
Chem 1010 Lecture Notes
The Periodic Table: How the elements are organized
Introduction
We have seen that atoms are the building blocks of all matter, and that different kinds of atoms are called elements. There are 90 naturally occurring elements and (at present) 25 artificial ones. We've learned a little about how they are named, which are the most common in the universe, one the earth, and in our bodies, and we've seen how to write their symbols.
In this lecture, we will discuss how the Periodic Table of Elements was organized, and what it can tell us about the elements.
First, we will talk about different ways to organize the elements, and why a new way was needed.
Then we will discuss what it means to say that the elements are periodic. This is important to understanding how the elements can be organized into a Periodic Table.
We will then talk about how the Periodic Table itself was originally organized, and how it looks today.
Finally, we'll talk about the different parts of the Periodic Table that you’ll need to know.
Organizing the Elements
With 115 elements and counting, it is very useful to have some way to organize them.
The elements could be organized in alphabetical order by their names, like the list of the elements and their symbols that we used in the last lecture. This sort of organization is useful for finding an element, but it doesn't tell us anything about their properties or how we would expect them to behave. Hydrogen, which is the lightest atom, appears in the middle of the list. Also, you can't tell by looking at this list that sodium and potassium have similar properties, or that strontium and barium do also.
When Dalton published his atomic theory, he listed the known elements in order of their masses, from lightest to heaviest. This seemed like a logical thing to do, since the mass is an important property, and one that is easy to quantify and put in order. This kind of a list at least shows us which elements are lightest and heaviest, and how they compare to each other in mass. But it still doesn't help us predict their behavior. Dalton tried to show some similarities in the symbols that he created; the symbols for sodium and potash (later named potassium) both have vertical lines, and the symbols for strontian (later strontium) and baryes (later barium) both have similar hash marks. But this kind of system would be difficult to apply to all 115 elements, especially since we now use letters from their names as symbols.
As the number of elements grew, it became important to find a way to organize the elements that showed how their properties were related. In 1869 a Russian chemist named Dmitri Mendeleev discovered the key (note that he is not the same person as Gregor Mendel, who discovered principles of genetics using pea plants).
Mendeleev was a card player. He wrote the names and properties of the 63 known elements on playing cards, then shuffled and dealt them again and again, looking for an arrangement that made sense. Eventually he made a critical discovery – when the elements are listed in order of their mass, their properties are periodic.
What is periodicity?
If something is periodic, then first of all, it has some order to it. Secondly, it has some properties or characteristics. And finally, it has a repeating pattern.
For example, let's consider some things you are very familiar with: numbers, letters, Legos, and weather.
Numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 23, 23, 24, 25, 26, 27, 28, 29, 30, etc.
The numbers have a natural order – they go from smallest to largest, increasing by one each time. One property they have is what digits they are made of, and it what order. Most importantly, they have an obvious repeating pattern. There are different symbols for 0 to 9, but then it starts over with 10 and goes to 19, then 20 to 29, 30 to 39, and so on.
You can make a table to show how this pattern repeats: put 0 to 9 across, and then on the next row put 10 to 19, and so on.
As you go across a row in the table, the first digit is the same, and the last digit is in order of size (0 to 9). Then you start on the next row, and it repeats. Because of this pattern, as you go down a column, all of the numbers end in the same digit. The pattern of digits repeats itself over and over, so that you can always predict how the number in any column will end.
The numbers are completely periodic. They have a regular pattern that repeats exactly with out fail. Not all patterns are so exact, as we shall see, but you can get a good idea of what a repeating pattern is.
Letters: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z.
The letters of the alphabet have a tradition order. They also have properties – some are consonants, and some are vowels. Some of the letters are rounded (C, D, O, S, etc) while some have only straight lines (A, E, K, M, T etc). The sounds they represent are also made in different ways: some sounds are made with the lips (m, b, p, etc), others with the top of the mouth (n, t, j, etc), some are made with the voice and some are not (f vs. v, s vs. z).
However, it is difficult to find any pattern to these properties. Vowels show up every so often, but not in a predictable order. Letters with rounded shapes and straight shapes are pretty random. And the way in which the sounds are made isn't in any particular order either.
You could make a table for the letters, like the one for numbers above, by starting a new row every time there is a vowel. The rows go up one letter each time, but the letters in each column except the first one have nothing in common.
Letters are NOT periodic. They have an order, and some properties, but there is not repeating pattern.
Legos:
We used Legos to show how matter is made of tiny, separate pieces. But are Legos periodic? The answer is no – they fail the first test, since there is no order to them. You could make a chart with all the same colors in each column, and different shapes in each row, but there's no reason to put a certain shape or color first. Therefore, there can't really be a repeating pattern.
Weather:
If you took the outside temperature every three hours for several days, you would see a pattern. The temperature goes up in the morning, reaches a peak around mid afternoon, and then falls again, reaching a low early in the morning.
If you made a graph of this data, here is what it would look like. You can easily see the repeating pattern.
You can therefore conclude that temperature is periodic. The pattern isn't perfect – the same high and low isn't reached each day – but you can use the pattern to make some predictions about what would happen next.
If you did the same with wind speed, however, the graph is much more random. There isn't any visible pattern. Therefore wind speed is not periodic.
Notice that both graphs and charts can be used to show periodicity, or lack of it. Both can show a repeating pattern.
Elements:
The elements are also periodic. This means that they must have an order, and some properties, and show repeating patterns in those properties. The most logical choices for order are mass (lightest to heaviest) or atomic number (1 proton, 2 protons, etc). As it turns out, these are give an almost identical order of elements. The elements have lots of properties, including physical properties that can be measured, and chemical properties that can be observed.
If you look at the following graphs showing three different properties (atomic radius, first ionization potential, and electronegativity), you can see some interesting repeating patterns. (Don't worry about understanding what these properties are – just look at the patterns).
Since the elements are periodic, there must be a way to make a chart which will make us of these repeating patterns.
How was the Periodic Table created?
The fact that the properties of the elements are periodic can be used to organize them into a table. To illustrate, we'll use a set of colored cards with numbers on them; the colors represent the properties of the elements, while the numbers represent their mass.
Step 1: Random order – if the cards are distributed randomly, you can't see any pattern to the colors and numbers. These cards are actually periodic, but you can't tell that yet; there isn't any obvious pattern. This would be like a random (or alphabetical) list of the elements.
Step 2: In numerical order – if the cards are lined up in order of their numbers, you can begin to see some patterns. This is like a list of the elements in order of their mass. The pattern isn't totally clear yet, but you can see some things that always happen. For example, a blue card always has an orange card before it; green is always followed by yellow; a purple is always between a blue and a green; a yellow always has an orange after it; and so on. These are the same kinds of patterns that Mendeleev saw in the elements.
Step 3: Make a table – You can arrange these cards into a table by starting a new row every time there is an orange card. Now the patterns become much clearer.
Step 4: Make the columns match – In order to make the colors line up in rows, you have to slide some of the cards to the right, leaving some open spaces. Now the cards with the same color are all in the same column. This is why the Periodic Table has the shape that it does. The atomic mass increases as you go across each row and then down to the next one, and the elements with similar properties are in the same columns.
Here is Mendeleev's actual original table. He did it sideways from how it is arranged now, with columns having increasing mass and rows having the same properties.
The amazing thing about Mendeleev's table is that it only contains about 63 elements – all the ones that were known at the time. So his cards would actually have looked a little more like this:
With only 2/3 of the naturally occurring elements, he was still able to see enough of the patterns to put a reasonable table together. He also used the holes to predict the existence of elements that hadn't been discovered yet. If you look at the cards above, you can tell what should go in some of the holes. Where there was an obvious hole in Mendeleev's table, he predicted that elements would be found to fill them, and could even tell what their properties would be before they were discovered! Within a few years, the elements that he predicted were found in nature.
It was also very exciting for chemists about 50 years later when they discovered a whole new column. Note that the yellow cards are missing above – none of the elements in the last column of our current table were known in Mendeleev's time. When helium was discovered, they realized that it didn't fit into any of the existing columns, and its mass was in between the first and second rows. Once they realized that another column existed, scientists went on to discover the other elements in this column.
What does the Periodic Table of Elements look like today?
About 50 years after Mendeleev, chemists discovered protons and the atomic number. Today we use the atomic number rather than mass to determine the order of the elements. The atomic number is always a whole number (you can't have half a proton!) and there is an element with every atomic number up to 116, so this is a very easy way put the elements in order. All of the naturally occurring elements have been filled in, and artificial ones are added as they are created. To print out a copy, please use this link: Periodic Table of Elements
You may be wondering why there are two rows at the bottom which aren't connected to the others. They are there because they have been pulled out of where they should go in order to make the table smaller and easier to print on a page. The first picture below shows what the table should really look like. The two rows that have been moved are circled. Notice that if you follow the atomic numbers on a regular table, they go 55,56,57,72,73,74. This is because you have to jump down to the bottom, go across, and back up again. The first row on the bottom starts with 58 and goes to 71. The same thing happens on the last row. You can think of it as a footnote sending you down to the bottom of the page, after which you go back up to where you left off.
What are the parts of the Periodic Table?
It will be important for you to understand the parts and structure of the periodic table. To print out a blank Periodic Table to write on, use this link: blank Periodic Table.
The rows of elements are called periods, and they are numbered from 1-7 going from the top down (notice that the two separate rows at the bottom are part of the 6th and 7th periods).
The columns are called groups or families. Some of the families have names – the first family is called the alkali metals; the second family is called the alkaline earth metals. The second to last family is called the halogens; the last family is called the noble gases. All other families are called after the name of the first element in that family - for example, the carbon family, the chromium family, and so on.
There are also some larger sets to be aware of. All of the elements to the upper right of the bold, stair-step line (as well as hydrogen) are nonmetals, and everything to the lower left of the line (except hydrogen) are metals. All the metals have metallic properties in their elemental state (they are shiny, ductile, conduct electricity, and so on). The nonmetals are gases, liquids, or crystalline solids in their elemental state.
The metals are further divided into three groups. The elements in the block beginning with the scandium family and ending with the zinc family are called transition metals. The two rows at the bottom are the inner transition metals. All the rest of the metals are called main group metals.
Copyright 2006 Sarah Morgan Black