Bohr Model Guided Practice – Keep this Resource Sheet – Not Homework
Now that we know how to find the number of protons, electrons, and neutrons there are in an element, it is time to talk about how to represent this visually.
One of the most common ways to represent this visually is by drawing a Bohr model. A Bohr model has all the protons and neutrons inside the nucleus in the center. We will then put the electrons on circles around the nucleus (the orbitals.)
Step 1: Determine the number of protons, electrons, and neutrons for the element
As we have learned previously the atomic number is the number of protons. This is also normally the number of electrons as well, unless it is an ion. For our number of neutrons, you subtract the number of protons (atomic number) from the mass number.
For example: Sulfur Atomic Number 12, Mass Number 24 therefore Protons=12 Electrons = 12, and neutrons = 12.
Step 2: Place the appropriate number of protons and neutrons in the center, and draw a circle around it to represent the nucleus.
This is very simple as you will draw one symbol for each proton (probably a +), and one symbol for each neutron (probably an o)
For example this is how magnesium would look: 12 protons, and 12 neutrons.
Step 3: Determine how many rings there will be around the nucleus for electons. The first ring can only hold 2 electrons, while the 2nd, 3rd, and 4th rings can each hold 8. You are not expected to learn Bohr models past 4 rings. Electrons always go single first, and then pair up, starting at the top and working clockwise.
For example we know that magnesium has 12 electrons. Two will go in the first ring, leaving us with 10. 8 go in the second ring leaving us with 2, and those 2 go in the third ring. Therefore it will have 3 rings.
Step 4: Put the electrons on the rings. Place the electrons on the ring one at a time. For the first ring, place the electrons opposite one another on the top and bottom. For all the other rings, place a single electron at the top, and then every electron you add goes a quarter ways around the circle until you end up back at the top. Electrons are placed singly up to 4, but then will pair up starting again at the top and going around.
We already know that magnesium has 12 electrons, and have determined that it will have 3 rings. We put the first two on the first ring across from each other on the top and bottom. The second ring we fill in with 8 electrons putting them by themselves first, but then pairing them up as we come back around. Finally we do the third ring, which will just have two electrons apart from each other. It will look like this:
In the space provided below, let’s work through Sulfur together.
Sulfur: Protons ___ Neutrons___ Electrons___
Work through the following on your own:
Calcium: Protons ___ Neutrons___ Electrons___Boron: Protons ___ Neutrons___ Electrons___
Li1+: Protons ___ Neutrons___ Electrons___Si4-: Protons ___ Neutrons___ Electrons___
Calcium - 42: Protons ___ Neutrons___ Electrons___Boron -8: Protons ___ Neutrons___ Electrons___