Video Transcript
In this video, we will learn about
the periodic table of elements, how it’s laid out, the names for its parts, and the
neat ways that the position of elements on the periodic table indicates some of
their properties.
Chemistry is all about chemicals
and how they react. But if you try to remember every
detail of every chemical, you’d have a massive job on your hands. There are about 160 million
recognized chemicals and an infinitely greater number that we could imagine. Even if all you looked at were the
named chemicals in use today, we’d still have too many to remember. However, all chemicals are made out
of the chemical elements and there’s a limited number of these.
We can use our understanding of the
elements to predict the type of bonding in chemical compounds. We can then predict the reactivity
and behavior of chemicals that we haven’t directly studied. However, 118 elements is still
quite a lot. So to help chemists out, there is a
tool called the periodic table of elements, which helps chemists remember lots of
details about the properties and behaviors of the elements. In this video, we’re not going to
look at the history of the periodic table. Instead, we’ll look at the theory
behind the modern periodic table and how you can best use it to become a better
chemist.
Each element is a type of atom or
ion, based on the number of protons in the nucleus. The number of protons in the
nucleus determines the number of electrons needed to make it neutral and form an
atom. The number of protons and electrons
an atom or ion has determines how it reacts. So the number of protons in nuclei
is really important. It’s taken us a long time to
isolate and identify the chemical elements we know now. What we have is an understanding of
nuclei with between one and 118 protons. To start off our periodic table, we
need to have some basic information about each element.
A name and a chemical symbol have
been given to each element. We can put these in a cell or
box. The most basic piece of information
we have about each element is the number of protons in the nuclei of that
element. This is the atomic number. You’ll usually find this above the
chemical symbol, but sometimes you’ll see it below or elsewhere. The other piece of information
that’s really useful is the element’s average atomic mass. This is the average mass of an atom
of that element in unified atomic mass units. This number is a measured property
of each element, based on the abundance of the isotopes of that element on
Earth.
Not all elements have average
atomic masses because not all elements are found on Earth in large enough quantities
for these measurements to be made. So what you may see instead is the
average atomic mass of the most common isotope or nothing at all. You need not worry about this in
practice because it’s highly unlikely you’ll be asked about an element which doesn’t
have a defined average atomic mass. If you’re ever worried about
telling the atomic number from the atomic mass, just remember the atomic number is
always smaller than or exactly the same as the atomic mass. Once we have all the sounds for the
elements, it’s helpful to put them in order by the atomic number of the element.
But chemists also want to be able
to compare elements based on how their atoms or ions react and how their electrons
are arranged. For now, to keep things simple,
let’s only look at the first 20 elements, hydrogen to calcium. The first thing we could do when
comparing these elements is look at the form they take when they are pure at room
temperature and pressure.
In their elemental form, some of
these elements are gases like hydrogen, helium, nitrogen, and so on. The rest are solids and we don’t
happen to have any elements that are liquids at room temperature and pressure. There’s no clear pattern here, so
let’s have a look at the elements that are metals and the elements that are
nonmetals. Metallic elements include lithium,
beryllium, and sodium, and we don’t include carbon even though it’s generally
conductive for other reasons. All the other elements are
nonmetals, and we still don’t have a particularly clear pattern. We’ll have to dig a little
deeper.
The next candidate is
reactivity. The element hydrogen is
reactive. It forms compounds with other
elements. For instance, hydrogen reacts with
oxygen to form water, so we know oxygen is reactive as well. However, helium is the least
reactive of all the elements, while there are many compounds for the elements
lithium to fluorine. But like helium, neon is
unreactive. The elements sodium to chlorine are
reactive, but argon is not. And finally, potassium and calcium
are reactive, and we won’t go further for now.
What we’ve identified is a
repeating pattern of chemical behavior. But more importantly, we see
similar periodic behavior for the elements between helium and neon and those between
neon and argon. We even see the same pattern
continued beyond argon with potassium and calcium. And we even have a place for
hydrogen, which is more similar to lithium, sodium, and potassium than it is to
fluorine and chlorine. A nice way of representing this
information is to group hydrogen, lithium, sodium, and potassium together in a
single column or group. And we can do the same with helium,
neon, and argon. And we can do the same with the
intervening elements, leaving a gap between hydrogen and helium because the data
suggests that hydrogen should be above lithium and helium should be above neon.
In this arrangement, elements left
to right are still arranged by atomic number, but they’re vertically aligned with
elements that they share chemical behavior with. The chemical behavior happening in
a set period as we increase atomic number is the origin of the name for the periodic
table. Beyond calcium, things get a little
bit more difficult. We have to make a big jump in
atomic number to 31 to see the periodic behavior of boron and aluminum continue. So we have another gap in our
periodic table. In between calcium and gallium, we
place 10 metals that have more to do with each other than they do with previous
elements: scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel,
copper, and zinc.
This is the first time we’ve
encountered elements that behave the way these do. So these elements sit at the top of
their group. Beyond gallium, germanium, arsenic,
selenium, bromine, and krypton continue the pattern of their groups. The modern periodic table is built
up through extension of these principles, although it’s not possible to go through
all the detail in this video. If we expand to include all known
elements, we get a table like this. As we get closer and closer to the
end of the periodic table, the difference in reactivity because of electronic
structure becomes less distinct. So the electronic structure itself
becomes what determines the position of an element on the periodic table.
This form of the periodic table is
known as the long form. And it’s not often used because
it’s very wide and awkward to use. This is why this selection of
elements is often moved below the table. This gives us the form of the
periodic table we’re most used to seeing. This form is easier to print and
use. In this floating block of elements
at the bottom, the element in the top left is lanthanum, so the elements along that
row are called the lanthanides or lanthanoids. In the row below, the first element
is actinium, so this row is known as the actinides or actinoids.
Now let’s go through some of the
labels that are applied to areas of the periodic table which help us group elements
by behavior or electronic character. The simplest division on the
periodic table is the line between metals and nonmetals. In their elemental form, metallic
elements tend to be shiny and electrically conductive, while nonmetals are
neither. Elements to the left of the
periodic table tend to be more metallic, but hydrogen is an exception. It’s a nonmetal. To the right on the nonmetal side
of the boundary, we have elements like boron, silicon, arsenic, and tellurium. As we get to the bottom right of
the periodic table, elements become rarer and more radioactive, so the
nonmetal-metal distinction isn’t as important.
On the left side of this exact
boundary, we have the metals aluminum, germanium, and antimony. If you examine the properties of
these elements close to the line, some elements seem to be like metals and like
nonmetals at the same time and have properties that are in between the extremes of
the two groups. These are called metalloids. Depending on who you ask and how
specific you’re being, some elements might be described as a metalloid or as a metal
or a nonmetal.
For instance, silicon is broadly
classified as a nonmetal, but it’s more specifically classed as metalloid. On the other hand, germanium is
more specifically classed as a metalloid but also broadly classed as a metal. These are the elements generally
considered to be metalloids, but some people include polonium and astatine and even
aluminum and carbon. Remember that these words are used
to categorize elements. And therefore, we’d expect some
disagreement. The important thing is that you
understand which elements you’re talking about when you use these words.
The next keyword applied to the
periodic table is group. A group on the periodic table is
simply a column, with the collection of chemically or electronically similar
elements. For instance, this column to the
right is one group. Some of the groups are given
numbers, and we count left to right. Some people only count the two
columns to the left and the six columns to the right, giving eight numbered groups
in total. This is useful when we’re
determining valency, but we’ll come to that in a moment.
The alternative scheme uses 18
numbered columns. Some collections and groups of
elements have specific names. The metals in group one are known
as the alkali metals — lithium, sodium, potassium, rubidium, cesium, and
francium. But hydrogen, a nonmetal, is not an
alkali metal. The elements in group two of the
periodic table are otherwise known as the alkaline-earth metals. These are beryllium, magnesium,
calcium, strontium, barium, and radium. Some groups don’t have specific
names, and some have lesser used names like the group 15 and group 16 elements that
are known as the pnictogens and chalcogens, respectively.
But the group 17 elements,
otherwise known as group seven elements, are generally known as the halogens. These are fluorine, chlorine,
bromine, iodine, astatine, and tennessine. And lastly, the elements in group
18, otherwise known as group eight or even group zero, are known as the noble gases:
helium, neon, argon, krypton, xenon, radon, and oganesson. Each group has its own distinctive
chemical character. For instance, noble gases are
unreactive, while alkali metals are highly reactive.
The group is useful for predicting
the valency of atoms of elements in the first few rows of the periodic table. The early view of valency was the
combining power of an element. Atoms of hydrogen have a valency of
one, while atoms of oxygen have a valency of two. This means when hydrogen and oxygen
combined form water, we need twice as much hydrogen. These’re the common valencies of
atoms of elements in groups one and two and 13 to 18. However, valency is more commonly
associated with the number of electrons in the outer or valence shell of an
atom. In which case, the valency of
elements in group one and two is one and two and the valency of elements in group 13
to 18 are three to eight.
The number of valence electrons in
an atom are a large part of what determines how that atom reacts. So it makes sense that elements
whose atoms have the same valency would have similar chemistry.
Now that we’ve looked at vertical
groups, let’s have a look at horizontal rows. Rows on the periodic table are
known as periods, which are collections of elements with the same highest occupied
electron shell. That sounds more complicated than
it is. So let me just explain. Every atom consists of a nucleus
surrounded by a cloud of electrons. In a fairly simple model, we can
describe electrons as occupying electron shells at various distances from the
nucleus. An atom of hydrogen has only one
electron, and we usually find that electron in the first electron shell. So hydrogen is to be found in the
first period of the periodic table. Helium is also in period one
because atoms of helium only have two electrons which sit in the first electron
shell.
Lithium however is in period
two. Atoms of lithium have three
electrons, and there’s only space in the first shell for two electrons. So the third electron goes in the
second electron shell. We see something similar when we
get to sodium. The highest occupied electron shell
for sodium atoms is the third electron shell. This pattern continues all the way
down the periodic table. In atoms of francium, the highest
occupied electron shell is the seventh electron shell. So we can use the periodic table
and electron shell diagrams to convert the period number and the shell number of the
highest occupied shell.
The electron shell model doesn’t
adequately account for all the electronic behavior of atoms of the elements, so we
need to go a little bit deeper. Shells of electrons can actually
contain what we call subshells, which contain orbitals of different shapes. The common types of subshell are s,
p, d, and f. Shells further from the nucleus can
fit more subshells. The first electron shell contains
just an s-type subshell. The second contains an s- and a
p-type subshell. The third contains s-, p-, and
d-type subshells, while the fourth electron shell contains s-, p-, d-, and f-type
subshells.
You needn’t worry about the impact
of other types of subshell on the periodic table. We can group elements based on
which type of subshell the valence electrons for atoms of that element are in. For instance, hydrogen and helium
are said to be in the s-block because the outer electrons of hydrogen and helium
atoms are in an s-type subshell. And elements in group one and two
are also said to be part of the s-block.
All the elements in group 13 to
group 18 apart from helium are part of the p-block. In between the s and the p-block,
we have the d-block, which in some interpretations also includes lanthanum and
actinium. And the majority of the floating
section below the table is the f-block. You might see lanthanum and
actinium as part of the f-block. Generally, it’s not that important
a distinction when it comes to practical applications.
The last important collection of
elements we need to address is the transition metals or transition elements. Elements in this particular group
are hard and dense in their elemental form and tend to form colored compounds and
multiple types of ion. Iron is a good example of a
transition metal. It’s hard and dense, and it’s
commonly to be found in colored compounds containing Fe2+ or Fe3+ ions. Generally speaking, these are the
elements considered transition metals, most of the d-block and all of the
f-block. Some descriptions of transition
metals exclude scandium and yttrium, and some definitions would include extra
elements as well.
But the most important distinction
to make is that often only the transition metals in the d-block are inferred when we
say transition metal. When we’re talking about transition
metals in the f-block, we’ll say inner transition metals. Yes, it’s confusing, but
unfortunately it’s the result of various different definitions of transition
metal. Just be sure you know which
definition you’re using.
So, to sum up the key points, the
periodic table condenses lots of information about the elements. Each element is given a cell
containing its atomic number, its element symbol, the element name, and the atomic
mass. The elements are then arranged by
atomic number and chemical and/or electronic characteristics. Elements can be classified as
metals, metalloids, or nonmetals. The group and period can be used to
determine the valency and the highest occupied electron shell for atoms of the
elements. And the regions of the periodic
table can be broken into s-, p-, d-, and f-blocks. And we’ll find transition metals in
the d- and f-blocks.
