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..
The Structure of the Periodic Table
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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>
Rencontrer les enseignants
Prof. Dorian A. CanelasAssistant Professor of the Practice
Chemistry
At this point, you know about protons, neutrons, and electrons.
So, it's a great time to begin discussing,
the organization of the periodic table of elements.
As you continue to work on chemical
problems, the periodic table will come in very
handy, for calculating masses and predicting properties, such
as polarity and relative reactivity, of different atoms.
For this
reason, there's a periodic table on the inside
cover or the back of most general chemistry textbooks.
Therefore, I encourage you to keep your favorite
version of the periodic table handy, for quick reference.
Perhaps, even printing it out, so that you can customize it.
Moreover, I want to call your attention to the fact, that
you will find multiple links to both simple bare bones periodic
tables, as well as, much more elaborate interactive periodic
tables, on the sidebar of the course main page.
I encourage you to check out some of those interactive periodic tables,
that can be found on the internet, because those are really fun.
This slide shows, a small piece of the modern periodic table.
The modern periodic table is often credited to Mendeleev.
But it's important to realize that, the most popular modern arrangements of atoms
in the periodic table, was built upon
observations, by several scientists in the mid-1800s.
They noticed, that there was periodic repetition of similar chemical
properties and reactivities, even as the
elements get progressively larger and heavier.
So for example, there's a cycle of reactivity that only comes up
once every few elements, if you arrange the elements, from lightest to heaviest.
Several chemists, notably Newlands in England, Meyer in
Germany, and Mendeleev in Russia, are credited with arranging
the elements in tables.
So that either the rows or the columns
of elements, exhibit similar properties, such as valence.
Now, in the example shown here, we can see that there
weren't all of the elements known at the time, in the mid-1800s.
In fact, about 50 or 60 elements were known, at the time.
And new elements were being discovered, every year.
Mendeleev decided to arrange the
known elements, according to increasing approximate
atomic mass, and he was not the first person to do that.
But what he did, when he presented his paper
in the 1860s, was he made some pretty bold predictions.
Now, he was going along, arranging the elements according to approximate
mass and he was lining them up, so that the elements
that were similar, were together, he was actually putting
them into rows, but now they're put into columns.
And some of the elements weren't known.
All of these elements, that are shown
in gray here, were actually, not yet discovered.
So for example, scandium over here, that I've
shown as a gray box, wasn't yet discovered.
He actually predicted that, that element would be discovered.
And here's how he did that.
For example,
as he was going along, and he had them arranged from lightest
to heaviest, he noticed that the next heaviest element after zinc, was arsenic.
But arsenic, if you put it in the next spot
in the periodic table, doesn't have similar properties to aluminum.
In fact, arsenic behaves much more like phosphorus.
So Mendeleev said, well,
we know, we keep discovering more elements, so what I'm going to
do in my periodic table, is group the arsenic with the phosphorus.
So, he went ahead and he moved it, over to where he thought it was going to belong.
And then he predicted, in a presentation that he gave, that there were two
undiscovered elements, that would have atomic masses,
between the mass of zinc and the mass
of arsenic, that had not yet been discovered.
There were some other people who had predicted this before him and they hadn't
been taken very seriously, but he presented
it quite convincingly, and he was correct.
Later on, two more elements were discovered, that fit very
neatly into his arrangement of elements, in the periodic table.
Gallium and germanium were discovered. Scandium was also, later
discovered.
He had actually already given these
elements slightly different names, based upon where
they were located on the periodic table, and I think, that's pretty neat.
Now, I state that Mendeleev arranged the
elements, according to approximate known atomic masses.
Because he was putting them into an order that made sense, based on reactivity.
Because of this, he looked at the trends in reactivity, and
he decided, that the order of tellurium and iodine, wasn't correct.
Tellurium is actually heavier than iodine. But he said, well, that's not correct.
That puts things in the wrong location, on my table.
So, he actually said that those masses must
be incorrect, because they didn't follow his trend.
That was a pretty bold thing to assert, at that time.
In fact, the masses were approximately correct, because there happens
to be some heavier isotopes of tellurium, because of the neutrons.
But, he had them in the correct order, for the periodic table.
Because now, the periodic table has arranged
the elements according to approximate atomic number.
Remember, the atomic number, is the number of protons.
And even though,
iodine has more protons than tellurium, it is lighter.
So he actually, by moving things around a little
bit, was able to put things in the correct grouping.
And that's how it's now arranged, is by atomic number.
But using the basic idea that Mendeleev had, so
he's often called the father of the modern periodic table.
Let's look at some of the ways that,
elements can be grouped on the periodic table,
in the most broad sense.
If you look carefully at the periodic table, the
way it's drawn here, a common way of depicting it.
There are other ways of depicting it.
In fact, there's a spiral periodic table that you
can find online, that I think is pretty neat.
But most commonly, we'll see it look exactly the way it looks here.
It will have columns and rows.
18 columns across, with some columns down underneath, that are sort of, cut out.
They actually go in, right here.
So, these two elements and these two elements, are the same.
I think, part of the reason that those are
cut out, besides the fact that they were some
of the latest elements to be discovered, is that,
that would make the periodic table really wide, wouldn't it?
We would have to cut right here, and move
all of this over, insert these 14 columns, and
that would make it too wide to fit conveniently
in your textbook or on a piece of paper.
So, I think
that's why it's still presented this way.
So, there's 18 columns and there's 14 more columns, underneath.
The vertical columns, which are commonly called
groups or families, can be numbered, different ways.
And you see, I've shown several different ways of numbering those here.
I've shown one common method, that uses Roman numerals and letters.
I'm particularly fond of this method, that gives the main group elements,
numbers one through eight, with Roman numerals.
And that's because I know for example, that
carbon, which is right here, has four valence electrons.
And by numbering that way, it puts it into group four.
That makes it really easy to remember.
And over here, where fluorine is, it has
seven valence electrons and it's in group seven.
So that makes that, really easy to remember.
But, in different countries,
they label things, different ways.
In the United States, you will see these Roman numerals
with A's and B's, you'll see the B's are for
the transition metals, and for a little more confusing, in
the way that's numbered, if you've never looked at it before.
Those are very commonly, posted that way, on periodic tables that are
on posters, on the walls of universities in the, in the United States.
But in Europe sometimes, they might even have it
numbered, differently.
So internationally, chemists got together and decided,
that the correct way to do it now,
the modern way to do it, is the way that's shown up, across the top.
And that is to just number those 18 columns,
with one through 18 in Arabic numerals, not Roman numerals.
They're sort of ignoring the Lanthanides and Actinides, down at the bottom.
This is the Lanthanides and this is the Actinides.
But that's the way this internationally agreed upon now.
But I did want to show you the
Roman numerals, because it does show up, frequently.
Now, you can see, that some of the
elements of the periodic table, I've given different colors.
For example, most of the elements are kind of this lavender color,
and tha, that's the color that I'm using here, to show metals.
So most
of the elements that are known, are metals.
Some elements of course, are not metals.
Water for example, is made of nonmetals.
It's a molecule made of hydrogen and oxygen, and those are non-metals.
And the nonmetals on this periodic table, are over here in
the far right, in the upper part, and they're colored pink.
Between the metals and the non metals, are these orange atoms,
I'll just draw X's on them, sort of on the border, of
the non metals and non of the metals and non-metals, excuse me.
And those are called the semimetals or the metaloids.
So, those are the elements that bridge the metals, to the non metals.
Now, there's more known elements, which are often drawn down here.
In fact, I think the most recent one that's known at this time,
because we've been discovering more over the last two decades, is right here.
Okay, in the seventh period.
So you see, I have the periods numbered
one through seven, along the left and the right.
I've left the one off, on the right, but you get the idea, and that's the rows.
Sometimes, elements in the same row have
similar reactivity, particularly in the transition metals.
But more often we talk about, groups having similar reactivity.
And the groups remember, are in the columns.
Okay, now let's talk about the properties of
metals, non metals, and semi metals, to compare,
what's different about these types of elements and
why have we decided to group them together.
In other words, why are so many of the elements drawn here, as this light purple.
Most of the elements are drawn that way. Well, all those elements which are metals,
have similar properties.
So, here there's some nice photographs of some metals.
You could, if I asked you to name a metal, you might say, iron, or steel, or silver.
Right, you know things that are metals. Maybe, aluminum.
But all metals are solids at room temperature, except mercury.
Here's a picture of mercury.
Mercury is a liquid, at room temperature. We used to use
mercury a lot in thermometers, to measure
the temperature and manometers, to measure the pressure.
And in fact, even in little children's toys.
I remember having a little maze when I was a child, that was made of plastic.
That had little beads of mercury in it, and I would
try to get the me-, beads of mercury through the maze.
But then of course, invariably the plastic maze would
break and the mercury would get on the floor.
And then it was kind of fun, to push it around and play with it.
But
I think they don't allow that anymore, because mercury is a neuro-toxin.
So, that, by the time that I was, even five
or six, I think those toys were no longer allowed.
Anyway, that's an aside. So, metals.
Most metals are solids. Mercury's kind of, an anomaly there.
In fact, mercury they thought, had magical properties.
Some of the emperors of China thought that, by ingesting
mercury, they would live longer, and perhaps even become immortal.
So mercury is very interesting, for a number of reasons.
I encourage you to read more about it.
All right, all metals are reflective.
They have a reflective surface, if the surface is freshly cut.
Here's a picture of cutting a piece of, of sodium metal.
Now, sodium oxidises, to makes a white, sort of, a greyish surface [SOUND], here.
But if we slice
it with a knife, which we can do because sodium is actually very
soft, you can see it looks shiny, just like a nickel looks shiny.
Metals in fact, are generally prized because
of that reflective quality, that they have.
Metals are also known, to conduct heat well.
That's why we make cooking utensils out of metal.
And, they also conduct electricity well, which is why we have metal wires.
This is very convenient, because metals can be easily shaped into objects.
They're malleable, which means they can be hammered into shapes.
Sometimes, they're even molded into
shapes by melting them and then letting them
cool back off, after they're in a mold.
And they're also ductile, which means they can be pulled into wires, which
is very handy, when we're trying to take advantage of their electrical properties.
Finally, one of the things we'll talk about quite a lot in chemistry, is that
metals have a common property; that they
don't hold on to their electrons very tightly.
So, all of the atoms of course, have the protons
in the nucleus, and the protons are
attracted to the electrons, because of coulombic attraction.
Remember, opposites attract.
But the metals do not hold on to their electrons as tightly, as the non-metals.
So, they tend to lose their electrons, to
form something that's overall, positively charged, called a cation.
Most elements are metals, it's about 75% of the elements.
Okay, so what's different about the non-metals?
Here's a picture of a couple of non-metals.
There's some chlorine gas in that flask.
Here's an example of some carbon, which is a non-metal.
This a diamond, but carbon can also be graphite
or, or there's other forms of carbon known, as well.
So, unlike metals, which are almost all solids,
the non-metals can be found in all three states.
Some of them are gasses, like helium, neon, argon, oxygen,
nitrogen; those are things you know of, that are gasses.
Some of them are liquids, like bromine and even, and
other non-metals at room temperature and pressure, are solid, like carbon.
They differ from the metals because they are poor conductors of heat.
Okay?
So, materials that are made of all non-metals, for
example a piece of wood, doesn't conduct heat very well.
So you can use a wood spoon to stir a hot liquid, and the spoon doesn't get
very hot to conduct that heat to your hands,
the way that a metal spoon would, over time.
In addition, they're also poor conductors of electricity.
We use them as insulators. We make rubber, for example.
Mostly, it's made almost entirely, out of non-metals.
And so, it is an insulator, not a conductor.
The solids of nonmetals, are not malleable.
In other words, they can't be hammered into shape.
They tend to be brittle, in fact. Non metals, in converse to metals,
the non metals are very attractive to electrons.
Their nucleus has very high effective nuclear charge.
We'll talk about how that works later, but the point
is that, non metals, the net effect of that, is
that non metals do not tend to lose their electrons
very easily, except for hydrogen, which can lose or gain electrons.
Hydrogen is kind of in the middle, we'll talk about that in a minute.
Instead, the non metals in general, tend to gain electrons to become anions.
So, they tend to get more electrons than a
neutral atom would have, so that the net effect
is that, the atom now has a negative charge
over all, because there's more electrons than neu, than protons.
They're on the upper right-hand side of the
table, except for hydrogen, which is sometimes drawn on the upper right, right
above fluorine, but it's sometimes drawn way over on the upper left, above lithium.
And sometimes hydrogen's drawn right in the
middle of the top of the periodic table,
which is where I think it should be, and I'll show you that, in a minute.
Okay, let's move on to semi metals or metalloid.
Those are the ones that bridge the
metals to the non metals. The semi
metals, as the name implies, show some of the properties of
metals, and some of the properties of non-metals.
They're very, very important to modern society, because it is
the semi metals or the metalloid, that are good semiconductors.
And we use those, in our computer chips.
And so for example, if you're watching this on a laptop or on a, a smartphone.
You have a little silicon chip in there, probably, that
is a semiconductor, that is allowing you to watch this video.
Let's look at examples, of properties of silicon.
So, here's a nice picture of a silicon wafer, that
has some inte, tiny integrated cirfit-, circuits, etched onto it.
You see those little squares.
Those are little integrated circuits.
And later, they will cut that giant wafer up, to have dozens of those
little circuit chips, which can then be
put onto the motherboard, for your computer, perhaps.
It's really, really cool technology.
So, let's use the properties of silicon as an example, of a semi-metal.
Now, silicon is shiny, as you can see from the picture.
Kind of looks like a
piece of silver, actually.
And it also conducts electricity, not quite as well as a
metal, but it does conduct, particularly in som, some, under certain conditions.
So that conducting can be turned on and off, which allows
signals to be turned on and off, in the computer chip.
It does not conduct heat well, which is good.
As it's conducting the electricity, it doesn't overheat.
It is
not malleable or ductile, in fact, it is quite brittle.
And these large silicon wafers, are very fragile.
Okay, so that was a good review, of
some of the properties of metals, non-metals, and semi-metals.
And hopefully now you understand, why the periodic table of elements,
can be broken apart and grouped, into those three classes of materials.
Now that we reviewed some of the properties and
differences between metals, non metals and semi metals, let's go
back to looking at the periodic table, and see
some of the other ways, that elements have been grouped.
The periodic table has a lot more elements on it now,
then it did, when Mendeleev and Meyer were creating their periodic tables.
In fact,
now there are 98 elements that are
known to exist naturally, on the Earth's surface.
There are, at this point in time, 118
elements, that people have reported in the literature.
The heaviest elements, are not known to occur naturally on the Earth's surface.
Rather, they have to be synthesize, by man in a lab and they tend to be quite
unstable, and fall apart quickly.
For example, Livermorium, which is element number 116, right
there it has been synthesized several times,
so several atoms of livermorium have been
created by man, but it only exists for say, a few decades of milliseconds.
So, not very long, not even one second and then it falls back apart because when you
pack that many protons together, into the nucleus, so in livermorium's
case, you'd have to pack in, 116 protons.
Remember, those protons repel each other, like charges repel.
They don't want to be close together.
So, even with some neutrons in there, to
cushion them, they tend to fall apart, pretty quickly.
The heaviest element that has been reported in the literature right,
to date, is 108, has an atomic number of 118, right there.
So, we have these very unstable metals, that
tend to exist for very short periods of time.
What else can we do, when we talk about the arrangement of the periodic table?
Well, the periodic table also has in addition to numbering of the
periods, the different rows, we have some common names for many of the groups.
Remember, in the periodic
table, things that are in the same column, tend
to behave similarly, to other elements in that column.
For example, the alkaline metals, lithium, sodium, potassium, etc.,
they all tend to lose one electron, to have a charge of plus 1.
That's what they most commonly do, in their chemistry.
What's this weird box over here, that's colored half-green?
So it looks half, like an alkali metal.
Did you notice, when I drew this periodic table, I didn't draw hydrogen over here?
I didn't draw hydrogen above fluorine because
hydrogen really doesn't behave, similarly to fluorine.
And I also didn't draw it over here, where you sometimes see it, above lithium.
In this periodic table, it's drawn right in the middle.
Sometimes, hydrogen does lose one electron,
just like lithium and sodium tend to do, and it becomes H1 plus.
But other times, hydrogen gains one electron, to make a hydride anion.
And in that case, it behaves more like the halogens.
So, I like to have hydrogen in the middle.
Because really sometimes it loses electrons, to become positively charged,
and other times it gains electrons, to become negatively charged.
So, it behaves sometimes,
a little bit like a metal and other times, more like a non-metal.
Although, it's officially classified as a non-metal.
So, the alkaline metals tend to lose one
valance electrons, to have a charge of plus 1.
The next column over, the second column, the
purple column there, is the alkaline earth metals.
That's the column with calcium and magnesium in it.
And then, if we continue on to the right,
the next thing we encounter, is the transition metals.
Now, the transition metals is an area of
study of chemistry, that's, sort of, its own
entire branch of chemistry, studying the complex behavior,
of some of the very important transition metals.
They sometimes, can lose different numbers of electrons, depending upon what they're
bonding with and what their environment is, and that can be really interesting.
We have some very important transition metals in our body.
For example, we need to have copper in our body, and we
need to have iron in our body, to carry oxygen into our cells.
Down below the periodic table, there are some heavy atoms, that are
generally, not very abundant on Earth, but are still really interesting to study.
The inner transition metals, and remember I
said, that the inner transition metals should be
inserted right here, and if we moved all of this over, right, that would make
the periodic table really wide. So generally, they're shown down below.
Next, as we move across, let's just go ahead and skip this area
because in this area we have, remember, we have some metals down here.
We've got semi metals along this diagonal, and
then we have some non metals over here.
So, these groups are a little bit squirrely, but if we move
over one more from that, we have the group called the halogens.
The halogens all, are attractive to bonding electrons,
and they tend to gain one extra electron,
to have an overall net charge of minus 1.
And finally on the far right, is the column called the noble gasses.
Sometimes, those are called the inert gasses, and they
don't like to interact with other elements, at all.
They do not like to form chemical bonds.
They like to be, by themselves. They like to be monotomic.
So helium, in your helium balloon, is just a helium atom floating around.
There are few known compounds that involve noble gasses, but not very many.
Let's end this lecture, with a note about grading.
You probably did some exercises last week, that
you found, went along fairly well, with the videos.
I hope that you've had a chance to look at the available, online book.
Or perhaps you've decided to purchase your
own copy, of an introductory general chemistry book.
This week you might have noticed, that there are two types of
problem sets, not just one.
There are foundational exercises, which I've made to accompany the video lectures.
And there's also something called, an advanced problem set.
The advanced problem sets are going to be appearing for
your problem-solving pleasure, during every other week of video lectures.
Please note, that these advanced problem sets do require,
somewhat stronger algebra background.
So, those are only required for students, who
hope to attain a statement of accomplishment, with distinction.
A regular statement of accomplishment can be achieved, even
if you're not able to complete the advanced problem sets.
You can achieve a regular statement of accomplishment, by doing the
foundational problem sets, taking the test, and doing the writing assignment.
But if you want to get a statement of
accomplishment with, with distinction, you're going to have to tackle
those advanced problem sets, which I think are really
fun, but might take little a bit longer to do.
And also might not be directly correlated to information,
you can just pick up by watching the video lectures.
They're going to require you to think, synthesize
some of the information that you know,
analyze some systems, and perhaps apply some
math skills, that you've gotten in other classes.
I hope, you'll go ahead and check out those advanced problem sets.
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