Middle and High School … from a Montessori Point of View
A quick electron configuration practice webpage that lets you enter the symbol for an element and see if you can write out the electron configuration in both the full and noble gas forms.
The table at the bottom is a guide to filling the electron shells and orbitals. You can click any of the blue squares to change the number of electrons in the orbital.
An improved version of the lower, table part is here.
These atom boards worked very well for practicing how to interpret atomic symbols. The protons (blue) and electrons (red) are magnetic so they snap into place quite satisfyingly. Their poles are oriented so that the electrons will only attach properly to the slots in the electron shells and the protons only attach the right way up to the nucleus. The neutrons are wooden and non-magnetic.
Step 1: Number of protons (+ charge).
Step 2: Number of neutrons.
Step 3: Number of electrons (- charge).
Step 4: Electron Shells
They’ve also turned out to be useful when explaining ionic bonding. Since it’s easy to add or remove electron shells, you can clearly show how many electrons can be donated or received to figure out how many atoms are involved in the reactions.
A couple of my students asked for worksheets to practice drawing atoms and electron shells. I updated the Atom Builder app to make sure it works and to make the app embedable.
So now I can ask a student to draw 23Na+ then show the what they should get:
Draw diagrams of the following atoms, showing the number of neutrons, protons, and electrons in shells. See the example above.
I guess the next step is to adapt the app so you can hide the element symbol so student have to figure what element based on the diagram.
Covalent bonding happens when atoms share electrons, unlike with ionic bonding where one atom gives electrons to another.
Why do some combinations of atoms make ionic bonds and others covalent bonds? The answer has to do with electronegativity, which is the ability of atoms to attract electrons to themselves. Atoms that have similar abilities to attract electrons to themselves will likely form covalent bonds.
For either type of bond, the atoms have the same objective. All atoms “want” filled outer electron shells. When sodium reacts with chlorine for example, sodium has one electron in its outer shell and chlorine is one short of a filled outer shell so it’s “easiest” for sodium to just donate its electron to chlorine to make them both happy.
However, when two similar atoms bond it’s often easier to share electrons.
Consider two hydrogens bonding covalently to form hydrogen gas (note: help on drawing atoms).
Each hydrogen has only one electron, and they both pull equally at the electrons so neither can give their electron away or take the other’s electron. Instead they share.
By sharing, they now each have two electrons in their outer shell, which is now full (since it’s the first shell), and both atoms are happy. This is covalent bonding.
The chemical reaction could be written as:
H + H –> H2
Now consider what happens when hydrogen atoms bond with oxygens. Oxygen atoms have 6 electrons in their outer shells, but they would like to have 8.
Oxygen atoms aren’t strong enough to take away the hydrogen electrons, so they share with covalent bonds. Each oxygen has to react with two hydrogens to get the two extra electrons it needs to end up with 8 electrons in its outer shell.
Thus we create water, which has the chemical formula H2O, and the chemical reaction can be written:
2 H + O –> H2O
Covalent molecules can be large and complex, in fact, one strand of your DNA will have somewhere around a billion atoms.
To make these easier to draw, you can represent each element by its symbol and each bond by a line. Remember, each covalent bond represents a pair of electrons that are shared.
So our water molecule would be drawn like this:
This is called a Lewis Dot structure. In addition to the lines showing the bonds, you’ll notice the dots that show the unbonded electrons: these dots are usually paired up.
The last thing I’ll point out here is that atoms can share more than just one pair of electrons. When they share four electrons that means there are two bonds, which is referred to as a double bond.
Oxygen atoms bond with each other like this to make the oxygen gas we breathe.
Now you can try drawing these covalent molecules:
Now that we’ve learned how to draw individual atoms (and have an online reference for the first 20 elements), let’s consider ionic bonding.
The key thing to remember is that atoms all “want” to have their outer electron shells filled. So while a sodium (symbol: Na) atom is happy* enough that it has the same number of protons and electrons (11 each) it could be happier if it got rid of the extra electron in it’s outer shell.
It can get rid of the electron by donating it to another atom that would be happier with an extra eletron. Something like chlorine (symbol: Cl) that only has 7 electrons in its outer shell, but wants to have 8.
When one atom donates electrons to other atoms this creates a bond called an ionic bond. The molecule created is called an ionic molecule. In this case, sodium and chloride react to produce sodium chloride (chemical formula: NaCl).
The chemical reaction can be written as:
Na + Cl –> NaCl
Now consider what happens when magnesium (symbol: Mg) reacts with chlorine.
Magnesium has two electrons in its outer shell that it wants to get rid of.
A single chlorine atom can’t take both, since chlorine only needs one electron to fill its outer electron shell. However, magnesium can give one electron to two different chlorine atoms to create a molecule with three atoms total.
The resulting compound is called magnesium chloride, and is written as MgCl2. The subscripted 2 indicates that there are two chlorine atoms in each magnesium chloride molecule.
The chemical reaction can be written as:
Mg + 2 Cl –> MgCl2
Notice that each magnesium atom reacts with two chlorine atoms (Mg + 2 Cl) to produce a compound with one magnesium and two chlorines bonded together (chemical formula: MgCl2).
Now, for homework, you can try figuring out what is the chemical formula for the following ionic compounds:
Be sure to:
Good luck. Next we’ll try covalent bonding.
* Think of happiness as energy. Like people, atoms are happier to be in low energy states.
When we looked at the patterns in the periodic table, one of the things I had my student graph was the electronegativity. Electronegativity is the ability of atoms to attract electrons to themselves. You’ll note that chlorine’s electronegativity is high, while sodium’s is low.
So chlorine will attract electrons to itself strongly, while sodium will not. This is why (more or less) sodium will end up donating its electron and why chlorine is happy to accept it.
When atoms with a large difference in electronegative bond together, the bonds tend to be ionic.
This app lets you drag and drop electrons, protons, and neutrons to create atoms with different charges, elements, and atomic masses. You can also enter the element symbol, charge and atomic mass and it will build the atom for you.
Note, however, it only does the first 20 elements.
Having demonstrated how to draw a few simple atoms, I had students fill out a periodic table template with drawings of the first twenty atoms. Actually, I only had them draw the electrons in their shells because it reduced the messiness of trying to fit in forty nuclear particles into a small tile, and the point I wanted to get at was the pattern of shells and valence electrons in the periodic table.
The end result looked something like this:
All the drawing only took about 15 minutes, and once they’d figured out the first half dozen or so it started to get a little boring. But that freed up the cognitive resources so they could notice the two key patterns.
It’s a first glimpse at the periodicity in the periodic table. And it sets us up nicely to be able to talk about chemical bonding.
This year, I’ve been basing my introduction to basic chemistry for my middle school students around the periodic table of the elements. The first step, however, is to teach them how to draw basic models of atoms.
I started it off by having the students memorize the first 20 elements (H through Ca), in their correct order — by atomic number — over their winter break.
So that they’d have a bit of context, I went over the basic parts of an atom (protons, neutrons, and electrons) and made it clear that the name of the element is determined solely by the number of protons. I even had them draw a few atoms with the protons and neutrons in the center and the electrons in shells. Since I’d dumped all of this on them in a single class period, it probably was a bit much, but since it was just to give them some context I did not expect the 7th graders, who had not seen this before, to remember it all; for the 8th graders it should have been just a review.
Most students did a good job at the memorization. Some found songs on the the internet that helped, while others just pushed through. Having the two weeks of winter break to work on it probably helped too.
When we got back to school, the first thing I did was give them an outline of the upper part of the periodic table and asked them to fill it in with the element names.
After they’d filled out their periodic table template, I went into the parts of the atoms in more detail, and had them practice. The key points I wanted them to remember were:
The atomic number is the number of protons. Since they memorized the elements in order, they should be able to figure this out on their own — but they could also look it up quickly on the periodic table, or look at the element symbol where the atomic number is sometimes written on the lower left.
The small atoms that we’re looking at tend to have the same number of neutrons as protons, but that’s not necessarily the case. So how do you know how many neutrons? You have to ask, or look at the atomic mass number, which is usually written to the upper left of the atom. Since the atomic mass is the sum of the number of protons and neutrons, if you know the atomic mass and the number of protons, you can easily figure out the number of neutrons. (Note that electrons don’t contribute to the mass of the atom because their masses are so much smaller than the masses of neutrons and protons.
Electron Shells: Electrons orbit around the nucleus in a series of shells. Each shell can hold a certain maximum number of electrons (2 for the first shell; 8 for the second shell; and 8 for the third). And to draw the atoms you fill up the inner shells first then move on to the outer shells.
So, if I wrote just the element symbol and its atomic mass on the board that students should be able to figure out the number of particles.
For example, the most common form (isotope) of carbon-12 is written as:
Carbon-14 is the radioactive isotope of carbon that is often used in carbon dating of historical artifacts. It is written as:
The only difference between carbon-12 and carbon-14 is that the latter has two more neutrons. These are therefore two isotopes of carbon.
Note: A picture of a hydrogen atom can be found here.
Update: I’ve created an interactive app that will draw atoms (of the first 20 elements), to go with a worksheet for student practice.
