The characteristics of an atom is pretty simple, once a handle on the size of the atom is in perspective. The size is small, so small that the human mind can not get a visualize of it, we can "see" it mathematically or through an artist's interpretation, but neither of these give a true idea of the size of the atom, let alone all the stuff the atom is made of. The atom as two main areas, that is the electrons clouds and the nucleus. The electrons clouds and nucleus vary in the type of atom being talked about. It all starts with a hydrogen atom which in it's basic form has one electron in its cloud and a nucleus, which is made up of one neutron and one proton. Then they start combining and more electrons and neutrons and protons, are added to make a different atom which is heaver and has different characteristics.
The electron cloud is the outer part and the nucleus is in the center. There might be a particle called an electron but it is so evasive that scientists had to invent a new way to look at things to undeerstand the electron better. Quatuam Mechanics can help in predicting and describing an electron. It is basically understood that we can know the mass of the electron, but we do not know its location, and vice versa. That is why it is called a cloud, the time and place of the elctron is always uncertian according to Heisenberg, who studies physics. It is not that we can not measuare it, the uncertiantiy is in the characteristics of the electron cloud the envolopes the neutron.
These electrons are classically described in orbits and levels, it is in these so called orbits and levels that a lot of the characteristics of the atom are found. Energy is what changes the levels and orbits of the eletron clouds. Eneregy is what changes the atoms in water to the atoms in steam, or the atoms in steel to the atoms in molten steel. Energy changes the levels and orbits of the electrons, which then changes the way the atoms interact with each other.
Inside the electon cloud is the nucleus. The nucleus is made up of the protons and neutrons which in turn are made up of several forces and types of quarks. The nucleus is responcible for the mass of the atom. Mass being the resistence to movement in space. There are several theories about what mass is, but mass resides in the nucleus, thus the hydrogen atom has a nucleus of one proton and one neutron and is lighter than air, where as lead as over 50 neutrons and protons and is a lot heaveir. Both the electrons and the neutrons of the lead atom and the hydrogen atom are identical, it is only in the amount of them that the lead atom has that makes it heavier.
For each proton and neutron there is an electron in most cases, though there are times when this does not hold. So the more neutrons and protons there are the more electrons and the more electrons there are the more possibilities there is for different characteristics to occur. That is weather the atom sticks to something or repels something, or when it breaks it bonds with other atoms, or how it interacts with time and space. The atom is very stable and lasts a very long time. In fact all the atoms in the universe were created a very long time ago and they are still around today, though the have combined and shared orbits with one another they are still the same atoms.