This is an introductory course for students with limited background in chemistry; basic concepts involved in chemical reactions, stoichiometry, the periodic table, periodic trends, nomenclature, and chemical problem solving will be emphasized with the goal of preparing students for further study in chemistry as needed for many science, health, and policy professions..
Orbital Energy and Ionization Energy Part II
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來自 Duke University 的課程
Introduction to Chemistry: Reactions and Ratios
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Matter and Energy
<p>If you are interested in significant figures in more detail, here are some <a href="https://www.khanacademy.org/math/pre-algebra/decimals-pre-alg/sig-figs-pre-alg/v/significant-figures" target="_blank">good videos</a> to follow on Khan Academy.</p><p>This week we will continue our explorations of matter and energy. We will discuss the sub-atomic particles that govern chemical reactions, isotopes, anions, and cations. We will learn how to name compounds, calculate formula masses, convert between grams and moles, examine periodic trends, and more! An advanced problems set is posted now; that is a longer assignment and is optional unless you would like to be eligible for the Honor’s Track. You can still earn a regular verified certificate without completing the advanced problem sets, so please be sure to keep working on the normal weekly exercises.</p>
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Prof. Dorian A. CanelasAssistant Professor of the Practice
Chemistry
[BLANK_AUDIO]
The ionization energy, or in this case the first ionization energy,
is the energy needed to remove one electron from a neutral atom.
So here's a generic atom A, it can be any element.
Okay, so A has the same number of electrons
as it does protons over here on the left.
So it's a number of electrons equals number
of protons.
Okay, that is really bad hand writing, but that's okay.
You can read it, I'm sure.
We are going to put some energy, the ionization energy amount into the atom.
So this number, this is an amount of energy that we're putting in.
Amount of energy,
that we are adding. So we're putting it into the system.
Okay, and because of that extra energy, the atoms has to react somewhow.
And one of the ways it can react, the most,
the most common way for it to react, is to
for the electron to move to higher energy stage, which
in an atom is further away from the nucleus, right?
So if we put exactly the right amount of energy in,
the ionization energy, which is an amount of energy we can measure.
Then what happens is one electron is ejected by the atom.
Okay, and the atom becomes positively charged.
This is one of the things that happens when you get a medical
diagnostic x-ray, if you've ever had one because they thought you had a broken
bone, for example.
Okay, this is one of the things that happens, is those x-rays, very high energy
waves, they hit your body's tissue and some
of the tissue in your body ejects electrons.
Okay?
And it depends on what type of element that particular tissue is composed of.
How many electrons are ejected from that particular type of tissue?
Well, for metals,
there's a small first ionization energy.
Doesn't actually take as much energy to eject an electron from a metal
It's easy to remove the electron when we compare that to a non-metal.
A non-metal has a relatively large first ionization energy.
It's difficult to remove electrons from non-metals.
So the trend for the ionization energy, the first
ionization energy now, taking the electrons
away from neutral atoms is that it
takes more energy to take electrons away from non-metals than it does for metals.
Now, if you know anything about Human Biology and you probably know
more than I do, because I certainly do not know very much.
I did try to take my colleague Mohamed Noor's
genetics and evolution course there which I enjoyed tremendously.
But even after
I did that you probably still know more about Biology than I do.
You probably know that your bones, your skeleton is,
has a lot of the element calcium in it.
Now it's compounds of calcium, but there's a
lot of the element calcium in your bones.
And other tissue in your body is comprised
mo, more of non-metals, there's water, there's carbon, right?
There's proteins, there's oxygen.
So when you get
an x-ray, think about which tissue shows up the brightest on the x-ray.
The tissue that ejects the most electrons is
the tissue that has the smallest first ionization energy,
which is the calcium, the metal that's in your body.
And so the bones show up brightly on the x-ray, other tissue not as bright.
And it's a very good diagnostic way to see if you have a fracture in your bone.
Because if there's a little fracture there'd be
a dark area where there's not any calcium.
Isn't that cool?
But that's also why you shouldn't have more x-rays than you need, right?
And we've,
we've gotten into a situation with the health community and I'm not an expert
certainly in, in recommendations for health, but
they've got into a situation where they determine
that sometimes they take so many x-rays of a patient, perhaps a cancer, a lung
cancer patient for example, that the x-rays
themselves probably start contributing to more cancer forming.
Because the x-rays do damage your tissue.
Every time you get an x-ray,
okay, you put in ionizing energy into your body, and electrons are being ejected.
Isn't that cool?
Chemistry is so cool.
There's so many things you can explain with chemistry.
All right.
Let's look at some actual ionization energies.
This is from a book I used to teach with.
And you see carbon, we're a carbon-based life form, right?
A lot of our tissue, our DNA, and our proteins, and our fats, carbohydrates,
have carbon backbones. So we can see how much energy it takes to
remove one electron from the outermost orbital of a carbon atom.
Now, I've done something a little fancier in this diagram, right?
And this diagram is showing more complicated types
of orbitals, is labeling them 2s or 2p.
The two is from the period that it's in, the row.
These, all these are second period elements.
The s or the p has something to do
with the shape, describes the shape of the orbital.
We are going to
learn more about that later, don't get bogged down with that.
The electrons that are shown here in this actual ionization
energy graph, which again, I said came from a textbook.
This is not my graph, it's my friend Denis Wirtz's graph.
The actual ionization energies here are only for
the ones that are in the outermost shell.
There are other electrons for fluorine, for example,
fluorine doesn't have just five electrons, it's got
nine electrons.
So there's some lower energy electrons for fluorine down here, right?
In a 1S and a 2S orbital. And I don't have those drawn here.
Because, when it comes to the first ionization energy, the only
thing we care about is removing the highest energy electron of fluorine.
How much energy does that take?
Well, they've measured it.
And for one mole of fluorine, it takes 1,681 kilojoules
to remove a mole of electrons from a mole of fluorine atoms.
So I'm measuring this distance really.
Here's the distance up to the energy of zero, you see that?
All these energies are negative.
See how they're all negative?
Because this is an interaction between protons of
the nucleus, like here's lithium, and negatively charged electrons.
Lithium takes
the smallest amount of energy to ionize, to make lithium one plus, so after
the electron left, once the electron gets up here to the zero value, right?
Once it gets way up here, it can no longer feel the positive pull of its nucleus.
It can't tell that there's any positive charges down there
in the direction of the nucleus, so it just floats away.
So for the metal, for the lithium, it doesn't take as much
energy to get the electron to no longer be associated with that atom.
It takes more energy to do that for the non-metal, for the fluorine.
Do you see how that works?
And what I'm doing here is I'm comparing the size of these red distances.
These, these, I can measure actually a distance in energy.
Here I measured, it's 520 for lithium,
do you see that? All right, now it's your turn to try one.
Are you ready?
Here are three fictional atoms.
Okay, so three different atoms, X, Y, and Z.
Y is not itrium, okay they're just all fictional, X, Y and Z.
And this is where, these are their outermost electrons.
So Z, you see, I've shown two sets of electrons
so this would be the outermost one right here, right?
Here's the outermost, we just circle it, here's the outermost
one for Y and here's the outermost one for X.
I want you to put these atoms in order of increasing first ionization energy.
Right?
So what you're going to be doing is [UNKNOWN],
okay, I've got the smallest first ionization energy.
Right? And you just list something.
And that's less than the one that's in the middle, which is less than the one
with the largest first ionization energy. All right.
So go ahead and do that.
Rank them.
Which one has the smallest, sometimes people say the lowest, although
I find that confuses students, which one has the smallest amount
of energy necessary to remove the outermost electron and which one
has the largest amount of energy needed to remove the outermost electron.
Okay, now that you've ranked them, go ahead and put
in your answer for which one has the smallest first ionization energy.
Right?
Answer this question.
You've ranked them all, now which one's the smallest?
Wonderful.
Hopefully you can see, looking back at it now,
that if the ionization energy is getting the electrons up
here to some value of zero energy, where they
can no longer feel the positive pull of the nucleus.
That's a pretty short trip for the electron that's outermost of Z.
That only needs to go that distance in energy.
I don't have units on my free energy over here,
but it doesn't matter what the units are.
All of these are on the same axis, so it's the same units for all of them.
Y has a slightly larger trip, and X has the largest trip of all.
So the smallest first ionization energy was Z, followed by Y, followed by X.
You see how that works?
Wonderful.
Now, I said that this lecture is about periodic trends.
So let's look at the periodicity of ionization energy.
So to review the periodic trend for ionization energy, we can
examine this plot of ionization energy as a function of atomic number.
So ionization energy is on the Y-axis,
right, it increases as we go higher on the graph.
Atomic number is on the X-axis.
Some of the elements are labeled.
But even the ones that aren't labeled, you can figure out what
they are just by looking up their atomic number on the periodic table.
Hydrogen is, of course, right here, because
that's got an atomic number of one.
So we could do that for all of them all the way across.
So, this must be lithium, down here,
if I was going to label things, right? This is lithium, right here, and
then the ionization H energy increases as I go across the second period,
okay, up to a peak of neon. So here's, so helium is the first peak.
I'm going to make that blue, I think.
Helium is the first peak, neon is the second peak.
And then,
when I get down to the next valley, which element do you think it is?
So I've gone across, this is the second period.
Then I, then I get to the third period which starts with sodium.
And then I move across the third period until
I get to argon, which is the next peak.
Once I get to the fourth period, where potassium is the metal.
Then I have to go across the transition metals.
So this little flat part here, that peaks right
there, that's going across the D-block the transition metals.
And then I go back to what's called the p-block, back to the
non-metals, and I, and I end up with a maximum at Krypton again.
So what do you notice is happening if I'm doing a trend?
What's this trend up here?
You see it? Helium, Neon, Argon, Krypton, Xenon.
Those are the ones that are at the peak for this trend in ionization energy.
And it's the first ionization energy, actually.
Because you can, actually there's another trend for
removing the second electron from a positively charged cation.
But we're looking at the first ionization energy here.
So you can see the peaks are the inert gases.
Down in the valleys, I had the alkali metals, right?
And
I can see similar types of patterns for transition metals.
So here is the first row that has transition metals in it, if you will.
And here's the next one further down. Okay?
And then I can see them again here. What do you think is right here?
What's right here where it's flat? What do you think is there?
Well, remember down underneath the periodic table, if you
take a look at it there's the lanthanides there.
So the lathanides don't change their ionization energy very much
as we go across the atomic numbers, but they all have
a fairly low ionization energy, which means it's fairly easy
to remove electrons from all the metals that are the lathanide.
It's fairly easy to remove electrons from
lithium and calcium and magnesium and other metals.
Iron, copper, lead,
tin, all those species are down here at low ionization energy.
It's relatively easy to remove electrons from them
to make them into what type of ion?
So if I remove an electron, this is review, if I remove an electron from an
atom, remove one electron from a neutral atom, what type of ion do I make?
A cation or an anion?
Wonderful.
Thank you for joining me for this lecture on An Introduction to Periodic Trends.
And hopefully, you can see that orbital energy
and ionization energy kind of fit together, don't they?
Tune into the next lecture to learn about two more periodic trends;
electronegativity and atomic radii. Thanks.
