Energy Levels & Photon Emission ( 2 )

Question 1

How do electrons exist in atom?

Answer 1

• Electrons in an atom can only exist in defined energy levels, where each energy levels is given a number.
n = 1, represents the ground state.
• Electrons in an atom are trapped by the electrostatic force of attraction of the nucleus.

Question 2

What is the ground state?

Answer 2

Lowest energy state of the atom.

Question 3

How do electrons move about in the atom?

Answer 3

• Allowed orbits ( shells ) surrounding the nucleus.
• Energy of an electron in a shell is constant.
• An electron in a shell is near the nucleus has less energy than an electron in a shell further away from the nucleus.

Question 4

What happens an atom in the ground state absorbs energy?

Answer 4

• One of its electrons moves to a shell at a higher energy.

* Atom is now in an excited state.

Question 5

What does an energy level diagram show?

Answer 5

• Allowed energy values of the atom.

* Each allowed energy corresponds to a certain electron configuration in the atom.

Question 6

Why is the electron configuration in an excited atom unstable?

Answer 6

• An electron that moves to an outer shell leaves a vacancy in the shell it moves from.
• The vacancy is filled by an electron an outer shell transferring to it
• When this happens, the electron emits a photon, the atom therefore moves to a lower energy level ( de-excitation ).

Question 7

What is de excitation?

Answer 7

When an atom moves a lower energy level.

Question 8

What happens when an atom moves to a lower energy level?

Answer 8

A photon is emitted.

Question 9

What is the formula for the energy of an emitted photon?

Answer 9

hf = E1 - E2
E1 = Energy level 1
E2 = Energy level 2, lower energy level.

Question 10

When can an electron in an atom absorb a photon?

Answer 10

• An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists.
• Only if the energy of the photon is exactly equal to the difference between the final and initial energy levels of the atom.
• If the photons energy is smaller or larger than the difference between the two energy levels, it will not be absorbed by the electron.

Question 11

What is the electron volt defined as?

Answer 11

Kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt.

Question 12

What is 1 Ev in joules?

Answer 12

1.6 x 10 ^ -19

Question 13

What is the excitation, and how can it occur?

Answer 13

• Electrons can move up energy levels if they absorb a photon with the exact energy difference between the two levels.
• The movement of an electron to a higher energy level ( excitation ).

Question 14

When does ionisation occur?

Answer 14

• When an electron is removed from an atom.

Question 15

How can you calculate the ionisation energy?

Answer 15

The energy of each energy level within an atom gives the amount of energy needed to remove an electron in that level from the atom.

Question 16

What is ionisation energy?

Answer 16

The amount of energy needed to completely remove an electron from the atom from the ground state ( n = 1 ).

Question 17

What do fluorescent tubes contain?

Answer 17

• Fluorescent tubes contain mercury vapour

Question 18

What is a fluorescent tube?

Answer 18

• Glass tube with a fluorescent coating on the inner
surface.
• Inner tube contains mercury vapour at low pressure, when the tube is on, it emits visible light.

Question 19

Why does a fluorescent tube emit visible light?

Answer 19

• They contain mercury vapour, which when an initial high voltage is applied, high voltage accelerates fast moving free electrons that ionise some of the mercury atoms, producing more free electrons.
• Flow of free electrons collides with other mercury atoms, they are excited to higher energy levels.
• When these excited atoms return to their ground states they emit photons in the UV range.
• A phosphorus coating on the inside of the tube absorbs these photons, exciting its electrons to much higher orbits.
• Electrons cascade down the energy levels, emitted many lower energy photons in the form of visible light.
• Coating atoms de-excite in steps and emit visible photons.

Question 20

What is an advantage of fluorescent tubes over filament lamps?

Answer 20

More efficient