Dr. GURU speaks,

An atom is around 10-10m (an angstrom) in diameter. This is equivalent to 0.1 nanometre.

10-10m = 1 / 1010m = 1/10000000000m = 0.0000000001m

The nucleus of an atom is only 10-15m wide. This makes it 100,000 times smaller.

Atoms of different elements have different sizes. Caesium has the largest atom at 0.5 nanometres. Hydrogen has the smallest atom at 0.05 nanometres. The Hydrogen molecule (H2) is only 0.15 nanometres wide. This means that a molecule can be smaller than an atom.

The volume of a spherical atom is given by the equation:

V = πd3 / 6

If we use an average diameter of 1 angstrom...

V = 3.14159 x 0.00000000013 / 6 = 5.235983333-31m3

The diameter of our planet Earth is around 12,000km. To aid with visualization this can be written as 1.2 x 107m, or to round down, 107m.

If an atom was the size of the World, its’ nucleus (being 100,000 times smaller) would only span 100m.

An atom, and thus all molecules, our physical world and the Universe, is at least 99.999% empty space. Exemption is given to Neutron Stars and Pulsars, which are only 10-30km wide, and Black Holes.

You cannot see through lead or a concrete wall, despite it being mostly empty space, because the wave-length of visible light is around 5 x 10-7m. This is in the range that will interact with the atoms and molecules in the lead or concrete, being refracted, absorbed, and reflected. None will manage to pass straight through. Gamma radiation has a wavelength of around 10-12m (a picometre). It is too small and fast to interact with a concrete wall or even a thin sheet of lead, and passes straight through. Radio-waves have a wavelength of around 103m (a kilometre). They are too large and slow to interact with thin concrete walls or sheets of lead, and also pass straight through. Both gamma radiation and radio-waves will be absorbed and refracted by greater thicknesses of lead, concrete and earth.

You can see through glass because it has a very random and spaced molecular structure, allowing a lot of visible light to pass straight through the gaps. This is also why you can see through most liquids and gases. Moreover, clear glass has relatively few electrons with energy levels that absorb or refract visible light. If you had an extremely thick block of glass it would refract a lot of the light and would be impossible to see through.

You cannot see directly through diamonds because their carbon atoms are regularly arranged into a compact tetrahedral lattice. Their electrons also refract a lot of the light, changing its path as it passes through.

The GURU must now sleep.