3.4 Energy Levels in Atoms

Question 1

What are the electrons in an atom trapped by

Answer 1

The electrostatic force of attraction of the nucleus.

Question 2

How do electrons move about the nucleus

Answer 2

In allowed orbits or shells surrounding the nucleus

Energy of an electron in a shell is constant

Question 3

An electron in a shell near the nucleus has ______ energy than an electron in a shell further away from the nucleus.

Answer 3

Less

Question 4

Each shell can only hold a ______ number of electrons. For example, the innermost shell can only hold ______ electrons and the next nearest shell can only hold ______.

Answer 4

Certain
Two
Eight

Question 5

What is the ground state of an atom

Answer 5

The lowest energy state of an atom

Question 6

What happens when an atom in the ground state absorbs energy

Answer 6

The atom changes to being in an excited state

Question 7

What does an energy level diagram show

Answer 7

The allowed energy values of the atom

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

Question 8

What is sometimes considered as the zero reference level for energy

Answer 8

The ionisation level rather than the ground state level

Energy levels below the ionisation level would then need to be shown as negative values

Question 9

Gases at low pressure emit ______ when they are made to ______ ______.

Answer 9

Light

Conduct electricity

Question 10

Why do gases at low pressure emit light when they are made to conduct electricity?

Answer 10

The atoms absorb energy as a result of excitation by collision but they do not retain the absorbed energy permanently

Question 11

The electron configuration in an excited atom is ______ because an electron that moves to an outer shell leaves a ______ in the shell it moves from. The vacancy is then filled by an electron from an outer shell ______ to it. When this happens, the electron emits a ______. The atom therefore moves to a ______ energy level.

Answer 11

Unstable
Vacancy
Transferring
Photon
Lower

Question 12

What is the process of de-excitation

Answer 12

When an atom moves to a lower energy level as a consequence of an electron (which has filled the vacancy) having emitted a photon

Question 13

What is the energy of the photon in de-excitation equal to

Answer 13

The energy lost by the electron and therefore by the atom

Question 14

In general, when an electron moves from energy level E1 to a lower energy level E2, the energy of the emitted photon hf =

Answer 14

W1 - E2

Question 15

In what situation can an electron in an atom absorb a photon and move to an outer shell where a vacancy exists

Answer 15

ONLY if the energy of the photon is exactly equal to the gain in the electron’s energy.
Ie. photon energy must be equal to the difference between the final and initial energy levels of the atom.

Question 16

What happens if a photon’s energy is smaller or larger than the difference between the two energy levels of the atom

Answer 16

It will not be absorbed by the electron

Question 17

An atom in an excited state can…

Answer 17

…de-excite directly or indirectly to the ground state, regardless of how the excitation took place

Question 18

An atom can absorb photons of certain energies and then…

Answer 18

…emit photons of the same or lesser energies

Question 19

Why do certain substances fluoresce or glow with visible light when they absorb ultraviolet light

Answer 19

Atoms in substance absorb ultraviolet photons and become excited. When the atoms de-excite, they emit visible photons. When source of ultraviolet radiation is removed, substance stops glowing

Question 20

What is the fluorescent tube

Answer 20

A glass tube with a fluorescent coating on its inner surface. Tube contains mercury vapour at low pressure

Question 21

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

Answer 21

• Ionisation and excitation of the mercury atoms occur as they collide with each other and with electrons in the tube
• The mercury atoms mit 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 visible photons

Question 22

A fluorescent tube can produce the same light output as a filament lamp with no more than…

Answer 22

… a few watts of power wasted as heat

Question 23

How to start a fluorescent tube

Answer 23

Has filament electrode at each end
Starter unit necessary as mains voltage is too small to ionise the vapour in the tube when the electrodes are cold
When tube first switched on, argon in starter switch unit conducts and heats bimetallic strip so bends so switch closes.
Current through starter unit inc. to heat filament electrodes.
When bimetallic switch closes, gas in unit stops conducting so strip cools and switch opens
Mains voltage now acts between two electrodes now hot enough for ionisation of gas

Question 24

Difference between low-energy light bulbs and filament bulbs

Answer 24

Much less power used but same light output

Light produced by folded up fluorescent tube instead of glowing filament so less energy wasted as heat