Electrons and Energy Levels

Question 1

What is a photon?

Answer 1

• It is the name given to a discrete packet of electromagnetic energy
• A photon has no mass or charge and is described by its energy wavelength or frequency

Question 2

What is a quantum of energy?

Answer 2

A quantum of energy is a small packet of energy

Question 3

What is the energy of photons proportional to?

Answer 3

• The energy of a photon is proporitonal to its frequency

- A photon of light carries less energy than an X-ray photon because the photon of light has a lower frequency

Question 4

Does an X ray or Radio Wave have a higher frequency?

Answer 4

X-ray has higher frequency than Radio wave

Question 5

Does an X ray or Radio Wave have a longer wavelength?

Answer 5

Radio wave has longer wavelength than X-ray

Question 6

How is the energy carried by each photon calculated?

Answer 6

E=hf or E=hc/lamda

Question 7

What is the intensity of EM radiation?

Answer 7

It is the energy transferred per unit time per unit area

Question 8

How does light source affect intensity?

Answer 8

1. When the light source is made more powerful, more photons are transferred per second
   - E.g. The intensity from a 100W bulb is greater than the intensity from a 10W bulb (if all other factors remain the same).

Question 9

How does photons affect intensity?

Answer 9

1. When each photon transfers more energy
   - E.g. A beam of ultraviolet photons is MORE powerful than a beam transferring the same number of infrared photons per second

Question 10

How does area affect intensity?

Answer 10

1. When the light is incident on a smaller area
   - E.G move closer to light source more light enters your eye each second and you sense a greater intensity of light
2. Intensity follows an inverse square Lae, so the intensity of light measured from a light source quadruples if you half the distance away from the same light source

Question 11

What happens to electrons inside atoms?

Answer 11

• Electrons inside atoms can absorb photons and gain energy

- They may gain enough energy to move further from the nucleus into a higher energy level or leave the atom all together

Question 12

What is an electron volt?

Answer 12

The unit of energy equal to 1.6 x 10^-19 J and it is the energy gained by an electron when it is accelerated through a potential difference of 1 volt

Question 13

How is work done in electron volts calculated?

Answer 13

W (energy transferred in electron volts) = Q (electron change (1.6 x 10^-19) x V (potential difference in volt

Question 14

How do you convert from joules to eV?

Answer 14

Divide J by 1.6 x 10^19

Question 15

How do you convert from eV to joules?

Answer 15

Multiply eV by 1.6 x 10^-19

Question 16

What happens to an electron when it absorbs a photon?

Answer 16

• An electron moves further from the nucleus if it gains the right amount of energy by absorbing a photon
• The electrons moves to higher energy levels

Question 17

What happens when an electrons drops from a higher energy level to a lower energy level?

Answer 17

-It loses its surplus energy by emitting a photon and moves closer to the nucleus

Question 18

Why does an electron have less energy closer to the nucleus?

Answer 18

• An electron in an atom gains and loses energy as it moves within the atom
• The electron has a combination of kinetic energy and electrostatic potential energy
• Because the electron has a -ve change and the nucleus has a positive change, the electron is attracted to the nucleus and work must be done to move the electron away from the nucleus

Question 19

Why can an electron only absorb specific amount of energy?

Answer 19

-Because the possible energies electrons in an atoms are not continues and only certain fixed energy levels are allowed and these energy levels are called quantised energy levels because they have fixed energy levels

Question 20

What is ground state?

Answer 20

When electrons in an atom are in their lowest energy state (n=1)

Question 21

When does excitation occur?

Answer 21

When electron absorb exactly the right amount of energy to move to higher energy levels

Question 22

When does ionisation occur?

Answer 22

Wehn an atom gains or loses and electron and becomes a charge particle. If an electron in atom absorbs enough energy to escape the atom completely, we say the atom is ionised

Question 23

What is ionisation energy?

Answer 23

The minimum energy required to remove the electron from the atom completely (from its ground state to infinity)

Question 24

When can excitation occur?

Answer 24

1. Absorbing a photon with the exact right amount of energy to move between two levels
2. Absorbing exact right amount of energy to move between two levels after colliding with a free electron that has energy equal to or greater than the energy required. The energy gained by the electron in the atom equals the energy lost by the colliding electron. The free electron’s kinetic energy after collision is equal to its kinetic energy before the collision minus the energy transferred to the excited electron in the atom

Question 25

What is a continuous spectrum?

Answer 25

• Is a spectra where all frequencies of radiation or colours of light are possible
• When a solid is heated a continuous spectrum is formed

Question 26

What is a diffraction grating?

Answer 26

• A piece of transparent material ruled with very closely spaced lines used to see the diffraction of light
• Light passing through each gap in the grating interferes with light spreading through neighbouring lined and this splits the light into a plectrum of colours it contains (the pattern of the coloured light carries information about the light source)

Question 27

What is a line spectrum?

Answer 27

• A spectrum of discrete coloured lines of light
• Light from a fluroecen tube produces a spectrum of coloured lines on a dark background
• The pattern of lines is characteristic for certain elements. All fluorescent bulbs have similar characteristics because electrons in mercury vapour inside them have the same excited states regardless of the shape of the bulb

Question 28

What is an emission spectrum?

Answer 28

It is seen when electrons in atoms fall from higher energy levels to lower energy levels releasing photons

Question 29

What happens when a gas is heated with an emission spectrum?

Answer 29

1. Electrons absorb energy and reach higher energy levels
2. They stay in these higher states for a short time until they fall back down
3. When an electron falls to its lower state, it emits a photon whose energy is equal to the difference in energy between the two energy levels
4. Since only a small no of possible energy levels exist, there is only a small number of possible transitions between them
5. The photons emitted have specific energies and therefore specific wavelengths and colours and those are the colours seen as light diffracts through diffraction grating

Question 30

What is an absorption spectrum? What does it look like?

Answer 30

• Can be seen when light shines through a gas and electrons in the atoms absorb photons
• A spectrum of dark lines can be seen on a floured background produces when a gas absorbs photons

Question 31

What do the dark lines on an absorption spectrum correspond to?

Answer 31

The wavelengths of possible energy transitions for the electrons of the gas atoms

Question 32

How do you figure out the energy of spectral lines?

Answer 32

1. When an electron moves to a lower energy level it emits a photon and the energy lost is equal to the energy of the photon
2. (energy in level 2) - (energy in level 1) = hf

Question 33

Why in spectral lines are the energies negative?

Answer 33

As we define zero point of energy when the electron is at infinity and this energy gets lower as the electron gets closer to the nucleus

Question 34

What is a fluorescent tube?

Answer 34

-A glass tube fulled with mercury vapour coasted inside with phosphorus

Question 35

What is fluorescence?

Answer 35

Is when a substance absorbs short wavelength electromagnetic radiation and emits it as longer wavelength radiation

Question 36

What happens in a fluorescent tube?

Answer 36

1. Light switched on, cathode is heated causing thermionic emission (free electrons realised from heated filament)
2. The free electrons have range of enemies and as a pd is applied across ends f glass tube this accelerates the electrons from the cathode to anode through mercury vapour
3. When free electrons collide with mercury vapour inelasticity some energy transferred from electron to mercury atoms
4. Atoms may become ionised (high energy electrons) and excited (lower energy electrons) provided the free electrons transfer enough kinetic energy
5. As mercury atoms in vapour become ionised plasma (a mixture of Iona and electrons in a gas) is formed
6. Atoms measles UV radiation when return to ground state
7. Photons strike the phosphorus coating and are absorbed
8. Energy is remitted as visible light and some energy is transferred as heat