Electronic Structure of Atoms

Bohr’s model of an atom

Bohr’s model of the atom Bohr described the atom as mostly being made up of empty space! The atom, he said, contains a positively charged nucleus surrounded by electrons which travel around the nucleus on circular orbits! HIS MODEL WAS BASED ON HYDROGEN AND DIDN’T REALLY WORK WELL TO DESCRIBE OTHER ATOMS!

Energy levels in atoms Electrons in atoms occupy energy levels that are outside the nucleus It is better to think of it like this An energy level REPRESENTS the definite energy that an electron has

Electronic configurations The way that electrons are arranged in an atom is called their electron configuration. You need to know this for the first 20 elements

Electronic configurations Element Atomic number Electron arrangement n = 1 n = 2 n = 3 n = 4 Hydrogen 1 Helium 2 Lithium 3 Beryllium 4 Boron 5 Carbon 6 Nitrogen 7 Oxygen 8 Fluorine 9 Neon 10 jus

Electronic configuration Element Atomic number Electron configuration n = 1 n = 2 n = 3 Sodium 11 2 8 1 Magnesium 12 Aluminium 13 3 Silicon 14 4 Phosphorous 15 5 Sulfur 16 6 Chlorine 17 7 Argon 18 Potassium 19 Calcuim 20 Just say 8 electrons go into the third shell and then the electrons start to go into the fourth shell! Activity – find all the elements of group 1 and draw out their electron configurations on a diagram, Then all the groups, what do you notice? Are there any exceptions?

A pattern in the periodic table! In general elements in the same group of the periodic table have the same number of electrons in the outermost energy level of the atoms. The exception is Helium, which just has just two electrons in its outer energy level, where as the rest of group 18 have 8! Lithium Sodium Potassium

Homework Revision – Exo/endo reactions, Chem formulas and equations HL 2011 Q6e,2007 Q3, 2010 Q6d,e OL 2011 q10c, q11b Electronic structure of an atom HL 2010 q5c, 2008 Q10c OL 2011 q10a Finish experiment report from today

Evidence for energy levels

Higher Level Spectroscope A spectroscope is used to analyse the light emitted by elements.

Higher Level White light If you look at white light with a spectroscope you’d see this: You can see the separation of all of the colours of which white light is made up of White light is made up of visible light of every wavelength so it is known as a continuos spectrum. Looks like a rainbow, which is when sunlight is seperated into all of its colours by a raindrop

Discharge tubes If an element (as a gas) is placed in a discharge tube, at low pressure, and electricity is passed through it will light! At high voltage

Hydrogen light emission Higher Level Hydrogen light emission If you look at the light emitted from a hydrogen discharge tube with a spectroscope you’d see this – a line emission spectrum Series of coloured lines of definitude wavelength – all of the wavelengths of light are not emitted from hydrogen! give the def of emission spectrum it is a series of coloured lines against a dark background

Higher Level Emission Spectra The emission spectrum of each element is characteristic of that element Thus different to every other element

Emission of different types of light Elements emit light that we can see (visible light) But they also emit light that we cannot see - called ultraviolet light and infrared light

The Hydrogen line spectrum In the Hydrogen line spectrum that we see with the spectrometer there are lines in the visible region, the ultraviolet region and the infrared region of the spectrum. The Balmer series contains all of the lines that are in the visible region of the hydrogen spectrum.

Evidence for energy levels existing What is an energy level ? Bohr came up with the idea of energy levels when he was studying the hydrogen atom.

Energy levels in the Hydrogen atom Bohr’s explanation: 1. The electron in the hydrogen atom must occupy an energy level. It usually occupies the n = 1 level – the “ground state” 2. When the electron absorbs energy it will move to a higher energy level – an “excited state”. The amount of energy it absorbs is exactly the energy difference between the two levels. Received energy - from a flame or light or electricity or heat etc Energy level - This means it must be found in a region of space where it has a definite energy. It could also move to the third energy level of fourth

Energy levels in the Hydrogen atom The electron is unstable at this higher energy level The electron falls back down to the lower energy level and gives out energy. The amount of energy it emits is exactly the energy difference between the two levels. E2 – E1 = hf

Energy levels in the Hydrogen atom When an electron falls from a higher energy level to a lower energy level it emits a definite amount of energy corresponding to the difference in energy between those two levels. We can see the definite amount energy that is emitted as a line in the emission spectrum of Hydrogen.

Energy levels in the Hydrogen atom The Balmer series This is the balmer series

Line spectrum are different for each element! Line spectrums are unique to each element because in atoms of that element have unique electronic configurations so different transitions can occur.

Flame tests

Discharge tubes If an element (as a gas) is placed in a discharge tube, at low pressure, and electricity is passed through it will light! At high voltage

A sodium discharge tube emits yellowy orange light! Discharge tubes A sodium discharge tube emits yellowy orange light! Yellow streetlamps are just sodium discharge tubes!

How to do a flame test You can do this test by introducing a sample of the substance to a non luminous flame and observing the colour that results. be careful not to leave the splint in too long as it may catch fire.

Different metals will give off different colour light!

2. Why do metals give off a characteristic colour in a flame? Questions about this experiment What colour is observed in each case when the following are heated in a flame: Copper Blue-green Lithium Deep red Sodium Yellow Strontium: Red Barium : Yellow-green Potassium: Lilac 2. Why do metals give off a characteristic colour in a flame? 3. What procedure is used to avoid cross contamination? The energy differences between energy levels in metal atoms vary from metal to metal. Using energy from the flame, electrons in the metal atoms move to higher energy levels, and then return to lower energy levels, emitting light whose energies in each case is equal to the energy difference between the higher energy level and the lower energy level. If platinum wire is used, either a separate platinum wire is used for each test, or the wire is thoroughly cleaned with concentrated hydrochloric acid after each test. If wooden splints are used, a separate splint is used for each test.

Fireworks When atoms of an element are supplied with energy under certain conditions they emit light. Chemists have used this to make fireworks! Strontium emits red light and Barium emits green light so chemicals containing these elements are put into fireworks!

Limitations of Bohrs theory

Wave nature of an electron De Broglie was a French physicist, who discovered that electrons have properties of waves as well as particles. Don’t need the name 1923 But bohrs model worked very well to decribe the emission spectra, so it is not worng, just another way of thinking about it. Since electrons move like waves they cannot really be travelling at a precise distance from the nucleus in orbits, as Bohr had suggested.

Electrons don’t really travel in orbits around the nucleus!

Heisenberg’s Uncertainty Principle Heisenberg found that you can’t see electrons just by using ordinary observation techniques To measure the velocity of an electron you can use light beams………………. but once the beam hits the electron it sends it off course! The Heisenberg Uncertainty Principle states that it is impossible to determine the exact position of an electron and its velocity at the same time!

Atomic orbitals Since it is impossible to determine the exact position of an electron at any one moment it is better to refer to the probability of finding an electron in a region of space within the atom Def An Atomic orbital is a region of space where there is a high probability of finding an electron They were worked out by Schrodinger! – whilst working in ireland. He was german,

The first energy level In the first energy level n=1 there is only one orbital in which electrons can be found. The orbital is called a s orbital. It is on the first energy level so it is called “1s” It is spherical in shape – there is the same probability of finding the electrons any direction form the nucleus. The orbital can hold two electrons. Ask, how many electrons can fit in total in the first energy level?

The second energy level In the first energy level n=2 there are four orbitals in which electrons can be found. There is one s orbital called “2s”, which can hold two electrons There are three dumbbell shaped p orbitals called “2p”, which can hold two electrons each – 6 in total Ask, how many electrons can fit in total in the first energy level?

There is one s orbital called “3s”, which can hold two electrons The third energy level In the first energy level n=3 there are nine orbitals in which electrons can be found. There is one s orbital called “3s”, which can hold two electrons There are three p orbitals called “3p”, which can hold two electrons each – 6 in total Ask, how many electrons can fit in total in the first energy level?

The third energy level Finally.. There are five d orbitals called “3d”, which can hold two electrons – 10 electrons in total Ask, how many electrons can fit in total in the first energy level?

The fourth energy level In the first energy level n=4 there are eighteen orbitals in which electrons can be found. There is one s orbital called “4s”, which can hold two electrons There are three p orbitals called “4p”, which can hold two electrons each – 6 in total Ask, how many electrons can fit in total in the first energy level?

The fourth energy level Finally.. There are five d orbitals called “4d”, which can hold two electrons – 10 electrons in total There are seven d orbitals called “4f”, which can hold two electrons – 14 electrons in total Ask, how many electrons can fit in total in the first energy level?

Energy sublevels Def An energy sublevel is a group of orbitals in an atom that all have the same energy Energy level Energy sublevels n=1 1s n=2 2s 2p n=3 3s 3p 3d n=4 4s 4p 4d 4f One sublevel Two sublevels An energy sublevel Is a group of orbitals in an atom that all have the same energy Three sublevels Four sublevels

The 4s sublevel is lower in energy than the 3d sublevel Energy sublevels The 4s sublevel is lower in energy than the 3d sublevel Go back over number of each orbital, and that each can hold two electrons

The electronic configuration of: Hydrogen It has one electron. The electron goes into the lowest energy level possible! The electronic configuration is 1s1

The electronic configuration of: Helium It has two electrons. The electrons go into the lowest energy level possible! Two electrons can fit into every orbital! The electronic configuration is 1s2

The electronic configuration of: Lithium It has three electrons. The electrons go into the lowest energy level possible! Two electrons can fit into every orbital! The electronic configuration is 1s2 2s1

The electronic configuration of: Silicon It has 14 electrons. The electrons go into the lowest energy level possible! Two electrons can fit into every orbital! Electrons fill orbitals of equal energy singly if possible! More excercises, all 36 elements! The electronic configuration is 1s2 2s2 2p6 3s2 3p2

Exceptions to the rule Copper has 29 electrons, and its electron configuration is : 1s2 2s2 2p6 3s2 3p6 4s1 3d10 This unexpected configuration is because there is extra stability with a full 3d sublevel! Chromium has 25 electrons, and its electron configuration is : 1s2 2s2 2p6 3s2 3p6 4s1 3d5 This unexpected configuration is because there is extra stability with a half full 3d sublevel!

Electronic configurations of ions They work the exact same way as the atoms….. but be careful to work out how many electrons the ion has before you start! Practise some first!

The electronic configuration of: Mg+2 Magnesium usually has 12 electrons The Mg+2 has 10 electrons. More excercises! Only on s and p black elements! The electronic configuration is 1s2 2s2 2p6

Arrangement of electrons within individual orbitals Electrons are represented by small arrows! Carbon has 6 electrons:

Arrangement of electrons within individual orbitals Only f you are asked! Arrows represent spin, they have to be going opposite directions in the same box! 3px 3py 3pz 3s Sulfur

Absorption of light You can also pass white light through a gas sample of an element and examine the light that comes out with a spectrometer

Absorption spectrum Def An absorption spectrum of an element is the spectrum that you see after light has been passed through the element An absorption spectrum is a series of dark lines against a coloured background!

The emission and absorption spectra of the same element are like a matching pair – the coloured lines in the emission spectrum match the black lines in the absorption spectrum

Atomic absorption spectrometer The amount of light absorbed by an element depends on the amount of element in a sample! The Atomic absorption spectrometer uses this principle to measure the amount of an element in a sample One such use is to measure the amount of lead in a blood sample