Middle and High School … from a Montessori Point of View
A neat stop-motion movie made by manipulating individual atoms.
This is a great spark-the-imagination video because you can use it to talk about the physics of atoms and molecules, and what is temperature — they had to cool the atoms down to 4 Kelvin to keep them from moving too much.
How they did it:
More detail from Slate, and NPR:
A look into the imaging of large molecules (chromosomes) and the arrangement of atoms in really small particles.
Steel is an alloy of iron and other elements in small amounts. The exact proportions of the small amounts of other elements can make the alloy stronger, more flexible, and/or more resistant to rusting among other things. Similar alloying is used to make aluminum stronger. You’ll often hear the saying, “Alloys are Stronger” (often used as an argument for more diversity). There is a lot of fascinating research and discoveries happening in the fields of metallurgical arts and sciences at the moment. However, YouTube user NurdRage demonstrates with some gallium and an aluminum can, alloys are not always stronger.
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.
Ms. Wilson’s chemistry class did a beautiful electrolysis experiment by mixing a universal pH indicator into the salt solution. The indicator changes color based on how acidic or basic the solution is; we’ve used this behavior to show how blowing bubbles in water increases its acidity.
In this experiment, when electrodes (graphite pencil “leads”) are placed into salt (NaCl) water and connected to a battery, the sodium (Na) and chloride (Cl) split apart.
NaCl –> Na+ + Cl–
The positive sodium ion (Na+) migrates toward the negative electrode, where it gets an electron and precipitates on the electrode as a plating. This is called electroplating and is done to give fake gold and silver jewelry a nice outward appearance.
Similarly, the water (H2O) also dissociates into hydrogen (H+) and hydroxide (OH–) ions.
H2O –> H+ + OH–
The positive hydrogen ions (H+) go toward the negative electrode where they get an electron from the battery and are liberated as hydrogen gas (when they bond to another hydrogen you get H2 gas). However, releasing the positive hydrogen ion, leaves behind hydroxide ions in the area around the positive electrode.
The opposite happens at the positive electrode, with hydrogen ions left behind in the solution.
Since acidity is a measure of the excess of hydrogen ions in solution (H+), the left behind hydrogen ions make the solution near the positive electrode acidic, which turns the indicator solution red. The OH– left near the negative electrode make the solution basic, which shows up as blue with the indicator.
If you gently shake the petri dish you end up with beautiful patterns like this:
And this:
Note: if the solution is mixed completely the hydrogen and hydroxide ions react with each other to make water again, the solution neutralizes, and becomes uniform again.
Note 2: This is an experiment that I should also do in physics. It should be interesting for students to see this experiment from two different perspectives to see how the subjects overlap.
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.
Why is the periodic table called the periodic table? Because of the periodic changes in the properties of the elements: there are patterns to the properties that repeat, time after time, as you go through the sequence of the elements. One key repetition, which affects the way different elements react, is in the electron configurations, however, other properties change as well. In fact, history of the periodic table
is a story of scientists trying to figure out the properties of unknown elements (not to mention figuring out that there were undiscovered elements) based on what they knew about the periodicity of the known elements.
File: periodic-table-properties.xls
In this exercise, we look at four different properties that students need to be aware of: density, melting point, ionization energy, and electronegativity. I’ve compiled the data in this spreadsheet: periodic-table-properties.xls; and I handed out the first page, with the properties of the first 38 elements (periodic-table-properties.xls.pdf).
I broke the class into pairs and had each pair graph one of the four sets of data. With 16 students that meant that we had a replicate of each graph, so I could use the redundancy as a quick check that they’d done them correctly.
As they put drew their graphs I went around the classroom, paying special attention to the students working on ionization energy and electronegativity. Especially for the latter, I’d picked pairs who I figured would be able to get the graphs done quickly but would appreciate the extra challenge of figuring out what electronegativity actually is. This way, when everyone was done, the students could use their graphs to look for the patterns and explain what they’d found to the rest of the class.
