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.