EL2 - Wave and Particle Models of Light Flashcards
How do we know so much about outer space?
What is relative atomic mass (Ar)?
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
How are models of the atom made + updated?
Tested using experimental investigations
Are revised when observations are made that aren’t predicted by model`
What were the different steps/models in the development of the current atomic model?
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’
What is nuclear fission?
The splitting of a large, unstable isotope triggered by bombarding it with smaller, high-speed particles (usually neutrons)
What conditions are needed for nuclear fission?
Why?
High temps and/or pressure to provide the energy needed to overcome the repulsion between the 2 positive nuclei
What is the general formula for calculating Ar?
(% abundance of xX isotopic mass of x) + (% abundance of yX isotopic mass of y) / 100%
The Ar of potassium is 39.1. Calculate the relative abundance of 39K and 41K
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%
What are isotopes?
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
What is Mr?
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))
What is the Avogadro constant (NA)?
The number of atoms/molecules in 1 mole of a substance
What does quantised mean?
Energy that can only take particular values (known as quanta)
What is the ground state?
The lowest energy level that an electron can occupy
What is a photon?
Quanta of energy in the form of electromagnetic radiation
Breifly describe Bohr’s model of the atom
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
What property does light have?
What does this mean?
Wave-particle duality
Means it can behave like both a wave and a particle…
What properties does light have the mean it can be described as a particle?
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
What equation links the wave + particle models of light?
ΔE = hv
ΔE = energy of photon (J) h = Planck's Constant v = frequency (Hz/s-1)
What equation explains the wave properties of light?
c = vλ
c= speed of light (ms-1) v = frequency (Hz/s-1) λ = wavelength (m)
Describe the appearance of an emission spectrum
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)
What is spectroscopy?
The study of how light and matter interact
Uses IR, visible, and UV light
Explain the formation of an emission spectrum
- 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
Why can emission/absorption spectra be used to identify different atoms from a compound/mixture?
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
What are flame tests?
Used to identify the presence of specific metals in a sample
Different metals give different coloured flames depending on their emission spectra
What is flame colour?
The light emitted by metal ions when a vaporised metal salt is heated up in a flame
What colour flame does Li+ give?
Bright red
What colour flame does Na+ give?
Yellow
What colour flame does K+ give?
Lilac
What colour flame does Ca2+ give?
Brick red
What colour flame does Ba2+ give?
Apple green
What colour flame does Cu2+ give?
Blue-green
Describe the appearance of an absorption spectrum
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)
How are atomic absorption spectra formed?
- 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
What are the similarities between emission and absorption spectra?
For a given element, lines appear at the same frequency
Lines converge at a higher frequency
Several series of lines are seen
What are the differences between atomic emission and absorption spectra?
Emission spectra show coloured lines on a black background
Absorption spectra show black lines on a coloured background
Why do the lines of emission/absorption spectra get closer together at higher frequencies?
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
Why are several series of lines seen on emission/absorption spectra?
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
