Elements and atomic number | Atomic structure | High school chemistry | Khan Academy

- [Instructor] We know that
everything in the universe

is composed of atoms,

but not all atoms are the same.

There are many different types
of atoms called elements,

each with a unique set of
physical and chemical properties.

Many elements are
probably familiar to you.

Oxygen and carbon, for
example, are elements,

and so is gold.

Others may be less familiar

such as bismuth, xenon, and osmium.

A particular element is identified

by the number of protons in its atoms.

And the number of protons
in the nucleus of an atom

is its atomic number,
represented by the symbol Z.

For example, helium has
an atomic number of two.

That means that every
helium atom has two protons.

And here's another example, iron,

its atomic number is 26,

so every iron atom has 26 protons.

An element's atomic
number can also tell us

the number of electrons in a
neutral atom of an element.

Remember, equal and opposite
charges cancel each other out

so to be neutral,

an atom must have an
equal number of protons,

which have a charge of 1+,

and electrons, which have a charge of 1-.

This means that our helium
atom must have two electrons

and our iron atom must have 26 electrons.

All of the known elements

are organized according
to their atomic numbers

in the periodic table.

The periodic table lists the elements

in order of increasing atomic number

as you go from left to right
and from top to bottom.

The elements are further
organized into columns

based on their properties.

Elements in the same column

tend to have similar physical
and chemical properties.

Every element is represented
by a chemical symbol,

a unique one or two letter abbreviation

that appears below the
element's atomic number

on the periodic table.

Many chemical symbols

are based on the English
name for an element.

For example, the symbol for hydrogen is H,

and the symbol for aluminum is Al.

But some chemical symbols

are based on the Latin
name for the element.

For example, the symbol for lead is Pb,

which actually comes from
its Latin name plumbum.

Notice that whenever the chemical symbol

for an element has two letters,

the second letter is written in lowercase.

All of this information is contained

inside of each element's
box on the periodic table.

For example, here's the box
for the element nitrogen.

At the top of the box

is nitrogen's atomic
number, which is seven.

Remember that this is equal

to the number of protons
in each atom of nitrogen

as well as the number of electrons.

Below that is nitrogen's
chemical symbol, N,

followed by its name.

Often, you'll also see another number

listed at the bottom of an elements box,

which has to do with atomic mass.

We'll cover details about
this and another video.

What's neat about having

all of this information in one place

is that if we know one
detail about an element,

either it's atomic number,
chemical symbol, or a name,

we can often use the periodic table

to find out the other details.

Let's take a look at a few examples.

Say we have the element phosphorus,

what is its atomic number
and chemical symbol?

Looking through the periodic table,

we can find phosphorus

and see that its atomic number is 15

and its chemical symbol is P.

Let's do another example.

Say we know the chemical symbol Hg,

what is this element's name

and how many protons does it have?

Here is the symbol Hg
on the periodic table.

Its name is mercury,

and its atomic number is 80

so it has 80 protons.

Let's reveal.

The universe is made up

of different types of
atoms called elements,

and these elements are
organized by atomic number

in the periodic table.

So, the next time that you
stare up at the starry sky,

you know that everything out
there is made up of elements.