A student asked this question about black holes during a discussion, and I didn’t have a good answer. Now there’s this:
A study last year found unusually high levels of the isotope carbon-14 in ancient rings of Japanese cedar trees and a corresponding spike in beryllium-10 in Antarctic ice.
The readings were traced back to a point in AD 774 or 775, suggesting that during that period the Earth was hit by an intense burst of radiation, but researchers were initially unable to determine its cause.
Now a separate team of astronomers have suggested it could have been due to the collision of two compact stellar remnants such as black holes, neutron stars or white dwarfs.
While long [Gamma Ray Bursts (GRBs)] are caused by the core collapse of a very massive star, short GRBs are explained by the merger of two compact objects … [such as] a neutron star with either a black hole becoming a more massive black hole, or with another neutron star becoming either a relatively massive stable neutron star or otherwise a black hole.
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.
Prep: Memorization over the Winter Break
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.
A diagram of an oxygen atom.
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.
Day 1. Lesson: The Parts of an Atom
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.
Template for the first 20 elements of the periodic table. (pdf)
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 written as a subscript to the left of the element symbol.
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 atoms have the same number of electrons as protons. Protons are positively charged, and electrons are negatively charged, so an atom needs to have the same number of both for its charge to be balanced. We don’t talk about ions –where there are more or less electrons– until later.
The atomic mass (4) is written as a superscript to the left of the element symbol. The atomic mass is the sum of the number of protons (2) and the number of neutrons (2).
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.
This oxygen atom has 8 electrons in two shells.
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.
Example: Carbon-12
For example, the most common form (isotope) of carbon-12 is written as:
Protons = 6: Since we know the atomic number is 6 (because we memorized it), the atom has 6 protons.
Neutrons = 6 : Since the atomic mass is 12 (upper left of the element symbol), to find the number of neutrons we subtract the number of protons (12 – 6 = 6).
Electrons = 6: This atom is balanced in charge so it needs six electrons with their negative charges to offset the six positive charges of the six protons. (Note: we haven’t talked about unbalanced, charged atoms yet, but the charge will show up as a superscript to the right of the symbol.)
Electron shells (2-4): We have six electrons, so the first two go into filling up the first electron shell, and the rest can go into the second shell, which can hold up to 8 electrons. This gives an electron configuration of 2-4.
Diagram of a carbon-12 atom.
Example: Carbon-14
Carbon-14 is the radioactive isotope of carbon that is often used in carbon dating of historical artifacts. It is written as:
Protons = 6: As long as it’s carbon it has six protons.
Electrons = 6: This atom is also balanced in charge so it also needs six electrons.
Neutrons = 8 : With an atomic mass of 14, when we subtract the six protons, the number of neutrons must be 8 (14 – 6 = 8).
The only difference between carbon-12 and carbon-14 is that the latter has two more neutrons. These are therefore two isotopes of carbon.
Diagram of a carbon-14 atom.
Example: Helium-4
Diagram of helium-4 atom.
Example: Sodium-23
Diagram of sodium-23 atom.
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.
The rate of radioactive decay of Chlorine-36 (blue x's) seems to be related to the distance between the Earth and the Sun (red line). (Image from Dekant, 2012).
In pre-Calculus, we’re figuring out how to match curves to data. The scientists in this study do something similar, trying to see what types of sinusoidal curves will match the data, then seeing what natural phenomena have the same period (the time it takes for one cycle).
Isotopes of hydrogen: hydrogen, duterium and tritium.
I have been told by reliable sources that the difference between atomic mass and atomic weight is that the atomic mass is the mass of a single atom (number of protons plus the number of neutrons), while the atomic weight is the averaged masses of all the different isotopes you would find in a natural sample.
This obviously requires a discussion of isotopes, which may be a topic best left for high school. However there are a number of interesting hooks that could capture the imagination.
It’s absolutely amazing how much the different numbers of neutrons in atoms can tell us about the ourselves and the world. Over 99% of the carbon in the atmosphere is carbon-12, with 6 neutrons and 6 protons, but the rest is made of carbon-12 (6 protons and 7 neutrons) or carbon-14 (6 protons and 8 neutrons).
Carbon-14 is radioactive and is used to date things for archeology and climate change etc. However, when it comes to our diet carbon-13 is a bit more interesting. Some plants, particularly grasses like corn, do photosynthesis a little differently so that they tend to have more of the slightly heavier carbon-13 isotope than the others. As a result, if you take a blood sample, you can tell (roughly) how much of your diet ultimately came from grasses.
Why is this interesting? Because when you eat meat, there is a good chance that the animal you are eating was fed with corn. If you look at the pre-packaged items in the supermarket, you’ll find that high-fructose corn syrup is an important ingredient on many of them.
The documentary King Corn, and the book The Omnivore’s Dilemma find that if you trace the modern industrial food chain much of it starts in the corn fields of the mid-west. We eat, in one way or another, a lot of corn. In fact, blood samples have found over 50% of the carbon in our bodies comes from corn and similar grasses (like sugar cane).
This article describes a number of other interesting applications of isotopes in investigating diet. A more technical description of carbon-13 and diet can be found here.
