Chapter 3 - Arrangement of Electrons in Atom

What did George Stoney do

George Stoney came up with the name electron

Neils Bohr

- Neils Bohr discovered the electron arrangement in atoms

- He did this by studying spectre

continuous spectrum

Contains many colours or wavelengths with no gaps

Emission line spectrum

Consists of coloured lines against a dark background emission line spectrum indicates the presence of energy levels in atoms

spectrometer

Instrument to examine the line specctrum of an element

Neils Bohr experiment

Bohr ran electricity through a vial of hydrogen noticed the hydrogen produced light then put this light through a prism

Result of Neils Bohr Experiment

Instead of a continuous spectrum Bohr saw a series of narrow lines he called this emission line spectrum

Evidence for energy levels

Elements produce a unique emission line spectra

Why do elements produce a different emission line spectra

Elements produce a unique spectrum due to the different number and arrangement of electrons

Bohrs Theory

1. Electrons orbit the nucleus along fixed paths called orbits or energy levels

2. Electrons in one orbit have a fixed amount of energy i.e quantised they have a fixed value

What happens if an atom is supplied with energy (Bhors Theory)

If an atom is supplied with energy in the form of heat electricity / heat electrons jump from low energy to high energy levels ( excited state)

What happens to excited state electrons

Excited state electrons are unstable and fall back down to a lower level after a short time

How is energy released (Bhors Theory)

Energy is released in the form of photon light and since electrons can only fall back to fixed energy levels only fixed amount of energy can be given off for that element

Energy level

The fixed energy value that an electron in atom may have

Ground level

Excited state

One which electrons occupy higher level than in the ground state

Frequency of light

The frequency of light emitted depends on the difference in energy between two energy levels

Frequency of light equation

E2 - E1 = hf

- E2 = Energy of higher energy level

- E1 = Lower energy level

- H - planks constant

What are energy levels represented by

The letter N

N = 1

N = 2

N=3

What are flame tests carried out with

Flame tests are carried out with salts of lithium, sodium , potassium, barium, strontum, copper and sodium

Method of flame test

1. Soak a wooden splint in some water

2. Drop a wooden splint in some salt

3. Hold the flint over a flame and observe the colour

Lithium

Crimson flame

Potassium

Lilac flame

Barium

Green flame

Strontum

Red flame

Copper

Blue green flame

Sodium

Yellow flame

Flame Tests Conclusion

Each metal provides a unique colour due to the unique electronic configuration of the elements and therefore different electronic transitions of energy levels

What is the second energy level called

Balmers theory (n=2)

What is frequency

Frequency is emitted as a photon of light

Light frequency (f)

Light frequency appears as a line of a particular colour of the emission line spectrum

What colour is emitted from electrons moving from energy level 3 to 2

Red

What did scientists find out

Scientists found out that the elements can absorb light

Experiment for absorption

White light is passed through a gaseous sample of an element and it was found that specific wavelengths were missing

What was observed in the continous spectrum

Dark lines were observed in the continuous spectrum that is called AAS these lines correspond exactly to the lines produced in the emission line spectra

AAS

Atoms of an element in the ground state absorb light of a particular wavelength unique to the element. This amount of light is directly proportional to the concentration of the element

Louis De Broglie

- Louis de Brogli discovered electrons wave nature

- He called this wave particle duality

- Electrons don't have a definite boundary because of wave particle duality which Heisenburg expanded upon

Hesienbergs UNcertainty

States that it is impossible to to measure the at the same time both the velocity and the position of an electron therefore it is impossible to specify the absolute boundary of an atom

Limitation of Bohrs theory

- Bohrs theory only worked for hydrigen and not multi electron atoms

- Bohr didnt take into account wave particle duality there is not a fixed distance between nucleus and electron

- Heisenbergs uncertainty principle is in conflict with Bohrs Theory

- Bohr didnt take sub levels into account

Atomic orbital

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

Erwin Schrodinger

Worked out the probability of finding an electron in a sub level of an atom

S sublevel

1 orbital, 2 electrons, spherical

Sublevel

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

p sublevel

3 orbitals, 6 electrons, dumbbell shape, PX PY PZ

d sublevel

5 orbitals, 10 electrons

How does Bohrs theory expplain the emission line spectrum of a hydrogen and the evidence for energy values

- In ground state electrons occupy the lowest available energy level, electrons jump and move to a higher energy level if it absorbs energy then it becomes instable and falls back down

- Energy is emitted as a photon of light thus giving rise to a spectrum light emitted depends on the difference of energy between the 2 energy levels