This is a page about basic chemistry. Here are some helpful hints about studying this material:
The atom is the basic structure of chemical elements. Atoms are made of:
Protons and neutrons, which are about the same size, are each much larger than electrons, which are tiny .
Protons and neutrons form the nucleus of an atom, in a dense clump.
Electrons are located around the nucleus in a structured cloud.
Most of the volume in an atom is empty space within the outer area where the electrons are located. Atoms have a "size" formed by the outside boundary of electron positions.
In the illustration above, electrons are shown in what may be called "orbits" around the nucleus. They really don't move that way, but they do move. At any instant, if you could say "stop action!," the electrons would be occupying specific regions around the nucleus, leading to the description that the electrons form a kind of structured cloud around the nucleus. We will return to the topic of the electrons when we talk about bonding in the next section, but in the following discussion we focus on the nucleus.
The number of protons in an atom determines its name. For example, for some geologically important elements:
The periodic table lists the elements in order of proton count, starting at upper left with hydrogen, going across to helium, then row by row, increasing toward lower right. The proton count is also called the atomic number, or Z. We'll talk about the periodic table later in more detail, but for now, simply learn that it is the proton count that is the key to an atom's identity.
When you see symbols such as 3 He, 6 Li, ... 12 C, 14 N, 16 O..., the number to the upper left refers to the mass number. The mass number is the sum of the count of protons and neutrons in the nucleus. These could also have been written as helium-3, lithium-6, ... carbon-12, nitrogen-14, oxygen-16, etc. Note: The numbers in the periodic table above are for the atomic number, which is the number of protons. You will note that the numbers in the periodic table are simply posted above the element symbol within the given square.
So, don't confuse this:
atomic number = number of protons
mass number = sum of the number of protons and neutrons
Take carbon, for example. You see in the table above and on the periodic table that carbon has 6 protons. When you see 12 C or carbon-12, you know the 12 refers to the mass number, the sum of the number of protons and neutrons. So, how many neutrons does carbon-12 have? 12 (mass number) - 6 (# protons, or atomic number) = 6 (# neutrons).
And for oxygen, we see above that oxygen has 8 protons. So, when you see oxygen-16 or 16 O, you can get the number of neutrons: 16 (mass number) - 8 (# protons, or atomic number) = 8 (# neutrons).
There can be a different number of neutrons in atoms of an element, leading to unique combinations of proton and neutron counts. These unique atomic configurations of an element are called its isotopes.
Proton and neutron counts for the first 10 elements, in the most commonly occurring forms, are shown in the following table:
And for the next 10 elements:
So, given a single atom with a given number of protons and neutrons, you can do this:
Some isotopes of a given element may be radioactive, or unstable. This means that some change in the proton and/or neutron count can happen as a result of several types of decay, and because the proton and/or neutron count changes, the atom "transforms" into another isotope. In the tables below for examples, you will see phrases like "decays to x in y yrs", and you will see the term half-life. These are ways of describing the changes that happen over time to unstable isotopes; some decay very, very rapidly, on the order of seconds, while others decay at slower rates of a few to thousands of years, while some decay at very, very slow rates on the order of millions or billions of years. We will return to the topic of radioactivity when we discuss age dating, but for now just appreciate that some isotopes are unstable, while others are stable.
Here is the kind of isotopic information you can easily find on the web:
Hydrogen has one proton. It exists in three isotopes, as follows:
* hydrogen-1 is normal hydrogen. hydrogen-2 is called deuterium. hydrogen-3 is called tritium. Note the very short half-life of tritium. Tritium can be used to date a bottle of wine, that is perhaps (reportedly, let's say) 100 years old, just to make sure the bottle of wine isn't a fake.
Carbon has 6 protons. Carbon exists in two common isotopes, carbon-12 and carbon-13, and one rare isotope, carbon-14, as follows:
* that's about one part in a trillion; Note that 98.9% ( 12 C) + 1.1% ( 13 C) = 100%, such that any other isotope of carbon, like 14 C, is going to have very low abundance.
Carbon-14 is used in carbon dating, which works to back about 50,000 - 70,000 years. For fossils or other carbon-containing materials older than that, the carbon-14 has decayed away.
Experimentally, other isotopes of carbon can be created: 9 C, 10 C, ... 15 C, 16 C, etc. These other isotopes are unstable (radioactive) and have very rapid decay rates of just a few seconds or so.
Magnesium has 12 protons. Magnesium exists in three stable isotopes, as follows:
Chlorine has 17 protons. At least nine isotopes have been recognized, ranging from 32 Cl to 40 Cl. Most of these are synthetic, having been observed through lab work only, and these are very unstable. We know that each chlorine isotope has 17 protons, and doing the math, we see that 32 Cl to 40 Cl would have 15 to 23 neutrons, as follows:
* These other isotopes are extremely rare or synthetic (lab-created only). 36 Cl can be used in age dating in the range of 60,000 to 1 million years.
Potassium has 19 protons. Potassium exists in two stable isotopes and one very long-lived isotope, as follows:
* Potassium-40 is very important in age dating. That half-life is 1.277 billion years, a very, very slow decay rate. The significance of the slow rate is that even for very old rocks and minerals, there is still enough original potassium-40 left over to measure with our atom-counting instruments, so we can do age dating.