The periodic table – classification of elements | Chemistry | Khan Academy

In this video, we’re going to
look at the periodic table. We’re going to classify
the elements into groups. And so as we go
through these terms, I’m going to be
checking them off. The groups are the vertical
columns on the periodic table. And so, if I go over here, I can
see that all of these elements are in the same vertical column. So all these elements
are in the same group. And we call this group 1. I can see that all
of these elements are also in the same column. So all these elements
are in the same group, and we call this group 2. I can continue
labeling my groups. This would be group 3, 4,
5, 6, 7, 8, 9, 10, 11, 12. And then I go back
up to here, and I can see I have another vertical
column, so group 13, 14, 15, 16, 17, and finally 18. So that’s one way to
number your groups. There is another way
to number your groups, and that would be to say
that group 1 is group 1A, group 2 is group 2A. And then kind of ignoring
groups 3 through 12, continue on with your
numbering system. So 1A, 2A– that would make
this group 3A, group 4A, group five 5A, group
6A, 7A, and finally 8A. And this second way of
numbering your groups is useful when you’re thinking
about valence electrons. And so let’s move on to
the concept of periods. A period is a horizontal
row on the periodic table. And so, if I look at
period 1, and I just move across my periodic table,
hydrogen is in the first period and so is helium. I move on to the second period,
so lithium, beryllium, boron, carbon, nitrogen, oxygen,
fluorine, and neon. And so I can continue
with numbering my periods, so this would be
period 3, 4, 5, and 6. Now notice I don’t have
the entire periodic table on this video. I didn’t have enough
room, and we’re not really going to talk about all
of those elements anyway. So let’s go ahead and
focus on metals next. Let’s talk about
the alkali metals. When I’m talking
about metals, I’m going to try to
write it in red here. The alkali metals are found
in group 1, or group 1A, so things like lithium,
and sodium, potassium. So here are my alkali metals. The alkali metals are
soft, silvery metals that are extremely reactive. And one nice thing about
organizing elements into groups is elements in the same
group have similar chemical properties. And so the alkali metals
react in similar ways. For example, all the alkali
metals will react with water. And the alkali metals
turn out to be so reactive that you’re not
going to find them in their pure state in nature. You’re not going to
walk outside and find some sodium lying on the ground. They’re found in
nature in combination with other elements. Let’s talk about hydrogen,
because hydrogen is also in group 1, but hydrogen
is not an alkali metal. Hydrogen is a nonmetal. So let me go ahead and
draw that in green here. I will represent
nonmetals in green. Hydrogen is the
exception in group 1. Next, let’s talk about
the alkaline earth metals. You find those in group 2, or
group 2A, so right in here. Things like magnesium
and calcium and strontium are your alkaline earth metals. Your alkaline earth
metals are reactive– not quite as reactive as
the metals in group 1, but you don’t find these
in the pure state, either. You find them in combination
with other elements. And so once again, the
alkaline earth metals are going to react
in similar ways. They have similar
chemical properties, and so that’s, again,
a convenient way of organizing the periodic
table into groups. For right now,
let’s just go ahead and say– groups 3 through 12–
these are all metals in here. And let’s just talk about
metals in general for a minute. Metals, the
properties of metals. Metals are solids
at room temperature, except for mercury. So here is mercury down here,
which is a liquid at room temperature. Metals are very
malleable, which means you can form them
into different shapes. They’re very workable. They’re not brittle. Metals are also
ductile, which means you can draw them into wires. You can form them into wires. For example, like copper. Here’s copper right here. Copper wires, of course,
carry current in homes. So metals are good conductors
of heat and electricity. And so those are the
properties of metals that most textbooks
will talk about. Let’s contrast those
with nonmetals. Nonmetals– if you
have a solid nonmetal, those solids would tend to
be brittle, not malleable like metals. Nonmetals are poor conductors
of heat and electricity. So you find nonmetals in
different states of matter. Let’s talk about one
of the nonmetals now, and that would be the halogens. Let’s find the halogens
on our periodic table. You find them in
group 7A, or group 17, things like fluorine,
chlorine, bromine. Here are your halogens
right in here. Halogens are very
reactive nonmetals. So they’re often very
colorful, very, very corrosive, and the name halogen
actually means salt former. We’re actually going to come
back to that in the next video when we look at some
electron configurations and we talk about why these
things are so reactive. And so those are the halogens. Next, let’s find
the noble gases. The noble gases are found
in group 8A, or group 18. Some of these are very famous,
like helium, neon, argon, krypton. Here are your noble gases. They’re colorless gases,
and they’re generally very unreactive. Once again, we’ll talk
about why in the next video when we talk about some
electron configurations. There are some other
nonmetals on here, which I will
identify in a minute. But first I want to
talk about the fact that you pretty much find
metals on the left side of the periodic table. So let me go back
to the red color. And you can see I have
all these metals over here on the left side. And then for my nonmetals,
in green, you’re going to find those over
here on the right side of your periodic table. The dividing line
between those– let me go ahead and
draw it in there– it’s kind of a zigzag line. Let me see if I can
sketch it in here. The dividing line would
go something like this. We’re going to go a zigzag
line down our periodic table. And some of the elements that
you find on this zigzag line have properties in between
those of metals and nonmetals, and we call those metalloids. Let’s go ahead and talk
about metalloids now. Metalloids– oid, of
course, being like a metal, so it’s similar to metals,
but, again, the properties are in between those of
a metal and a nonmetal. Some of the elements that are
considered to be metalloids would be boron– right in here–
silicon, germanium, arsenic, antimony, tellurium,
and sometimes you’ll see astatine listed as one. It depends on which textbook
that you’re looking in. So you can see that some of the
elements along this zigzag line are considered to be metalloids. And there’s no official, one
definition for which elements are considered to be
metalloids, and so you might see a little
bit of a discrepancy there for some of
these elements. But in general,
those are the ones that are considered to be
metalloids, silicon probably being the most famous one. Silicon is a semiconductor. It’s a metalloid, so
it’s like a metal, so it does conduct
electricity, but not to the same extent
that a metal would. And so these
intermediate properties are sometimes useful. Let’s go ahead and mark
some of the rest of these. These would be
some other metals. And then over here
on the right would be the rest of your
nonmetals here. Carbon is nonmetal,
nitrogen is nonmetal, oxygen is nonmetal,
phosphorus, sulfur. So that’s just a quick way to
divide the periodic table up with some simple definitions. In the next video,
we’ll talk more about the electronic
structure, and we’ll get into definition
of transition metals.

6 Replies to “The periodic table – classification of elements | Chemistry | Khan Academy”

  1. In my chemistry lecture, we actually discussed how hydrogen is indeed a metal. There is a reason it is placed where it is on the periodic table. 🙂

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