Month: February 2013

Introducing Covalent Bonding

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.

Sodium and chloride bond ionically when sodium donates an electron to chlorine.

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).

An hydrogen atom.

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.

Two hydrogen atoms bond covalently by sharing electrons.

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

H2O

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.

An oxygen atom.

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.

Bonding to form a water molecule.

Thus we create water, which has the chemical formula H2O, and the chemical reaction can be written:

2 H + O –> H2O

Drawing covalent molecules

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:

Drawing a water molecule. The lower drawing is called a Lewis-Dot structure.

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.

Double Bonds

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.

Oxygen gas.

Practice

Now you can try drawing these covalent molecules:

  1. A molecule with one nitrogen atom and some hydrogen atoms (can you figure out how many hydrogens)
  2. A molecule with the chemical formula: CH4
  3. A molecule with the chemical formula: C2H6
  4. A molecule with the chemical formula: C3H8
  5. A molecule with the chemical formula: C2H4 (hint there’s a double bond)
  6. A carbon dioxide molecule, which has the chemical formula: CO2
  7. An ozone molecule, which has the chemical formula: O3
  8. An alcohol molecule, which has the chemical formula: CH3OH

An Introduction to Ionic Bonding

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.

This sodium atom has one electron in its outer shell. It could be happier without it.

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.

Chlorine needs one more electron in its outer shell to be happy.

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).

Sodium and chloride bond ionically when sodium donates an electron to chlorine. This produces the ionic compound, sodium chloride (NaCl).

The chemical reaction can be written as:

Na + Cl –> NaCl

MgCl2

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 magnesium (Mg) atom, which has two electrons in its outer shell that it would like to, if possible, get rid of by bonding.

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.

Magnesium gives one electron to each of the two chlorines to create magnesium chlorine.

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).

Practice

Now, for homework, you can try figuring out what is the chemical formula for the following ionic compounds:

  1. Potassium and Florine
  2. Beryllium and Chlorine
  3. Sodium and Oxygen
  4. Magnesium and Oxygen

Be sure to:

  • Draw the atoms that you will be reacting,
  • Show how the electrons are donated,
  • Write the chemical formula of the resulting compound,
  • Write the chemical reaction.

Good luck. Next we’ll try covalent bonding.

* Think of happiness as energy. Like people, atoms are happier to be in low energy states.

Notes

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.

The repeating pattern in electronegativity shows up quite well in the first 20 elements.

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.

Dissecting Computer: Building a Hovercraft

Extracting the hard drive from an old computer.

Our school was recycling some old computers, so my students convinced me that it would be worthwhile o dissect a few of them to see if there was anything worth saving. It was quite remarkable to see just how interested they were in examining the insides of the machines — a few desktop computers and a monitor — but I guess I shouldn’t have been surprised. After all, it’s getting harder and harder to open up their iPods and other electronics, and even more difficult to repair and repurpose them, so I can see why students would jump at the chance of looking inside a device. Also, they tend to like to break things.

Pulling apart a monitor.

To get them to think a little more about what they were seeing, I got a couple students to draw a scale diagram of one of the motherboards, and write up a report on what they’d done.

Diagramming a motherboard.

Some of the other students spent their time trying to make all the motors, LED’s, and lasers work by hooking them up to 9-Volt batteries. Then they found the fans… and someone had the brilliant idea that they could use it to make a hovercraft. Using a gallon sized ziplock bag and some red duct tape, a prototype was constructed.

Hovercraft prototype.

The fan would inflate the bag which would then let air out the bottom through small holes. I convinced them to try to quantify the effectiveness of their fans before they put the holes in by hooking the bag up to one of our Vernier pressure sensors that plug into their calculators. Unfortunately, the sensor was not quite sensitive enough.

Attempting to measure the hovercraft’s bag pressure using a gas pressure sensor connected to a calculator.

This was not how I had planned spending those days during the interim, but the pull of following the students’ interests was just too strong.

Ski Trip (to Hidden Valley)

Approaching a change in slope.

We took a school trip to the ski slopes in Hidden Valley. It was the interim, and it was a day dedicated to taking a break. However, it would have been a great place to talk about gradients, changes in slopes, and first and second differentials. The physics of mass, acceleration, and friction would have been interesting topics as well.

Calculus student about to take the second differential.

This year has been cooler than last year, but they’ve still struggled a bit to keep snow on the slopes. They make the snow on colder nights, and hope it lasts during the warmer spells. The thermodynamics of ice formation would fit in nicely into physics and discussion of weather, while the impact of a warming climate on the economy is a topic we’ve broached in environmental science already.

The blue cannon launches water into the air, where, if it’s cold enough, it crystallizes into artificial snow. The water is pumped up from a lake at the bottom of the ski slopes.

The Edges of the Universe

… observed astrophysical black holes may be Einstein–Rosen bridges, each with a new universe inside that formed simultaneously with the black hole. Accordingly, our own Universe may be the interior of a black hole existing inside another universe.

— Poplawski, 2012: Radial motion into an Einstein–Rosen bridge (pdf) in Physics Letters B.

For some reason the Big Bang theory came up during a middle-school class discussion last week; specifically, the mind-bending question of what exactly was there just before and at the beginning of the universe. We also meandered into the question about what’s at the edge of the universe — and how can the universe have an edge where time and space end.

There really aren’t any satisfactory answers to these questions, especially not for middle schoolers. But it allowed me to talk about how science is really just the best explanations for the known observations. Unsatisfactory answers to these questions are why we have the scientific method.

A black hole passing in front of a galaxy acts as a giant lens. Image by Urbane Legend via Wikipedia.

To throw a little more mind-bending fuel on the fire, however, I’m going to show the class the article quoted above.

It’s an interesting example of how scientists see the universe through math. And how they strive to make sense of things they can’t see or touch.

Student-Led Classroom Design

When given the choice, the environments students choose to work in does not look like the typical classroom. Mrs. D., our head of school, shared a link to this article about the Swedish Telefonplan School that’s designed with the students’ preferences in mind.

From inside the Telefonplan School. Image via Zilla Magazine (hat tip Edudemic).
The inside of the Erika-Mann Elementary School. Photo by Jan Bitter.

It’s a lot like the Erika-Mann-Grundschule in Berlin, and the type of open-plan rooms that Montessori Middle Schools aim for. I particularly note the design gives lots of space for small group work. Adolescents tend to cluster, but seem to work most productively in smaller groups.

The group of Lamplighter Montessori students work in parallel but help each other out.

And given the choice, students often prefer the floor to the tables.

Fulton School students choose to sit in the window to work on their math.

Meteor Impact

The footage was taken [on February 14th] in the Urals, where over 200 were injured from the impact. The meteor was likely related to the asteroid 2012 DA14, which is scheduled to graze our planet [on the 15th] at 7.25 pm EST.

— Sullivan 2013: The Big Rock Heading Our Way on The Dish.

More details on Watts Up With That?

The meteor ended up hitting a factory. Photo from Twitter.com user @TimurKhorev via Watt’s Up With That.

Now think about the KT impact.

Update: More details and video at Sky and Telescope. Including this one:

Points for Gryffindor: Houses in the Middle School

Keeping score of the house cup.

We were discussing the rules about how students should act when they work in groups: work to find work to do; allow people to work; be respectful; be focused on the project at hand. As the discussion evolved into what we should do about enforcement, one student suggested that positive reinforcement would be better than negative, so maybe they should get points for good behavior. Being students raised on the Harry Potter series, it was perhaps inevitable that someone would come up with the idea of separating the class into houses that could compete to see who got the most marbles/points.

Support for an inter-house competition was unanimous — after a little more discussion and explanation — and they were able to persuade me to try it. Each group would be its own house — they got to choose the names — and would earn marbles as a group.

I dug up some marbles and a few old jars overnight. I realized, however, that I’d run out of marbles pretty quickly if I gave them out as liberally as I wanted to, so I whipped up a website to keep score long-term.

They chose names. The names all ended up being Harry Potter themed — over some opposition within the groups, however. No one wanted Hufflepuff, and one group flirted with Slytherin, but we finally ended up with Griffindor, Ravenclaw, Chudley Cannons, and S.P.E.W. (Society for the Promotion of Elvish Welfare). I think they may have, briefly, given each other individual names out of the books, but I was not privy to those deliberations.

How’s it working?

It’s working remarkably well so far. It was originally their idea, and they were forced to persuade me that it was worth trying, so I think they’re well invested in making it work. Our discussions have been much more organized, with fewer people speaking out of turn. And we’ve had much more discussion and questions among them since that’s one way to gain points.

I’ve made it a point to use the physical jars with marbles. They can hear the marbles clink when they fall in, so they get direct, unobtrusive positive feedback.

I’ve also made it a rule that they don’t get points if they ask for them — to reduce the lobbying — but they can still challenge if they did not get points they think they deserved; I always encourage them to think that they’re entitled to a reasonable response from me on any subject (its a good way of keeping me honest, and it helps them see the bigger picture).

The students were also able to change my mind about taking away points. I’d originally wanted to only give positive rewards, but they thought they could handle the negatives just fine, and were kind-of looking forward to them. And I have to admit they seem to work. Now, I’m not the only one trying to keep these adolescents in line. They’re getting pressure for good behavior from their peers; a much more potent source of influence for kids in a stage that features social development.

The inter-house rivalry is also healthy enough at the moment. They’re quite happy to see the other groups loose points, but seem to realize that openly advocating for it would not be a particularly advantageous move.

At the end of the quarter, the students want some sort of reward for the house with the most points. I told them that they should make proposals, because I had no good ideas.

In all, it has started quite promisingly. We’ll see how it goes.