Video Transcript
In this video, we will learn how to
describe and identify the rows, periods, and blocks of the periodic table and the
position of different types of elements. We’ll examine how the modern
periodic table is arranged and relate the position of an element on the periodic
table to its electronic configuration.
Elements are substances that
consist of one type of atom. There are 118 known elements that
each have their own unique set of properties. These elements have been arranged
to create the modern periodic table of elements, where each box of the periodic
table represents a different element. Of these elements, 92 occur
naturally on Earth in significant amounts. The rest, like curium and
einsteinium, are typically artificially created and only exist in the laboratory
setting, sometimes only for a very short amount of time.
The elements neptunium and
plutonium do occur naturally in small amounts. But their discovery was through
artificial production. The element technetium is also
often considered an artificially made element. While technetium does exist
naturally, most of it has decayed away. So, nowadays, it is typically
artificially created.
Most of the elements are solids at
room temperature. Most of these elements, like iron,
sodium, and gold, have a high melting and boiling point. Only two elements, bromine and
mercury, are liquids at room temperature. And the rest are gases at room
temperature, as these elements have very low melting and boiling points. The low boiling point of nitrogen
makes liquid nitrogen useful in medicine to preserve tissues such as the cornea of
the eye.
It’s worth mentioning that not much
is known about the elements shown here in pink. Because they have only been
artificially synthesized in very small quantities. We might expect them to be solids
or gases as shown. However, some scientists disagree
about how these elements will behave.
The elements of the periodic table
can take on several forms, where some of these forms are stable and others are
unstable. The unstable forms are radioactive
and give off radiation in the form of high-energy rays or particles. All elements have an unstable
radioactive form, though typically the most stable form of an element is the most
common found in nature.
The elements boxed in pink do not
have a stable form and are always radioactive. The radiation produced by a
radioactive element may be harmful. But in many instances, the
radioactive element can be used in a positive way. For example, cobalt-60, an unstable
form of the element cobalt, is used in food preservation. Cobalt-60 produces 𝛾-rays, which
prevent the reproduction of microbial cells without impacting human health.
Elements can exhibit other
interesting properties. For example, silicon is a
semiconductor. This means it can conduct
electricity differently depending on the temperature, which makes silicon useful for
creating computer chips.
Now that we’ve learned about some
of the properties of the elements, let’s take a closer look at the periodic table
and how it’s arranged.
Each of the boxes or cells of the
periodic table contains information about the element. This cell shows some of the most
common information that is typically represented. The chemical symbol is usually
written in the middle of the box. This symbol consists of a single
capital letter or a capital letter followed by a lowercase letter.
The name of the element often
appears below the chemical symbol. The atomic mass typically appears
at the bottom of the cell. The atomic mass of the elements
varies widely, with hydrogen having an atomic mass of one and lead having an atomic
mass of 207. Above the chemical symbol is the
atomic number. This number is unique for each
element. The atomic number indicates the
number of protons in the nucleus of an atom of that element. Hydrogen has an atomic number of
one. So an atom of hydrogen has a single
proton in its nucleus. Carbon has an atomic number of
six. So an atom of carbon has six
protons in its nucleus.
Atomic number is one of the
properties used to organize the elements in the modern periodic table. In the modern periodic table, the
elements are organized in order of increasing atomic number from left to right. But the periodic table isn’t just a
long list of elements. The elements are organized into
precise rows and columns.
A column on the periodic table is
known as a group. There are 18 groups that are
usually numbered sequentially from left to right. We may also encounter an older
numbering system for groups, shown here in pink. A row on the periodic table is
called a period. There are seven periods numbered
from top to bottom. The elements are organized into
these groups and periods according to their electronic configurations. To better understand this
organization, let’s take a look at the electronic configurations of the first 18
elements.
As we look at the electronic
configurations, we can see that atoms of elements in the same group tend to have the
same number of valence electrons or electrons in the outermost energy level. In addition, the A grouping numbers
indicate how many valence electrons atoms of elements in the group will have. So atoms of elements in group one A
have one valence electron. And atoms of elements in group six
A will have six valence electrons.
The atoms of elements in the last
column all have full outer shells or energy levels. Atoms of these elements all have
eight valence electrons, except for helium, which only has two. Because elements in the same group
have similar electronic configurations, these elements also have similar chemical
properties and reactivity. For example, the elements in group
seven A are highly reactive and can react with metals to form metal salts, while the
elements in group zero, with full valence shells, are stable and unreactive.
If we look at the electronic
configurations across a period, we see that atoms of elements in the same period
have the same number of occupied energy levels. The number of occupied energy
levels corresponds to the period number. In addition, atoms of elements in
the same period have the same outermost energy level. The outermost energy level in atoms
of elements found in period one is K. The outermost energy level in
period two is L. And the outermost energy level in
period three is M.
We can now use the periodic table
to determine how many valence electrons an atom has, how many energy levels the
electrons occupy, and which energy level is the outermost energy level. But the periodic table can provide
us with even more information. The periodic table can be further
divided to indicate which sublevel the valence electrons are found in. The sublevels are called s, p, d,
and f.
With all of this information in
mind, let’s consider the element phosphorus. Phosphorus is in period three. So the electrons in an atom of
phosphorus will occupy three energy levels, with the outermost energy level being
energy level M. Phosphorus is found in group five
A. This means that an atom of
phosphorus will have five electrons in its outermost energy level. This also means that the innermost
energy levels will be full, with two electrons in the K energy level and eight
electrons in the L energy level.
Furthermore, as phosphorus is in
the p block, some of its valence electrons will be found in the p sublevel. In addition to being divided into
s, p, d, and f blocks, the periodic table can also be divided into specifically
named groups and sections. The elements in group one,
excluding hydrogen, are known as the alkali metals. The elements in group two are
called the alkaline earth metals. The elements in group 18 are called
the inert gases. And the elements in group 17, or
seven A, are called the halogens.
The elements found in the middle
section of the periodic table are called the transition metals. The two rows that are separated
from the main portion of the periodic table each have their own name that comes from
the names of the first element in each row. The top row is called the
lanthanides after the element lanthanum, and the bottom row is called the actinides
after the element actinium.
The organization of the periodic
table reveals several patterns or trends in the properties of the elements. One of these trends is atomic
radius. Atomic radius describes the size of
an atom. It is typically measured in
picometers. A picometer is very small. One picometer is equivalent to 10
to the negative 12 meters. In general, atomic radius tends to
increase down a group and from right to left across a period. So elements with a small atomic
radius are found on the top right of the periodic table. And elements with a large atomic
radius are found on the bottom left.
We can explain this trend by
looking at the electronic configurations of elements in the same group and same
period. Hydrogen, lithium, and sodium are
all found in group one of the periodic table. We can see that an atom of each of
these elements has one more occupied energy level than the element above it. The more occupied energy levels an
atom has, the larger the atomic radius, as the occupied higher energy levels are
further from the nucleus.
Now let’s look at an atom of
lithium, carbon, and neon, three elements found in the same period. Atoms of these elements have the
same number of energy levels. So, to understand which atom has a
larger atomic radius, we need to look at the nucleus. An atom of lithium has three
protons in its nucleus, an atom of carbon has six protons, and an atom of neon has
10 protons. As the number of protons in the
nucleus increases, the attraction between the nucleus and the outer electrons
increases. The outer electrons will be pulled
closer to the nucleus when the attraction is stronger, making the atomic radius
smaller. So an atom of neon will be smaller
than an atom of carbon, which is smaller than an atom of lithium.
Another periodic trend involves the
metallic character of an element. Metallic character relates to the
tendency to lose electrons. The majority of the elements on the
periodic table are metals. Metals have a strong metallic
character. This means that during a chemical
reaction, atoms of these elements tend to lose electrons and form positive ions. The nonmetallic elements have a
weak metallic character. Atoms of these elements do not tend
to lose electrons easily. During chemical reactions, atoms of
nonmetals often share electrons or gain electrons to form negative ions. The metalloid elements can be
considered to have a metallic character between the metals and the nonmetals.
Lastly, we have the inert gases,
which are stable, unreactive, and do not tend to lose electrons or form ions. In general, we can see that
metallic character increases as we move down a group and from right to left across a
period. This is similar to the trend for
atomic radius. In fact, an increase in atomic
radius tends to correspond to an increase in metallic character. This is because as the atomic
radius increases, the electrons are less attracted to the nucleus, which means that
they can be lost more easily.
We’ve learned a lot about the
organization of the periodic table in this video. But before we summarize what we’ve
learned, let’s take a look at a question.
How many electrons in the outermost
level will an element in the fourth period and one A group in the periodic table
have?
The periodic table is a way to
represent and organize the 118 known elements. Each horizontal row of the periodic
table is called a period. There are seven periods numbered
from top to bottom. The period number indicates the
number of occupied energy levels in an atom of that element. The element in the question is
found in the fourth period. So an atom of this element has four
occupied energy levels.
Each vertical column in the
periodic table is called a group. There are 18 groups numbered from
left to right. The groups can also be numbered
using an A-B numbering system. For the A groups, the group number
indicates the number of electrons in the outermost energy level. The element in the question is
found in group one A. So an atom of this element has one
electron in the outermost energy level. This element happens to be
potassium. So the number of outermost
electrons in an atom of potassium, which is found in period four and group one A of
the periodic table, is one.
Now let’s review what we’ve learned
with the key points. The periodic table of elements is
used to organize the 118 known elements that each have a unique set of chemical and
physical properties. Each cell of the periodic table
provides information about the element, including chemical symbol, element name,
atomic number, and atomic mass. Each of the 18 columns are called
groups, and each of the seven rows are called periods.
Atomic number increases from left
to right across a period. Atoms of elements found in the same
group have the same number of outermost electrons. The period number is equal to the
number of occupied energy levels in an atom. The periodic table can be broken
down into different sections, including sublevels and named groups. The organization of the periodic
table reveals various trends. The atomic radius and the metallic
character of an element increase moving down a group and from right to left across a
period.
