How to Draw Bohr Diagrams and Lewis Diagrams

Bohr Diagrams Find your element on the periodic table. Determine the number of protons, which is given by the atomic number. Determine the number of electrons. In a neutral atom (no net charge) it is the same as the number of protons. 6 C Carbon 12.011 Carbon is element 6, so it has 6 protons and a neutral carbon atom has 6 electrons

Bohr Diagrams 6p 4) For carbon, write 6p where your nucleus is going to be. 6n 5) Next, figure out the number of neutrons. Protons and neutrons each have a mass of 1 atomic mass unit (a.m.u.) Carbon’s atomic mass rounds to 12, so 12 – 6 protons = 6 neutrons Write 6n 6)Draw a small circle around the numbers to indicate the nucleus.

Bohr Diagrams Find out which period (row) your element is in. Elements in the 1st period have one energy level. Elements in the 2nd period have two energy levels, and so on. www.chem4kids.com

Bohr Diagrams 6p 6n 7) Draw the shells around the nucleus. Carbon is in the 2nd period, so it has two energy levels, or shells.

Bohr Diagrams 6p 6n Add the electrons. Carbon has 6 electrons. The first shell can only hold 2 electrons. 6p 6n

Bohr Diagrams Since you have 2 electrons already drawn, you need to add 4 more. These go in the 2nd shell. Add one at a time -starting on the right side and going counter clock-wise. 6p 6n

Bohr Diagrams 6p 6n Check your work. Carbon 12.011 Check your work. You should have 6 total electrons for Carbon. Only two electrons can fit in the 1st shell. The 2nd shell can hold up to 8 electrons. The 3rd shell can hold 18, but the elements in the first few periods only use 8 electrons. 6p 6n

1 H Hydrogen 1.008 Bohr Diagrams Now lets look at a few examples Hydrogen Period 1, so 1 energy level, Group 1A, so 1 electron in the outer (in this case the only) shell 1p

Bohr Diagrams 2p 2n Helium 2 electrons fill up the first shell 2 He 4.003

Bohr Diagrams Lithium – Period 2, so 2 orbitals 3p 4n 6.941 3p 4n Lithium – Period 2, so 2 orbitals 2 electrons fill the first shell, so the third e- has to go in the next energy level

As you add e- to the 2nd shell, go counterclockwise through 3, 12, 9, and 6 o’clock positions. Li 3p 4n Each time you add an electron, draw a NEW Bohr diagram complete with the proton and neutron information in the nucleus AND the correct element symbol. Follow the example of Lithium, showing now. Click now to get started. When you get to N (nitrogen) use the same positions and PAIR the electrons.

8 O Oxygen 15.999 Bohr Diagrams Oxygen Element 8, so 8 protons Atomic Mass = 16, so 16 – 8 = 8 Neutrons If there are 8 protons, then a neutral atom has 8 total electrons Period 2, so 2 electron shells. Group 6A, so 6 electrons in the valence (outer) shell 8p 8n

10 N Neon 20.180 Bohr Diagrams Neon 10 electrons Period 2 = 2 shells Group 8A = 8 valence electrons (making that shell full, so any more electrons will have to start a new shell… 10p n

11 Na Sodium 23.990 Bohr Diagrams Sodium 11 electrons Period 3 = 3 shells Group 1A = 1 more electron than neon, so there is a new shell with 1 valence electron p n N

13 Al Aluminum 12.011 Bohr Diagrams Aluminum 13 electrons total Period 3, so 3 electron shells Group 3A, so 3 electrons in the valence shell (NOTICE that period 1 is full with 2 electrons and period 2 is full with 8 electrons)

19 K Potassium 39.098 Bohr Diagrams Potassium

What Happens Next? When we get to element #21 in Period 4 we see a more complex pattern develop. However, we will not try to draw Bohr diagrams for elements past #20 (Calcium) at this time.

Bohr Diagrams Your turn! Now: YOU draw a Bohr Diagram for each of the first 20 elements.

Lewis Diagrams When we talk about atomic bonding in chemical reactions (atoms sticking together to make all the chemicals that exist) we just need to look at the outer or valence electrons. That’s where the atoms bond.

1 2 3 4 5 6 7 8 1 A 2 A 7A 8A 6 A 5A 4A 3 A For A group elements, the number of valence electrons is the same as the group number  Group number   Number of Valence electrons 

Lewis Diagrams Take a look at the periodic tables on the next 2 slides. They are the same, of course, but the way that the GROUPS are numbered differs slightly on each one. You need to be able to recognize the differences.

1 2 3 4 5 6 7 8 I A II A VII A VIII A VI A VA IVA III A Many tables still use the Roman numerals, so you have to know them, at least up to VIII (8)  Group number (Roman)   Number of Valence electrons  B group

1 2 3 4 5 6 7 8 17 18 16 15 14 13 Some tables don’t separate A and B groups. Instead they just number all the groups 1 – 18. That’s no problem, because you just drop the “1” from the front of the number and you still have the A-group valence electron count!  Group number   Number of Valence electrons  9 10 11 12

So let’s get on with it! When we talk about atomic bonding in chemical reactions (atoms sticking together to make all the chemicals that exist) we just need to look at the outer or valence electrons. That’s where the atoms bond.

6 C Carbon 12.011 Lewis Diagrams In this Bohr diagram of carbon, you see all of the electrons. You only need to know about the valence shell to know how many bonds carbon can make So, next we’ll take away the drawing of the first (inner) energy level. (It’s still there…we just don’t have to worry about it for bonding!) C

Lewis Diagrams There! Notice that the only electrons you see are carbon’s 4 valence electrons. Carbon is in group 4A, so you can always know it has 4 valence electrons. While we’re at it… since you aren’t drawing all of the energy levels, there’s no point in drawing a circle for the valence shell anymore, is there? C

Lewis Diagrams What you have left is the Lewis diagram of your element – in this case carbon. Notice that you can now quickly diagram any main group (A-group) element just by looking at the periodic table and seeing what group it’s in! C

Lewis Diagrams Fill using the same order you did with Bohr diagrams. Every lonely (unpaired) electron is a bonding site! Here we see that carbon can make 4 bonds. C

Lewis Diagrams Your turn! Now: YOU draw a Lewis Diagram for each of the first 20 elements.

H He Lewis Diagrams Hint: The first two will look like this. 1 H Hydrogen 1.008 Lewis Diagrams H Hint: The first two will look like this. 2 He Helium 4.003 He Helium is kind of an oddball since its valence shell is full at 2 electrons. It’s the only one that looks like this.