what is an atom? How does helium in balloons contain atoms and how do they relate to each other?

I'm not exactly sure what you mean by your second question
"how does helium in balloons contain atoms and how do they relate to each other?" Please
rephrase the question if I wasn't able to answer to your
satisfaction.

An atom is a building block of matter so to
speak.  Just as living things are made of cells, all matter is made of atoms. Atoms have
three kinds of particles in varying amounts: protons (positively charged particles with
mass), electrons (negatively charged particles, but relatively no mass), neutrons (no
charge with mass).

Helium is an element, which is depicted
on the periodic table of elements.  Helium is a specific species of atom, having two
protons, two electrons and two neutrons. Because it has two protons and two neutrons,
helium's atomic weight is 4 (more or less). It is an inert gas and therefore one of the
noble gasses. A substance being inert indicates that it does not chemically
react.

When a balloon is filled with helium, it rises
because of the difference in densities between the inside of the balloon and the air
outside of the balloon. Density is the proportion between a subject's mass (weight [sort
of]) and volume. Because helium itself is a very light atom, the helium gas within the
balloon is lighter than the air around it, making its density relatively low and so it
rises.

If you were to heat a balloon filled with helium,
the helium atoms  become excited and so move around faster inside the balloon. The
movement of the atoms cause the balloon to expand despite the fact that gas is not being
added to the inside of the balloon. The heated balloon will also rise higher than the
rooom temperature balloon because of the decreased density (the volume increased without
an increase in mass). Similarly, if you were to cool down the balloon filled with
helium, the balloon would contract because the helium atoms would be slowing down. The
cooled balloon would sink because of the increased density (the volume decreasing
without a decrease in mass).

A balloon filled with helium
slowly deflates over time despite temperature and without puncturing the balloon because
the helium atoms are small enough to escape between the molecules of the balloon
itself.