For a very long time after atoms were first theorized, not very much was known about them, and they were believed in theory to be the smallest particles of all. We now know that this isn’t the case. Atoms have structures and are made up of yet smaller particles, which are themselves made up of smaller particles.
All atoms except Hydrogen are made of three particles; electrons, protons, and neutrons. Hydrogen alone has no neutrons, though “heavy” hydrogen or radioactive hydrogen does. It is often helpful to think of an atom as an incredibly small “solar system”.
At the center of this tiny system is the nucleus, corresponding to the “sun”. The nucleus consists of protons, with a single positive charge each, and a neutron, with basically the same mass as the proton, but with no electrical charge at all. In “orbit” around the nucleus are electrons, tremendously smaller in size (about 1,834 times smaller), but each of these has a negative charge. The orbit is so far away from the nucleus that about 90% of the atom is empty space. The most basic atom is hydrogen, with one proton and one electron.
Here is where the idea of the miniature solar system breaks down. The orbits of the electrons are more properly referred to as “shells”. The reason is that the electrons do not spin around the nucleus as planets do, but sort of bounce around within a narrow band, or shell. This makes it appear more like a sphere outside of a sphere.
The outer spheres can only a certain number of electrons in each. The first outer shell can only contain two electrons. Helium contains two electrons, so the shell is complete. If a shell is not complete, the element is very reactive, meaning that it can combine with other elements. However, if the shell is complete, it is very stable and does not easily combine with other elements. This is why hydrogen combines easily with oxygen to create water, and why it is so very combustible, while helium doesn’t naturally combine with any other element and is non-combustible.
Additional shells can contain more than two electrons. However, in each case, there is a limit to the number of electrons the shell can contain. Lithium, the third element, has three electrons, two in a complete inner shell, and one in a second shell, which makes Litium a reactive element. Regardless of how many shells an atom has, they try to balance their outer shell. Because of this, an atom with only one or two electrons in an outer shell that contains, for example six, will “donate” or “share” electrons in order to become more stable. An atom that is only lacking one or two electrons will accept electrons from another element, to become more stable. This is how most common compounds are created. For instance, Oxygen lacks two electrons to have a complete outer shell. So it combines easily with two hydrogen atoms, sharing the hydrogen electrons to create H2O, or water. Water is relatively stable and is non-reactive, because the oxygen and hydrogen have full outer shells when they are combined in this way.
There are other ways that elements can be combined, but this is the strongest sort of bond. Usually, in scientific script, the number of electrons needed to attain a complete outer shell is related as a superscript of the electrical charge of the atom (remembering that electrons have a negative charge and protons have a positive charge.) As we progress through the atomic chart, each successive element has an additional proton; hydrogen has one, helium has two, lithium has three, and so on. So by measuring the number of electrons in the outer shell of an atom, it is easy to measure the electrical charge each atom has.
Again, it is the outer shell that is the most important. Hydrogen only has one electron, but the shell can hold two, so it lacks one electron for a complete shell. Since the electron has a negative charge, and it is lacking an electron, it’s net charge to become stable is -1. (If it loses an electron, it becomes stable, so it needs to lose one electron, hence, minus one.) Helium, on the other hand, has a net charge of 0, since it has a full shell. The greater the net charge, the more reactive an element is. Helium, with a net charge of 0, is totally non-reactive, so it is called a “noble gas”. Other noble gases include Neon and Argon, which also have a net charge of 0. On the other hand, Oxygen requires two electrons to be complete, so it’s value is +2.
From this, we can see that Oxygen is more reactive than Hydrogen. It needs two electrons to become stable.
While this is a basic representation, this explains how atoms function and combine with other atoms. Naturally, there are also other forces at play, but none are more powerful than this simple concept. It makes possible every item we see every day, from the water we drink, to the car we drive, to the food we eat. Though not the smallest particles, atoms make up everything we observe on Earth.