Energy levels and spectra

Question 1

Electrons in the atom

Answer 1

• The electrons in an atom are trapped by the electrostatic force of attraction of the nucleus.
• They move about the nucleus in allowed orbits, or shells, surrounding the nucleus.
• The energy of an electron in a shell is a constant.
• An electron in a shell near the nucleus has less energy than an electron in the shell further away from the nucleus.
• Each shell can hold a certain number of electrons.
  
  • The innermost shell can hold 2 electrons
  • The next nearest shell can only hold 8 electrons.

Question 2

What is the lowest energy state of an atom?

Answer 2

The lowest energy state of an atom is called its ground state. When an atom in the ground state absorbs energy, one of its electrons moves to a shell at higher energy, so the atom is in an excited state.

Question 3

Why is the electron configuration in an excited atom unstable?

Answer 3

• When an electron has been excited to a higher level, the atom becomes unstable due to the vacancy in the lower level.
• An electron falls back (de-excites) from a higher fixed level to the lower fixed level.
• This releases energy in the form of a photon of electromagnetic radiation.
• Energy of the photon equals the energy lost by the electron
• A photon of definite frequency is emitted.

Question 4

The gas-filled tube used to measure excitation energies emtis light when excitation occurs.

Answer 4

This happens because the the atoms absorb energy as a result of excitation by collision, but they do not retain the absorbed energy permanently.

Question 5

The energy of the emitted photon =

Answer 5

hf = E₁ - E₂.

Question 6

When the fluorescent tube is on, it emits visible light because:

Answer 6

• Ionisation and excitation of the mercury atoms occur as they collide with each other and with electrons in the tube.
• The mercury atoms emit ultraviolet photons, as well as visible photons and photons of much less energy, when they de-excite
• The ultraviolet photons are absorbed by the atoms of the fluorescent coating, causing excitation of the atoms.
• The coating atoms de-excite in steps and emit visble photons.

Question 7

Excitation using photons

Answer 7

• An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists - but only if the energy of the photon is exactly equal to the gain in the electron’s energy.
• In other words, the photon energy must be exactly equal to the difference between the final and initial energy levels of the atom.
• If the photon’s energy is smaller or larger than the difference between the two energy levels, it will not be absorbed by the electron.

Question 8

How does an atom de-excite?

Answer 8

• An atom in an excited state can de-excite directly or indirectly to the ground state.
• An atom can absorb photons of certain energies and then emit photons of the same energy or lesser energies.

Question 9

Fluorescence occurs when an object ‘glows in the dark’.

Answer 9

• The object is illuminated with ultraviolet (UV) radiation.
• Atoms within the object absorb UV photons which excite the electrons.
• When they de-excite, the atoms emit photons of visible radiation (see the diagram showing the energy level of de-excitation on the previous screen).
• Removing the UV source stops the ‘glowing’.

Question 10

What is a fluorescent tube?

Answer 10

The fluorescent tube is a glass tube with a fluorescent coating on its inner surface. The tube contains Mercury vapour at low pressure.

Question 11

When we use a tube of glowing gas as the light source:

Answer 11

We see a spectrum of discrete lines of different colours instead of a continuous spectrum.

Question 12

No other element produces the same pattern of light wavelengths.

Answer 12

This is because the energy levels of each type of atoms are unique to that atom. So the photons emitted are characteristics of the atom.

Question 13

Line spectrum

Answer 13

• Each line in a line spectrum is due to light of a certain colour and therefore a certain wavelength.
• The photons that produce each line all have the same energy, which is different from the energy of the photons that produce any other line.
• Each photon is emitted when an atom de-excites due to one of its electrons moving to an inner shell.
• If the electrons moves from energy level E₁ to a lower energy level E₂.

Question 14

the energy of the emitted photon

Answer 14

hf = E₁ - E₂