Arrangement Of Electrons In The Atom Flashcards
(19 cards)
Who provided an insight into the arrangement of electrons in an atom
Niels Bohr
-Danish scientist
Bohr’s study of spectra
-White Light is broken up into an array of colours when passed through a glass prism
-array of colours is called a spectrum
-This spread of colours is referred to as a continuous spectrum (colours blend together) e.g Rainbow 🌈
-Bohr carried this exp again using a hydrogen discharged tube and noticed instead of seeing a continuous spectrum,he saw a series of narrow coloured lines (Line spectrum)
Hydrogen Discharge tube
A glass tube filled with hydrogen gas at low pressure through wch an electric current is passed
Distinguish btwn Continuous and Line spectrum
Continuous spectrum is obtained when white light is passed through a glass prism
-In it’s spectrum The colours blend together really well
Line spectrum is obtained when light from a hydrogen discharge tube is passed through a glass prism
-In its emission spectrum it contains lines
Instrument used to study spectra
Spectrometer
Emission line spectra of elements
-When light emitted by other elements was studied,they discovered the elements also produced line spectra.
-Its easier to study spectra in a dark room.
-Each element has itsown unique emission line spectra because each element has a unique number of electrons
Why do elements emit different colours when electricity is passed through them
Because of the unique number of electrons that each element would have.
E.g Sodium street lamps -Yellow colour
Mercury vapour lamps -Blue colour
Neon advertising signs -Red
What do you use to observe an emission spectrum
1.Spectrometer
-can be used to measure wavelengths of light
2.Hand-held spectroscope
How do you test for salts
Flame test
Process:
-Using a pre-socked wooden splint dip it into the salt and hold it over a blue flame on the Bunsen burner
-Observe colour produced by the salt and record results
-Repeat the exp with other elements
Lithium -Crimson
Potassium -lilac
Barium -green
Strontium -Red
Copper -Blue-grern
Sodium yellow
What did Bohr realise when he did the study of emission spectra
Realised any model of the atom must explain
1.why the emission spectra of the elements are line emission spectrum rather than continuous spectra
2.Why the emission spectrum of each element is unique to that element
Bohr theory
- The theory introduced the idea of quantisation of energy i.e An electron in an atom can only have a fixed amount of energy
-Fixed amount of energy is called a quantum of energy
Summary of theory
-Electrons revolve around the nucleus in fixed paths called orbits (orbit-energy level)
-Electrons in one orbit have a fixed amount of energy
-Energy levels are represented by the letter n
Lowest_____n=1
Nxt Highest______n=2. e.t.c
-energy of an electron in a particular orbit (energy level) is quantised
-an electron will have a certain amount of energy depending on the orbit or energy level in wch it is located
Energy level
Is defined as the fixed energy value that an electron in an atom may have
-an electron in any one particular energy level,it neither gains nor loses energy
Ground state of an atom
Is one in wch the electrons occupy the lowest energy levels
-when an atom in its ground state is provided with energy (by heating/electricity),
a specific amount of this energy is absorbed
and the Electronjump from lower energy levels to higher energy levels,The electrons are now in an excited state
Excited State of an atom
One in wch the electrons occupy higher energy levels than those available in the ground state
-Energy absorbed is equal to the difference in energy btwn the lower (grnd state) and a higher (excited state) energy levels
-Electrons in the excited state are unstable and fall back down to lower energy levels after a short period of time
(It wants to be stable again)
-As an electron falls back to a lower energy level, the excess energy is released in the form of a photon of light (or wavelength of light) wch has definite amount of energy.
-light of definite frequency is given off
Frequency of light emitted depends on the equation E2-E1 = hf
E2-E1= hf
E2 - Energy of higher energy level
E1 - Energy of lower energy level
h - number that never changes called Planck’s constant
f - frequency
-equation tells us that the energy diff is proportional to the frequency of light emitted
(Energy absorbed = energy given out)
-definite amount of energy emitted appears as a line of particular colour in the emission spectrum.
Therefore, an emission spectrum is formed.
Atomic Absorption Spectrometry
(AAS)
-If white light is passed through a gaseous sample of an element,it is found that the light that emerges has certain wavelengths missing
i.e Dark lines are observed in the spectrum.Such a spectrum is called an atomic absorption spectrum
-**The dark lines in the absorption spectrum of an element are exactly in the same position as the coloured lines in the emission spectrum of that element **
-Missing wavelengths are absorbed by the sample and correspond to the wavelengths that would be detected if an emission spectrum of the element were recorded.
Absorption spectrum is a photographic negative of an emission spectrum
-Emission spectrum consists of coloured lines against a dark background
Absorption spectrum consists of dark lines against a coloured background
AAS uses
1-instrumental technique for detecting the presence of certain elements in materials dissolved in water and measuring their concentrations
&
Instrument used is Atomic Absorption Spectrometer
2-Used in the analysis of water for metals like lead, mercury and cadmium.
Referred to as ‘heavy metals’
3-Measuring concentrations of lead in the blood
Lead is harmful to the body
4-used in forensic science to test for gunshot residue
Operation of Atomic Absorption Spectrometer
