So for example, all atoms of sodium have 11 protons in the nucleus.
And if you look
on the periodic table, the atomic number for
sodium, the symbol is Na, remember, is eleven.
Your probably going to need a periodic table to
follow this lecture and do some of the in-video excersises.
So, go ahead and pause the video to get out a periodic table if you need to do so.
So, for example, in the last lecture, I noted, I noted that the
alkali metals, such as lithium, sodium, and potassium, tend to lose one electron.
For example, if a sodium atom has only 10
electrons, and not 11, then its charge is +1.
And because it has a positive charge, more
protons than electrons, it is called a cation.
The cation is a species that has a positive charge.
The first thing is that when a species forms an
ion, it does so by gaining or losing electrons, not protons.
If the number of protons is changing, then the identity of the element would change.
But the number of electrons can change and
the identity of the element remains the same,
it just changes into an ion.
An anion always has more electrons than protons.
It could be a monatomic anion, meaning there's a single
atom that has more electrons than protons, or it could
be a collection of atoms where the total number of
electrons is greater than the total number of protons, and
that's called a polyatomic ion. Again,
anions form whenever a neutral atom gains electrons.
Non-metals tend to be more attractive to electrons than metals.
So, non-metals tend to form anions more readily than metals do.
So, in this table, which we're going to fill
out together, we're going to start by being given a species,
and then we're going to determine the number of
protons, electrons and neutrons than that, that that species has.
If we look at the mass of a zinc atom we can see it's 65.39 atomic
mass units, which seems a little strange, so let's just round it down to 65.
If the mass of zinc is 65 atomic mass units and
we subtract the number of protons, which should weigh 30 atomic mass
units, we see that there are probably 35 atomic mass units
left, and those all come from the 35 neutrons that are present.
Well, in this case, you have to look out
for, remember, you have to look up the atomic mass.
Yttrium's atomic mass is 89. So, if the mass is 89, and I
You probably remember from one of the previous lectures, that
all atomic masses are based on carbon-12 as a standard.
We, as humans, arbitrarily decided to do that.
We decided that carbon that has six neutrons and six protons has
a mass of exactly 12. Everything
else is relative to the carbon. But if you have a pretty
decent periodic table and you look at it, you might notice, that the
average atomic mass reported even for carbon which is our standard, is not
exactly 12.00000. In fact it's 12.01 atomic
mass units for the average atomic mass.
Remember, that's the average atomic mass of all of the carbon on Earth.
Isotopes are atoms of the same element.
So, they have the same number of protons,
but there are different in their number of neutrons.
So, for example, I might have two different isotopes of carbon.
One of the carbons is called carbon-12. It has a mass number of exactly twelve.
So, it's got six neutrons and six protons.
But there's another stable form of carbon called carbon-13.
And you might have heard of yet another type of carbon called carbon-14.
Carbon-14 gets used in radioactive dating
of archaeological samples but that one's not
actually stable, so since we're saying stable
isotopes, I'm going to cross Carbon-14 out.
Let's do our calculation using just the isotopes
that don't decay over time, Carbon-12 and Carbon-13.
To calculate the number of neutrons, recall that we just take the
mass number and subtract the number of protons,
because the electrons don't have, a mass that matters.
The mass of the electrons is negligible.
So, Carbon-12 then, has six neutrons, and Carbon-13 has seven neutrons.
So, there's two different isotopes of carbon, that are stable on Earth.
Most of the carbon on earth has six neutrons.
But there's a little bit of carbon on earth,
about 1% of the carbon on earth, and that is in the plants we eats, in the
carbon dioxide in the air, has this extra neutron
and weighs 13 atomic mass units instead of 12.
Average atomic masses do vary a little bit from
place to place on the face of the planet Earth.
And that can be very useful sometimes when
we're trying to determine the origin of a sample.
In fact, the relative amount of Carbon-13 in a sample
depends not only on where it was found on Earth
but what time of day it is, if it is a plant.
When plants are actively breathing in carbon dioxide during the day they have a
different amount of Carbon-13 in their system
than they do at night when they're resting.
Isn't that interesting?
We can use the amount of Carbon-14 in the sample to determine
how old the sample is because once a species is no longer animate,
it's no longer consuming any other Carbon-14, samples like eating food,
and so the amount of carbon 14 in that sample decays extremely slowly over time.
The chemist can use that to determine how long the sample has been inanimate.
So, this is used all the time to date
things like mummies or plant fossils that are found.
Another time that isotopes comes in handy, this is part of the why do I care
portion of chemistry class Is when we're trying
to determine if someone has taken some artificial hormones.
For example, one of the things that can make you build muscle
mass more quickly is having testosterone in your system.
Testosterone is a naturally occurring steroid, that your body can synthesize
and men have more testosterone in their system, generally, than women do.
As it turns out, you can look at a sample of someone's blood and determine
if the testosterone in the sample is testosterone that their body manufactured
because then it has a certain ratio of Carbon-12 to Carbon-13.
Or, is it testosterone that was manufactured
in a laboratory, not by a human body?
That sample has a slightly different ratio of Carbon-12 to Carbon-13.
Now, we had a student at Duke many years ago, Derrick Lowe, who wrote a
wonderful blog post about this. Around the time that Floyd Landis
got in trouble for taking artificial testosterone, when he had the
fastest time in the Tour de France, which happened in 2006.
Now, shortly thereafter they stripped him of the title.
They determined that some of the testosterone in
his bloodstream was not testosterone that his body
had created and they did that using mass
spectrometry to look at the ratio of Carbon-12
to Carbon-13. Isn't that interesting?
If you are really interested in that, I encourage you to read more.
You can read more on, on Dr. Lowe's blog or you
can read more on other sites, but it is really interesting.
Let's do another example of isotopes.
We've talked a lot about carbon, but carbons,
most of the carbon on earth is one isotope.
Most of it's Carbon-12.
99% is Carbon-12.
There are other types of elements that have
a bigger split between different types of isotopes.
For example, chlorine.
a mass of 35 atomic mass units and some of the chlorine has a mass of 37
atomic mass units.
Remember, this is the mass number of a specific atom.
So, if we wanted to determine the number of neutrons.
Well, we know the number of protons because that's the atomic number.
We can just look that up on the periodic table.
For chlorine it's 17.
If we take 17 and subtract it from
35 we can determine that Chlorine-35 has 18 neutrons.
Chlorine-37 then, has two more neutrons. It's got 20 neutrons.
It's pretty simple math to do.
abundant, but this time the abundance isn't 99%, it's 70%.
So, these are things we've determined by
making observations of the world around us.
The abundance of Chlorine-37 is 30%.
Well, that makes the calculation for the
average atomic mass of chlorine a little strange.
It's the same
calculation that we did before. I'm going to take the average abundance.
If I had a 100 and 70% of them are Chlorine-35 then the abundance
of chlorine-35 is 0.7. That's 70 divided by 100, right?
And this is the mass Chlorine-35.
I have 30% Chlorine-37, so I take 30 divided by 100.
Multiply that times the mass of Chlorine-37.
I add those two numbers together, and I get that the average
Introduce students to the periodic table.
Project the image Periodic Table.
Tell students that this is the periodic table. Explain that each box contains information about a different atom. The periodic table shows all the atoms that everything in the known universe is made from. It’s kind of like the alphabet in which only 26 letters, in different combinations, make up many thousands of words. The 100 or so atoms of the periodic table, in different combinations, make up millions of different substances.
Note: It is often confusing for students to see the terms “atom” and “element” used interchangeably as if they are the same thing. Explain to students that an atom is the smallest particle or “building block” of a substance. An element is a substance made up of all the same type of atom. For instance, a piece of pure carbon is made up of only carbon atoms. This piece of pure carbon is a sample of the element carbon. The people who developed the periodic table could have called it the Periodic Table of the Atoms but they did not have a firm understanding of atoms at that time. Since they were working with actual samples of elements such as copper, mercury, sulfur, etc., they called it the periodic table of the elements.
Play one or both of the following songs.
- The Elements by Tom Lehrer with animation by Mike Stanfill
- Meet the Elements by They Might be Giants
Explain the meaning of the numbers and letters in the boxes in the periodic table.
Tell students that the class will focus on the first 20 elements over 2 days. On the first day, they will look at the number of protons, electrons, and neutrons in the atoms of each element. On the second day, they will look at the arrangement of electrons in the atoms.
- Give each student a copy of the periodic table of the elements, the periodic table of elements 1–20, and the activity sheet.
- Students will use the periodic table of elements 1–20, along with the activity sheet, in the lesson they will do today.
Project the image Periodic Table of the First 20 Elements.
Project the image Element explanation.
Explain what the numbers and letters in each box on the periodic table represent.
Explain atomic mass.
The atomic mass of an element is based on the mass of the protons, neutrons, and electrons of the atoms of that element. The mass of the proton and neutron are about the same, but the mass of the electron is much smaller (about 1/2000 the mass of the proton or neutron). The majority of the atomic mass is contributed by the protons and neutrons.
For any element in the periodic table, the number of electrons in an atom of that element always equals the number of protons in the nucleus. But this is not true for neutrons. Atoms of the same element can have different numbers of neutrons than protons. Atoms of the same element with different numbers of neutrons are called isotopes of that element. The atomic mass in the periodic table is an average of the atomic mass of the isotopes of an element. For the atoms of the first 20 elements, the number of neutrons is either equal to or slightly greater than the number of protons.
For example, the vast majority of carbon atoms have 6 protons and 6 neutrons, but a small percentage have 6 protons and 7 neutrons, and an even smaller percentage have 6 protons and 8 neutrons. Since the majority of carbon atoms have a mass very close to 12, and only a small percentage are greater than 12, the average atomic mass is slightly greater than 12.
Describe the activity students will do to learn about the first 20 elements of the periodic table.
Show students that you have 100 cards (5 for each of the first 20 elements). Explain that each card contains information about one of the first 20 atoms of the periodic table. The students’ job is to read the card carefully, figure out which atom the card is describing, and put the card at the spot in the room for that atom.
Review the information about protons, electrons, and neutrons students need to know in order to match the cards with the correct element:
- Positively charged particle in the nucleus of the atom.
- The number of protons in an atom’s nucleus is the atomic number.
- Negatively charged particle surrounding the nucleus of the atom.
- The number of electrons surrounding the nucleus of an atom is equal to the number of protons in the atom’s nucleus.
- Particle in the nucleus that has almost the same mass as a proton but has no charge.
- For the atoms of the first 20 elements, the number of neutrons is either equal to or slightly greater than the number of protons.
To match the number of neutrons listed on your card to the correct element, look for an element on the periodic table so that if you add the number of neutrons on your card to the protons of the element, you will get close to the atomic mass for that element. For example, you may have a card that says that the atom you are looking for has 5 neutrons. You would look at the periodic table to find an atom that you could add 5 to its number of protons that would give you a sum close to the atomic mass given for that element. The answer is beryllium (Be), which has 4 protons and an atomic mass of 9.01.
Note: There are a few neutron cards that have two possible correct elements instead of just one:
- 6 Neutrons—Boron or Carbon
- 10 Neutrons—Fluorine or Neon
- 12 Neutrons—Sodium or Magnesium
- 16 Neutrons—Phosphorous or Sulfur
- 20 Neutrons—Potassium or Calcium
Have groups work together to place each card with its correct atom.
Distribute the cards to groups. If you have 10 groups, each group will get 10 cards. Be available to help students who have trouble with the neutrons and atomic mass.
Discuss the placement of the cards for two or three atoms.
Select two or three atoms and review whether the cards were placed correctly. This review will help reinforce the concepts about the structure of atoms and help students determine the number of protons, electrons, and neutrons in each type of atom.
Have students begin filling out the activity sheet with the following information:
- Number of protons
- Number of electrons
- Number of neutrons (usually)
Introduce students to their element project and an online resource that they can use.
Assign each student to an element. Include the first 20 elements and any other elements that you find interesting so that each student can research and present their own.
Each student should find and present some basic information about their element to the class. The presentation can be in the form of a poster, pamphlet, PowerPoint presentation or other form. The presentations should be short and can include: atom name, atomic number, derivation of name, when and where discovered, natural sources of the element, major uses, and any other information you find important.
Some Internet sources for this information can be overwhelming. They can also contain advertising that you may not want students exploring. For basic information about the periodic table, including some images and video, The Journal of Chemical Education’s Periodic Table Live is an excellent resource.
If there is time available, have students work on this atom project during the week.