Both [thermometer and proxy] records also show that the global warming in the last 15 years of the record (1980–1995) is significantly faster than that of the long-term trend (1880–1995).
To figure out what the weather and climate were like in the past, before things like thermometers were invented, scientists use proxies such as: the change in tree ring thickness; differences in the isotopic composition of shells and rocks; records of species change in the oceans; gases in bubbles trapped in glacial ice (as well as the character of the ice itself). Paleoclimatologists at NOAA have analyzed 173 different proxy records to provide a lot more evidence that the increase in temperature we’ve been measuring for the last 150 years (with thermometers) is real.
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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:
A molecule with one nitrogen atom and some hydrogen atoms (can you figure out how many hydrogens)
A molecule with the chemical formula: CH4
A molecule with the chemical formula: C2H6
A molecule with the chemical formula: C3H8
A molecule with the chemical formula: C2H4 (hint there’s a double bond)
A carbon dioxide molecule, which has the chemical formula: CO2
An ozone molecule, which has the chemical formula: O3
An alcohol molecule, which has the chemical formula: CH3OH
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:
Potassium and Florine
Beryllium and Chlorine
Sodium and Oxygen
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.
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.
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.
… 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.
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.
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.
The universal pH indicator turns red for acids and blue for bases.
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.
Changing colors of universal indicator show how blowing bubbles acidifies water (light green-second beaker) from neutral pH (dark green-third beaker) standard. For comparison, the first beaker (red) is acidified while the last beaker (blue) is made alkaline.
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–
Hydrogen bubbles forming at the negative electrode.
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:
Swirls.
And this:
After the electrodes have been disconnected.
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.
