You already know that atoms are the basic building blocks of matter and that they are made of subatomic particles: protons, neutrons and electrons. But what happens when we change the number of these particles? In this section we’ll look at just that - how subatomic particles change the atom and what makes the atoms different.
The number of protons in an atom is called its atomic number. Atoms with the same atomic number (i.e., the same number of protons) are considered the same element, no matter how many neutrons or electrons they have.
There are quite a few known elements (as of writing this article, we have 118) and they all have names. You have most likely heard of some already, like oxygen, iron, or gold. If I say the atomic number of oxygen is 8, you should know that each atom of oxygen has 8 protons in its nucleus. It also works the other way around – if an atom has 8 protons in its nucleus, it is definitely oxygen.
Some elements have longer names, and the names aren’t always consistent across different languages. That’s why we use chemical symbols instead. Chemical symbols are usually one or two letters long. We use them when writing molecular formulas or chemical reactions. For example, you have probably seen \(\ce{H2O}\) written in this form. The H represents hydrogen and O represents oxygen.
You can find all known elements organized in the periodic table of elements, which you can read more about in other pages.
Atoms can also have different numbers of neutrons. They can still be the same element, since we define an element based only on the number of protons. When two atoms of the same element have different numbers of neutrons, we call them isotopes. Another way we can say this: two atoms of the same element with different mass numbers are isotopes. The mass number is the total number of nucleons (protons + neutrons) in an atom's nucleus. Since all atoms of the same element have the same number of protons, any difference in mass number must come from different amounts of neutrons.
Some isotopes are more stable than others and can be more commonly found in nature, while others don’t exist naturally at all. If we calculate what percentage of an element is made up of a specific isotope, we call it its relative abundance.
There are two common ways to write isotope notations. If we want to use a chemical symbol, we write the mass number to the upper left corner. We can also write the atomic number in the lower left corner to make it more obvious, for example \(^{12}\)C is an isotope of carbon with 12 nucleons. It has 6 protons, so this isotope has 6 neutrons.
Another way to write isotopes is by using the name of the element. This isn’t used in equations, but in written text. That same carbon would be called carbon-12.
Hydrogen has two stable isotopes:
There is one more isotope of hydrogen you may come across. \(^3_1\)H - tritium or hydrogen-3. It is not stable; it has a half-life of 12.33 years.
Notice the 1s in the lower left corner? They’re redundant, because all hydrogen atoms have an atomic number of 1 but it makes it even clearer to see the number of neutrons. You don’t need to include them, though.
The fact that some isotopes are unstable is key to radioactivity, which you might learn more about later. Isotopes and radioactivity play a major role in daily life — from medical imaging and treatment, to disinfecting and sterilizing equipment, to nuclear power and weapons, all the way to carbon dating. The existence of isotopes makes a lot of modern science possible, so it makes sense to know something about them.
Neutral atoms have an equal number of protons and electrons. However, taking or giving an atom electrons isn’t very hard to do – it’s basically the whole point of most chemical reactions. Changing the number of electrons gives you charged atoms, also called ions. We have cations – positively charged ions and anions – negatively charged ions. A change in charge doesn’t always affect properties, but sometimes it does. For example, different cations of manganese have different colours. Some later pages discuss the topic of exchanging electrons more.
Written by Tereza Vargová