That’s one of the first things I tell my chemistry students. They react by looking at me as if I have three heads. Still, without exception, every single one of my students will at some point over the course of the year express their awe at the genius of the periodic table and of the minds that put it together.
Nearly everyone has at least some knowledge of the periodic table. We’ve seen it in textbooks, in classrooms, and even in nerdy science memes. But many are surprised to discover just how much information is contained in this modern scientific marvel. In this post, I will briefly summarize how the periodic table is arranged and some of the information you can learn from the table, once you know where to look.
What are Elements?
Let’s begin our study of the periodic table by defining some terms.
You probably know that all matter is made up of tiny building blocks called atoms. How many different TYPES of atoms do you think exist? Believe it or not, there are only 92 different types of atoms found in nature*. We call those different types of atoms elements, and it is these elements that are listed on the periodic table.
Stop and let that sink in just a second. Everything in the universe is made using only 92* different types of atoms. Butterflies, amoebas, planets, sea water, volcanoes, humans, chocolate, stars—you name it. Everything in nature is made with only those basic building blocks. Incredible, huh? How is that possible?
The different types of elements can combine to form compounds, and these compounds can have drastically different properties than the elements that they are composed of.
Sodium (Na), the 11th element in the periodic table, is commonly found as a soft metal that reacts violently with water. Chlorine (Cl), the 17th element in the periodic table, most commonly exists as a yellowish-green toxic gas. But when one atom of sodium combines with one atom of chlorine, you get sodium chloride (NaCl), commonly known as table salt: a tasty solid that is neither toxic nor reacts violently with water.
In a similar way, different combinations of the 92 natural elements join to form compounds and it is these arrangements of atoms in compounds that gives us the incredible variety we find in the world. Water is made up of hydrogen and oxygen. Table sugar is made up of carbon, hydrogen, and oxygen. Wood alcohol is also made up of carbon, hydrogen, and oxygen, but the elements are arranged differently in each substance. Baking soda is made up of carbon, hydrogen, oxygen, and sodium. Graphite (used in pencil “lead”) is made of only carbon.
Wait a minute. Why do I keep referring to 92 natural elements when modern versions of the periodic table list 118 elements? Do you have any guesses?
Well, it turns out that some of the elements listed on the periodic table are not found in nature at all. Instead, they were created by scientists in a lab setting. How did they accomplish such an incredible feat? The process is complex and involves particle accelerators, and some of the elements (especially the ones shaded in gray in the figure above) were only present for such minute periods of time that their presence was only detected through the use of computers. You can get an idea of how it works by watching the following video. (By the way, the newest elements have their permanent names now. The names in this video were only temporary names).
How is the Periodic Table Arranged?
If you examine a copy of the periodic table, you will see that the elements are all listed by number, and the numbers increase from left to right and from top to bottom along the table. Hydrogen (H), the first element on the table, is number one. Helium (He), the second element on the table, is number two, and so on. The numbers assigned to each element, the atomic numbers, are not arbitrary by any means. The atomic number of each element not only tells you where to find each element in the table, but also corresponds to the number of protons in an atom of that element.
Since Lithium (Li) has the atomic number of 3, we know that a Lithium atom has three protons. Beryllium (Be) with atomic number 4 has 4 protons. Calcium (Ca) is the twentieth element listed in the periodic table and contains 20 protons per atom. Pretty neat, huh? So just by looking at the periodic table, you can determine how many protons are in the atoms of each element.
What’s important to realize is that it is the number of protons an atom has that defines what element it is. Carbon (C) atoms always have 6 (and only 6) protons. Silicon (Si) atoms always have 14 (and only 14) protons. Cobalt (Co) atoms always have 27 (and only 27) protons. So if I were to tell you that I isolated an element made up of atoms with 50 protons, we would know that the element I’d isolated had to be tin (Sn), because only tin atoms contain 50 (and only 50) protons.
Even cooler, once you know the number of protons in the atom of an element, you can figure out the number of electrons, too. You may recall that atoms contain three subatomic particles: protons, neutrons, and electrons. Within an atom, protons and neutrons reside in the center (the nucleus of the atom) while the high-energy electrons are in constant movement in regions called electron orbitals (or electron clouds) that surround the nucleus. Two of the subatomic particles have electrical charges: protons have a positive charge and electrons have a negative charge. Neutrons have no charge and are electrically neutral.
Even though two of the subatomic particles of an atom are charged, the atom itself has no net charge. Why? Because in an atom, the number of protons and electrons are equivalent. So the number of positively-charged protons and the negatively-charged electrons cancel each other out. So if Lithium with atomic number 3 has 3 positively-charged protons, it must have 3 negatively-charged electrons to remain electrically neutral. And Barium (Ba), the 56th atom in the periodic table with an atomic number of 56 has 56 positively-charged protons, so it must have 56 negatively-charged electrons to exist as an electrically-neutral atom.
The Building an Atom online simulation is a fun way to become familiar with the makeup of atoms. Like all of PhET’s simulations, it’s free. You can check it out here:
The other number listed for each element is the relative atomic mass.
An element’s mass is determined by the number of protons and neutrons it contains. Electrons are so small that their mass isn’t included in the atomic mass. While atoms of an element always have a constant number of protons, the number of neutrons can vary. For example, Carbon atoms always have 6 protons, but can have 6, 7, or 8 neutrons, depending on the atom. We call atoms of the same element but with different numbers of neutrons isotopes.
In the figure above, we see three isotopes of carbon: Carbon-12, Carbon-13, and Carbon-14. All three of the isotopes have 6 protons, because carbon atoms always have 6 protons. The difference between the isotopes are the number of neutrons within the atoms, and because the number of neutrons is different between the isotopes, so are the atomic masses. For a given isotope, the atomic mass is found simply by adding the number of protons and the number of neutrons. Take Carbon-12, for example. Its atomic mass is listed as 12. Since we know that carbon atoms always have 6 protons, we can calculate that atoms of Carbon-12 have 6 neutrons (atomic mass of 12 – 6 protons in carbon atom = 6 neutrons). Similarly, atoms of Carbon-13 must have 7 neutrons (atomic mass of 13 – 6 protons in carbon atom = 7 neutrons) while atoms of Carbon-14 must have 8 neutrons (atomic mass of 14 – 6 protons in carbon atom = 8 neutrons).
Often, the atomic mass of elements on the periodic table is listed as the relative atomic mass. This number is calculated by taking the average of all of the available isotopes of each element, weighted by their natural abundance. If that’s hard to understand, don’t worry about it. The takeaway is that just by looking at the periodic table, you can learn a lot about the atomic makeup of each of the elements.
Before moving on, I just wanted to note something about element names and symbols. Some of the element names and symbols seem to make more sense than others. Most of the time, the element symbol is simply the first letter of the element’s name. This is the case for hydrogen (H). But since helium also starts with an h, the symbol for this element is He. The symbol for carbon is C, while calcium is Ca. But what about sodium? Why is its symbol Na? Na is the abbreviation for natrium, the Latin term for sodium. Likewise, the abbreviation for potassium (K) is based on its Latin name kalium. The symbols for antimony (Sb), copper (Cu), gold (Au), iron (Fe), lead (Pb), mercury (Hg), silver (Ag), and tin (Sn) are all based on the Latin names for each element.
The History and Brilliance of the Periodic Table
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Clearly, great minds went into designing the periodic table. But how did it happen? Most people associate Dmitri Mendeleev with the creation of the modern periodic table, and rightly so. But he wasn’t the first one to undertake the organization and categorization of the known elements. You can read more about early versions of the periodic table here.
Mendeleev was able to realize that groups of the known elements of his day had similar properties. What’s more, if the elements were arranged in order of increasing mass, a pattern emerged— a periodic pattern, so to speak. To summarize, Mendeleev recognized that by arranging the elements in order by atomic mass, he could create columns of elements that had similar properties. Elements in the first column were chemically similar, as were elements in column 2, 3, and so on. He was so certain that the pattern held true that he left empty spaces in his table for elements which had yet to be discovered.
Watch this really great video to learn more.
Brilliant, right? Are you starting to realize how cool the periodic table is?
We now know that elements in the same column (known as groups or families) have similar chemical properties because they have the same number of reactive electrons (called valence electrons) in their outermost electron clouds. It is the valence electrons of an atom that dictate the types of chemical reactions a given element will engage in. Elements in Group 1A (the first column) are all metals with one valence electron. Elements in Group 2A (the second column, the Alkali Earth metals) have two valence electrons, and so on.
Atoms are most chemically stable when they have eight valence electrons in their outermost electron level, and the majority of chemical reactions take place as atoms seek to obtain this desired electron configuration. The noble gases in Group 8A are often described as chemically inert. With eight valence electrons in their outer electron cloud, they are atomically stable and therefore don’t take part in chemical reactions.
So not only can you determine the number of protons, electrons, and neutrons by looking at the periodic table, you can also predict the types of chemical properties each element will have based on how many valence electrons it contains. As students learn to interpret the periodic table, they are able to predict the outcomes of chemical reactions. They understand that elements in Group 1A will react with elements in group 7A (the halogens) in a one to one ratio to form ionic compounds, while elements in group 2A will form ionic compounds with the halogens in a one to two ratio.
As students progress through chemistry, they also learn the general trends in the periodic table. For instance, as you go from top to bottom or from right to left in the table, the atomic radius (a measure of the size of the atoms) increases. Likewise, one can predict how easily a given element will attract electrons to itself to achieve the desired eight valence electrons (electron affinity) and how much relative energy is required to remove a valence electron (ionization energy) just by looking at the table.
Believe it or not, this isn’t even all of the information contained within the periodic table!
If you’d like to learn more about the periodic table and the elements it contains, I recommend the book The Disappearing Spoon. It weaves together the history, science, and trivia of the periodic table in an enjoyable way. I also HIGHLY recommend the series Mystery of Matter, which originally aired on PBS but is now available to rent or buy on Amazon (both as a DVD set or as digital download). This three-part documentary begins with the discovery and isolation of the first known elements, highlights Mendeleev’s creation of the periodic table, and ends with the discovery of radioactive elements, the subatomic particles, and how scientists split the atom. Everyone I’ve ever recommended it to—no matter what age—has loved the series.
A fantastic resource is available from the Royal Society of Chemistry. Click on any element from the periodic table and learn all sorts of facts, including the element’s history, melting and boiling points, chemical properties, and how it is commonly used.
For More Information:
Periodic Videos: a video for every element in the periodic table
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