Question

How small is an atom

Answer 1

An atom must be smaller than any THING you ever saw,
because every THING is made of many atoms.

An atom is too small to see.  Because if you want to see something,
you have to shine a light on it, and the light that bounces off of it
toward your eye is what you see.  But the wavelength of light is
larger than the atom, and the energy in some light waves is enough
to tear the electrons away from the atom.  So it would be like trying to
see a mosquito by bouncing rocks off of it !

Some simple atoms, with very few particles in them, are bigger.  And
some complicated atoms, with hundreds of particles in them, are smaller ! 
That's because more positive protons in the nucleus attract more negative
electrons outside the nucleus with more force.

An atom is so small that it's impossible to say what size it actually IS.
That's NOT because we can't measure it.  It's because of a weird law
of nature, called the "Uncertainty Principle".  It says that the smaller
something is, the less possible it is to KNOW its location.  The best
we can ever say about the electrons that orbit an atom is how LIKELY
they are to be at a certain distance from the nucleus in the center.

Here are the measurements of atoms of a few familiar substances.
The number is the MOST PROBABLE diameter of the sphere.

After I give you the numbers, I'll tell you the UNIT of measurement.

Hydrogen atom     106
Oxygen atom          96
Helium atom           62
Neon atom              76
Lead atom             308
Iron atom               312
Silver atom            330
Gold atom             348
Sodium atom        380

Now ... these numbers don't mean anything until you know the unit.
I mean, you have to know the size of ' 1 ' of them.  So here it is:

The unit of size for that list is 1 "picometer".  (Say  "PEEK-o-MEE-ter")

1 picometer means  1 x 10⁻¹² meter, or  0.000 000 000 001 of a meter.

So the biggest atom on the list ... Sodium, with a diameter of 380 ...
is 380 picometers across.  That's

                                 0.000 000 000 38  of a meter. 

If your pinky fingernail is 1/4 of a centimeter across,
then you could line up this many Sodium atoms across it,
side by side:

                       6,578,947 Sodium atoms.    (about 6 and 1/2 million !)

The answer to your question is:  Awesomely small.