A is for Atom
Suppose you took a cookie and cut it into little pieces, and then you cut those pieces into crumbs and those crumbs into littler and littler crumbs, and so on. Would you ever get to a point where it became impossibleno matter how good your knife, your hand, or your eyesto cut any further? Would you ever reach the smallest possible piece of cookie? Or could you keep dividing it into smaller and smaller cookie bits...forever?
The ancient Greek philosophers wondered about things like this. One fellow, Democritus, said that all matter (that's what scientists call the "stuff" of the world) was made of tiny bits. He called these bits atoms, from the Greek word for "not cuttable." He believed the bits were put together in different ways to make different kinds of matter. Another philosopher, Aristotle, didn't think matter came in bits. He thought it all flowed together, like water running through your fingers.
Well, it turns out that Democritus was right. But it took many more centuries for scientists to prove the existence of atoms. They have since identified about 90 different kinds of atoms that occur naturally. Atoms are the building blocks of all matter. Matter can be made up of just one kind of atom, or different kinds of atoms joined together.
Atoms are tiny. A million atoms stacked on top of one another wouldn't be quite as thick as a hair on your head. About 100 billion of them would cover the period at the end of this sentence. An atom is so small that a single drop of water contains more than a million million billion atoms. That's 1,000,000,000,000,000,000,000! Of course their exact size depends on the atom. Some are smaller, some bigger. The smallest kind of atom is hydrogen, which also happens to be the most abundant atom in the universe. Uranium is the largest kind of atom, except for a few really big ones that scientists have made in laboratories.
Most of the time, atoms really are "not cuttable," as Democritus said. But scientists have special ways of breaking them apart in order to study them. (Please don't try this at home.) They have found that atoms themselves are made up of smaller parts.
Most of the space in an atom is a "cloud" of incredibly tiny electrons. Electrons whiz around at millions of miles per hour (and yet never get stopped for speeding). Because the electrons are going so fast, they're practically everywhere at once, and so scientists think of them as a cloud.
Somewhere deep inside the electron cloud is a nucleus. If the electron cloud were the size of a baseball stadium, the nucleusfloating somewhere in the middle of the stadiumwould be smaller than the baseball. The nucleus of an atom is made from two types of particlesprotons and neutrons. They give the atom almost all of its mass (that's a measure of how much matter is in somethingsee G is for Gravity).
Aside from mass, there's another big difference between an atom's electron cloud and its nucleus. The electrons have a negative electric charge, and the nucleus has a positive charge. Actually, it's the protons in the nucleus that are positively charged. As their name suggests, the neutrons are neutral (they have no charge). These positive and negative charges hold the atom together. This is because things with unlike charges are attracted to each other, while things with the same charge repel each other. (If you've ever played with magnets, you know that the negative and positive poles of two magnets are eager to know each other and you can tell because they stick together when they get close, but you can't force the two negative poles or the two positive poles to get personal.) An atom has an equal number of positive and negative charges, so it must have the same number of protons in its nucleus as electrons outside the nucleus.
The number of protons in the nucleus of an atom is called the atomic number, and since we know an atom is neutral, the atomic number also tells us the number of electrons outside its nucleus. A hydrogen atom has one proton and one electron, so its atomic number is 1. An oxygen atom has eight protons and eight electrons, for an atomic number of 8. With 47 protons and 47 electrons, silver has an atomic number of...you figure it out! Scientists organize atoms according to their atomic number (see E is for Element). Depending on how many protons and electrons they have, atoms behave in different ways, just like peopleexcept atoms are easier to predict than people!
Though it appears that the electrons of an atom form a cloud around the nucleus, it turns out that there are actually "mini-clouds" within the big cloud. The mini-clouds are called shells. Each shell holds a different number of electrons. The first shell (which is closest to the nucleus) can hold 2, the second can hold 8, the third can hold 18, and the fourth can hold 32. These numbers turn out to be really important because atoms "want" to have their outer shells completely full or completely empty, and they are always trying to fill or empty them. (Of course electrons aren't human and they don't have feelings or desires, but they are definitely strong-willed when it comes to filling or emptying their shells.)
Atoms fill or empty their shells by finding other atoms to give electrons to, or accept electrons from, or share electrons with. When two or more atoms get together to share electrons, the result is a molecule and the process is called a chemical reaction. An entire branch of science, chemistry, is devoted to studying what happens when atoms get to know each other.
Since the nucleus of an atom is like a ball inside a stadium, and since practically all the matter of an atom is in its nucleus, an atom is mostly empty space. And since you're made of atoms and the chair you're sitting on is made of atoms and the floor the chair is resting on is made of atoms and the earth supporting the floor is made of atoms and so forth...you might be wondering this: Why don't you (mostly empty space) just fall right through the chair (mostly empty space) and continue falling through the floor (mostly empty space) and the earth (mostly empty space)? If you weren't wondering, please start wondering now!
The answer is electrons. Though an atom is mostly empty space, that space has negatively charged electrons whizzing through it all the time. The electrons of the chair are negatively charged and the electrons of your bottom are also negatively charged. Remember what we said about opposite charges attracting and like charges repelling? Because the negative charges of your bottom are in contact with the negative charges of the chair, they push against each other. So, you stay on the chair. It's a good thing, too, because it would be hard to read this book while plunging toward the center of the earth.