EL2 - Wave and Particle Models of Light

Question 1

What is relative atomic mass (Ar)?

Answer 1

The mass of one atom of an element relative to 1/12 the mass of carbon-12

Is an average of relative isotopic masses, taking into account abundance

Question 2

How are models of the atom made + updated?

Answer 2

Tested using experimental investigations

Are revised when observations are made that aren’t predicted by model`

Question 3

What were the different steps/models in the development of the current atomic model?

Answer 3

Dalton model - simple ‘billiard ball’. Particles cannot be divided, created, or destroyed. Are unique.

‘Plum pudding’ model - electrons embedded in sea of positive charge. Discovered by firing cathode rays (electrons) in air - discovered e-. Introduced idea that atoms made of smaller particles

Nuclear model - Geiger-Marsden experiment showed some alpha particles deflected at large angles by small, dense area of positive charge surrounded by e-

Bohr model - Evidence from atomic spectra + patterns of ionisation enthalpy. e- arranged in shells - ‘planetary model’

Question 4

What is nuclear fission?

Answer 4

The splitting of a large, unstable isotope triggered by bombarding it with smaller, high-speed particles (usually neutrons)

Question 5

What conditions are needed for nuclear fission?

Why?

Answer 5

High temps and/or pressure to provide the energy needed to overcome the repulsion between the 2 positive nuclei

Question 6

What is the general formula for calculating Ar?

Answer 6

(% abundance of xX isotopic mass of x) + (% abundance of yX isotopic mass of y) / 100%

Question 7

The Ar of potassium is 39.1. Calculate the relative abundance of 39K and 41K

Answer 7

Make 1 isotope abundance x so the other = 100-x
(39x + 41(100-x))/100% = 39.1

Multiply both sides by 100 + multiply out brackets
39x + 4100 - 41x = 3910
-2x = -190, x =95
39K = 95%, 40K = 5%

Question 8

What are isotopes?

Answer 8

Atoms of the same element with a different number of neutrons

This causes mass number to be different

Their relative abundances are used to calculate Ar

Question 9

What is Mr?

Answer 9

Relative molecular mass

The ratio of the average mass of one molecule of an element or compound to one twelfth of the mass of an atom of carbon-12.

(Ar but for molecules… (not elements))

Question 10

What is the Avogadro constant (NA)?

Answer 10

The number of atoms/molecules in 1 mole of a substance

Question 11

What does quantised mean?

Answer 11

Energy that can only take particular values (known as quanta)

Question 12

What is the ground state?

Answer 12

The lowest energy level that an electron can occupy

Question 13

What is a photon?

Answer 13

Quanta of energy in the form of electromagnetic radiation

Question 14

Breifly describe Bohr’s model of the atom

Answer 14

Electrons in an atom occupy discrete, quantised energy levels/shells

Electrons in an energy level have a specific amount of energy

Hence the energy of the electron is said to be quantised

Question 15

What property does light have?

What does this mean?

Answer 15

Wave-particle duality

Means it can behave like both a wave and a particle…

Question 16

What properties does light have the mean it can be described as a particle?

Answer 16

Made up of ‘tiny packets of energy’ called photons

The energy of a photon corresponds to its position in the EM spectrum

Increased freq. = increased energy + decreased wavelength

Question 17

What equation links the wave + particle models of light?

Answer 17

ΔE = hv

ΔE = energy of photon (J)
h = Planck's Constant
v = frequency (Hz/s-1)

Question 18

What equation explains the wave properties of light?

Answer 18

c = vλ

c= speed of light (ms-1)
v = frequency (Hz/s-1)
λ = wavelength (m)

Question 19

Describe the appearance of an emission spectrum

Answer 19

Consists of coloured lines on a black background

The lines become closer at higher frequencies

There are several series of lines (although some may fall outside visible part of spectrum)

Question 20

What is spectroscopy?

Answer 20

The study of how light and matter interact

Uses IR, visible, and UV light

Question 21

Explain the formation of an emission spectrum

Answer 21

• Electrons in the ground state absorb energy
• This promotes them to a higher energy level - excited state
• Electrons then drop back down to lower energy levels.
• The energy lost (ΔE) is emitted as a photon of light
• The frequency of the photon is related to the energy lost by ΔE = hv
• Different energy gaps produce photons of different frequencies
• This produces different coloured bands on the emission spectrum

Question 22

Why can emission/absorption spectra be used to identify different atoms from a compound/mixture?

Answer 22

Because each element has a unique configuration of electrons, therefore has a unique emission/absorption spectrum

The energy levels of the electrons are discrete + quantised means only certain freqs. emitted/absorbed - it’s not continuous

Question 23

What are flame tests?

Answer 23

Used to identify the presence of specific metals in a sample

Different metals give different coloured flames depending on their emission spectra

Question 24

What is flame colour?

Answer 24

The light emitted by metal ions when a vaporised metal salt is heated up in a flame

Question 25

What colour flame does Li+ give?

Answer 25

Bright red

Question 26

What colour flame does Na+ give?

Answer 26

Yellow

Question 27

What colour flame does K+ give?

Answer 27

Lilac

Question 28

What colour flame does Ca2+ give?

Answer 28

Brick red

Question 29

What colour flame does Ba2+ give?

Answer 29

Apple green

Question 30

What colour flame does Cu2+ give?

Answer 30

Blue-green

Question 31

Describe the appearance of an absorption spectrum

Answer 31

If white light is passed through a sample of vaporised atoms, an absorption spectrum is seen

Shown by black lines on a rainbow background (showing all colours of visible light)

Question 32

How are atomic absorption spectra formed?

Answer 32

• Electrons in the ground state absorb photons of light
• The energy from these photons causes the electrons to be excited to higher energy levels
• The electrons drop back down to the ground state and a photon/light is emitted
• The energy of this photon is related to the frequency/energy of light initially absorbed as ΔE = hv
• Light of the frequency doesn’t pass through the sample (as it’s absorbed) so a black line is seen in the spectrum

Question 33

What are the similarities between emission and absorption spectra?

Answer 33

For a given element, lines appear at the same frequency

Lines converge at a higher frequency

Several series of lines are seen

Question 34

What are the differences between atomic emission and absorption spectra?

Answer 34

Emission spectra show coloured lines on a black background

Absorption spectra show black lines on a coloured background

Question 35

Why do the lines of emission/absorption spectra get closer together at higher frequencies?

Answer 35

Higher frequency lines are caused by translations of electrons with large ΔE values

These are produced from translations from higher energy levels

Higher energy levels are much closer together than lower energy levels

Translations from adjacent energy levels will have similar ΔE values and hence produce light of similar frequencies

Question 36

Why are several series of lines seen on emission/absorption spectra?

Answer 36

Lines are produced when electrons drop to a lower energy level

Different series of lines are produced by electrons dropping to different ground states/electron energy levels