Arrangement of Electrons in the Atom

Bohr's Study of Spectra

When white light is passed through a glass prism, the white light is broken up into a array of colours (Spectrum)

 A continuous spectrum contains all wavelengths of from red to violet.

In place the white light, if you used light coming from a (hydrogen) discharge tube instead of seeing a continuous spectrum, you see a series of narrow coloured lines 

A gas filled (hydrogen) discharge tube is simply a glass tube filled with (hydrogen) gas at low pressure through which an electric current is possed.

Since the light is emitted when an electric current is passed through gas (hydrogen), the spectrum is more accurately called on emission line spectrum

 Spectra are usually studied using a spectrometer.

Each element has its own unique emission line spectrum. The emission line spectrum of an element is often used by chemists to detect its presence.

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Mandatory Experiment 1 (Flame Test) Results

Lithium -- Crimson

Potassium -- Lilac

Barium -- Green 

Strontium -- Red

Copper -- Blue/Green

Sodium -- Yellow

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Bohr realised that any model of the atom must explain:

1. why the emission spectra of the elements are line spectra rather than continuous spectra 

2. why the emission spectrum of each element is unique to that element

The Bohr Theory

 Bohr introduced the idea of quantisation of energy ie that an electron in an atom can only have a fixed amount of energy. (Quantum)

Bohr stated:

1.Electrons revolve around the nucleus in fixed paths called orbits.

2.Electrons in any one orbit have a fixed amount of energy. Therefore the energy of an electron in a particular orbit is quantised.

3.As long energy as an electron is in any one particular level, it neither gains nor loses energy.

An Energy level is defined as the fixed energy value that an electron in an atom may have.Energy levels are represented by the letter n'.

Spectroscopic Evidence for energy levels 

1.Atoms normally exist in the ground state. The electrons have the lowest amount of energy possible

2. When energy is supplied to an atom, a specific amount of the energy is absorbed and the electrons jump from a lower energy level to a higher energy level (excited state)

3. The energy absorbed is equal to the difference in energy between ground + excited state.

4. Electrons in excited state care unstable, hence they fall back down to lower energy level after a shark time

5. When this occurs, excess energy is released in the form of a photon of light which has a definite amount of energy (light of a definite frequency is given off)0

6. E2 - E1 =hf

7. The definite amount of energy emitted (light of a definite frequency) appears as a line of a particular colour in the emission line spectrum is formed.

The Ground State of an atom is one in which the electrons occupy the lowest available energy levels.

The Excited State of an atom is one in which the electron occupy higher energy levels than those available in the ground state

Balmer Series

The Balmer series is the set of electron transitions from higher energy levels which fall to the n=2. 
Lyman and Paschen appear in the ultraviolet and infrared regions of the spectrum and hence are invisible.

Atomic Absorption Spectrometry (AAS) 

As well as emitting light of a characteristic freque atoms can also absorb white light.

If white light is passed through a gaseous sample of an element, it is found that the light that emerges has certain waveligths missing (dark lines in Spectrum) This is called an atomic absorption spectrum. The absorption spectrum is like a photographic negative of an emission spectrum!

AAS is a useful instrumental technique for detecting the presence of certain elements in material dissolved in water and for measuring the concentrations of these elements.

Atomic Absorption Spectrometer

Principle

1. Atoms of an element in the ground state absorb light of a particular wavelight that is characteristic of the element.

2.The amount of light absorbed is directly proportional to the concentration of the element present in the sample.

Process

1. Sample Solution is sprayed into a flame, and the sample element is converted into atoms of the element. 

2.Ground State atoms absorb radiation from a special lamp whose cathode contains the element whose concis being measured.

3.Absorbance is measured.

Energy Sublevels 

A sublevel is a subdivision of main energy level and consists of one or more orbitals of the same energy.

On clover inspection of emission line spectra, what is appeared originally to be a single line actually consisted of a number of lines very close together. It was proposed that each main energy level (bar the first) was made up of a number of sublevels all of which were close in energy. 

The number of sublevels was the same as the value of 'n' for the main energy level.

n=1 --- 1s

n=2 --- 2s 2p

n=3 --- 3s 3p 3d 

n=4 --- 4s 4p 4d 4f

Wave Nature of Electron 

Louis de Broglie stated that all moving have a wave motion associated with them. This was subsequently proved correct and electrons were said to have 'wave-particle duality'

Uncertainty Principle

Werner Heisenberg/ put forward a very famous principle called Heinsenberg's Uncertainty Principle.
It states that it is impossible to measure at the same time both the velocity and position of an electron.

Limitations of Bohr's theory.

1.Observations only apply to emission line spectrum of hydrogen (failed to account for many lines in emission spectra of other elements) 

2. Didn't account for 'wave-particle' duality (electron isn't travelling in fixed path at precise distance from nucleus)

3.Didn't account for energy sublevels 

4.In conflict with Heisenberg uncertainty principle (we can only refer to probability of finding an electron).

Atomic Orbitals

There is a high probability of finding the electron near the nucleus of the atom and the probability gets less as the distance from nucleus increases

An orbital is a region in space within which there is a high probability of finding an electron

Erwin Schrödinger calculated probability of finding an electron in any particular sublevel in an atom, which gave rise to the shape of orbitals.

S orbitals are spherical and differ only in size (the probability of finding an electron in a particular s sublevel is identical in all directions at a fixed radius from the nucleus

P orbitals are dumbbell-shaped Each p orbital points along one of 3 axis (x, y, z). The three corresponding P orbitals are of equal energy and only differ in their orientation.

Each orbital only holds 2 electrons.