Most atoms don't ride solo,

instead they bond with other atoms.

And bonds can form between atoms

of the same element

or atoms of different elements.

You've probably imagined bonding as a tug of war.

If one atom is really strong,

it can pull one or more electrons

off another atom.

Then you end up with one negatively charged ion

and one positively charged ion.

And the attraction between these opposite charges

is called an ionic bond.

This is the kind of sharing

where you just give away your toy to someone else

and then never get it back.

Table salt, sodium chloride,

is held together by ionic bonds.

Every atom of sodium gives up one electron

to every atom of chlorine,

ions are formed,

and those ions arrange themselves

in a 3D grid called a lattice,

in which every sodium ion

is bonded to six chloride ions,

and every chloride ion is bonded

to six sodium ions.

The chlorine atoms never give

the sodium atoms their electrons back.

Now, these transactions aren't always so cut-and-dried.

If one atom doesn't completely overwhelm the other,

they can actually share each other's electrons.

This is like a pot luck

where you and a friend each bring a dish

and then both of you share both dishes.

Each atom is attracted to the shared electrons

in between them,

and this attraction is called a covalent bond.

The proteins and DNA in our bodies,

for example,

are held together largely by these covalent bonds.

Some atoms can covalently bond

with just one other atom,

others with many more.

The number of other atoms

one atom can bond with

depends on how its electrons are arranged.

So, how are electrons arranged?

Every atom of a pure, unbonded element

is electrically neutral

because it contains the same number

of protons in the nucleus

as it does electrons around the nucleus.

And not all of those electrons are available for bonding.

Only the outermost electrons,

the ones in orbitals furthest from the nucleus,

the ones with the most energy,

only those participate in bonding.

By the way, this applies to ionic bonding too.

Remember sodium chloride?

Well, the electron that sodium loses

is the one furthest from its nucleus,

and the orbital that electron occupies

when it goes over to chlorine

is also the one furthest from its nucleus.

But back to covalent bonding.

Carbon has four electrons

that are free to bond,

nitrogen has three,

oxygen two.

So, carbon is likely to form four bonds,

nitrogen three,

and oxygen two.

Hydrogen only has one electron,

so it can only form one bond.

In some special cases,

atoms can form more bonds

than you'd expect,

but they better have a really good reason to do so,

or things tend to fly apart.

Groups of atoms

that share electrons covalently with each other

are called molecules.

They can be small.

For example, every molecule of oxygen gas

is made up of just two oxygen atoms

bonded to each other.

Or they could be really, really big.

Human chromosome 13 is just two molecules,

but each one has over 37 billion atoms.

And this neighborhood,

this city of atoms,

is held together by the humble chemical bond.