Spectra (note pictures page 887 in text)

• Visible light is produced when energy is absorbed
  by a substance and then re-emitted in the form of
  EM radiation.
• Visible light is not of particularly high energy,
  so the jumps between energy levels that produce
  visible light are relatively small jumps.
• Each line in a given spectrum represents a jump
  between specific energy levels.

Demos Diffraction Gratings for analyzing spectra
3
Electron Shells - Definitions

• Electron Shells are the grouping of electrons in
  the rows as you go down the periodic table.
• These shells roughly represent increasing average
  distances of electrons from the nucleus.
• Each time you go down a row in the periodic
  table, a new shell is being filled. (note that
  shells are not always completely filled before
  electrons start filling the next shell. This is
  due to the order of filling the orbitals.)
• Labeling Shells
• o       K holds 2 electrons (1s)
• o       L holds 8 electrons (2s 2p)
• o       M holds 18 electrons (3s 3p 3d)
• o       N holds 32 electrons (4s 4p 4d
  4f)
• o       O holds 50 electrons (5s 5p 5d
  5f 5g)
• o       P holds 72 electrons (6s 6p 6d
  6f 6g 6h)

Recall Orbital Filling Order
1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p7s5f6d Electrons
are added according to this order
4
Electron Shells - Examples
Examples Write down how many electrons are in
each shell for the following elements Ca, Ba,
W, Pu. First write the electron configurations,
then fill in the table.
Electron configurations
5
X-Ray spectra and their relation to shells X-Ray
spectra are produced when high energy projectile
electrons from an accelerator (X-Ray tube)
bombard a target containing large atoms (usually
a heavy metal like W)
M Shell
L Shell
K shell
W
Target electron
6
X-Ray spectrum components

• Characteristic X-Rays spiky peaks
• o  Produced when projectile electrons hit
  electrons in target atoms strongly
• enough to cause a jump between shells.
• o  When electron in atom jumps down to lower
  shell it emits an X-Ray
• o  K edges removal from K shell or jump
  to K shell
• o  L, M, N. edges also exist, but not
  important for X-Ray production

• Bremsstrahlung X-Rays smooth curve between
  spikes
• o  Produced when projectile electrons have
  collisions with target atoms that do not
  dislodge an electrons from a shell.
• o  Energy lost by the projectile electron in
  collision is emitted as X-ray
• o  Low energy X-rays come from weak
  collisions with other electrons
• o  High energy X-rays come from strong
  collision like with a nucleus