Many people do not understand the importance of the periodic table of elements. They may think it is nothing more than a way of organizing the natural elements. To be able to appreciate its genius and glean the wealth of information that it holds – you need to know how to read it.
Dmitri Mendeleev, who developed it, organized the elements in a way that captured their properties. He relied on all the information available in 1869 about the known elements, and looked for a way to contain all the knowledge of these elements and set it out before the readers by the very form of the table.
Many of the elements known today were not discovered in Mendeleev’s time, and the information available on the known ones was limited. In devising the table, Mendeleev drew on the work of John Dalton, one of the first scientists to determine the atomic mass of elements, and of Johann Wolfgang Döbereiner, on trends in certain properties of selected groups of elements.
What’s the best way to look at the periodic table?
Let’s start with the most obvious arrangement – the atomic number. The elements are arranged in order of increasing atomic number. Each element has a unique atomic number, which indicates how many protons it has. For example, oxygen’s atomic number is 8, telling us that its nucleus contains eight protons.
An atom’s nucleus (almost always) holds protons and neutrons. The proton’s mass is very similar to that of the neutron, while the mass of the electron, which surrounds the nucleus, is much smaller. Therefore, the combined mass of the protons and neutrons make up the mass of the atom. The mass of one proton or one neutron has been set as one atomic mass unit.
For example, a fluorine atom has 9 protons and 10 neutrons. Its atomic number is 9, while its atomic mass is 19 atomic mass units. So we can say that its atomic weight is 19, or its mass number is 19.
Another level of arrangement are the table’s columns. Each column is a chemical family – the chemical elements in it have similar chemical properties. Why? Because each element in the column has the same number of electrons in its outer shell. These electrons have the most significant influence on how the element reacts with other elements or substances. All the elements in the first column, for example, have one electron in their valence shell, and therefore will have a very strong reaction to water.
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Four of the columns – the first two and the last two – have the closest family resemblances:
The table’s first column lists hydrogen and the alkali metals, all of which have one electron in their valence shell. This electron makes them highly reactive, and thus they must be stored in oil, to prevent them from reacting with air or water vapor. These metals are very soft, highly conductive thermally and electrically, have a low melting point, and are malleable. As we descend the column, the metals become even softer and their melting points – lower.
The second column includes the alkaline earth metals, which have two electrons in their outer shells. They have similar properties to the alkali metals, but they are less reactive chemically. Because they have two electrons in their shells, they tend to produce different compounds from the alkali metals.
The seventh column is home to the halogens. These are nonmetals with seven electrons in their outer shells, and so are one electron short of a full shell. Halogens are very chemically reactive because they “want” to attract that extra electron from another element to complete their shell. All halogens have a very strong smell and are found naturally in a diatomic state – for example chlorine Cl2.
The eighth, and last, column contains the noble gases – all of which are colorless, odorless, and tasteless. These elements have a full outer shell, with eight electrons (except for helium, which has a full shell with two electrons). They don’t “need” to obtain electrons from any other element or share their electrons in a chemical reaction, and therefore usually do not form compounds. This is where the name “noble gases” came from – because they don’t like to mix with anyone.
The table’s rows are called periods, since each starts with an element in the first column which behaves as an alkali metal, goes on to an alkaline earth metal, and ends with a halogen and a noble gas. Each row expresses a whole range of elements, with their typical chemical properties.
The period number signifies the number of electron energy levels, or the highest level of energy an electron occupies. Take the alkaline metal column for example: lithium (Li) is on period 2 and contains two electron energy levels; sodium (Na), period 3, has three levels; potassium (K), period 4, has four; and so on.
Now that we have examined the arrangement of the atomic numbers, the columns, and rows, we can look at other general trends that predict the chemical behavior of the elements according to their location in the table. .
Moving down a particular column, the atomic number goes up – i.e. the element’s nucleus gets larger and has more electrons at different energy levels, resulting in a larger distance between the nucleus and outer electron shell (in other words, a larger atomic radius).
This arrangement also enables us to offer some general rules, for instance, about ionization. Ionization is the level of energy needed to remove an electron from an atom in a gas state. It can indicate the atom’s reactivity level, as well as its chemical and physical properties.
If we look at a specific column for example, we see that the further down we move, the farther away is the electron shell from the nucleus – and thus it is easier to remove an electron from it. So the ionization energy gets smaller moving down a column. Now, looking at a specific row, we can see that the atomic number increases as we move along (left to right) the period, and thus the nucleus becomes larger and more positive ‒ making it more difficult to remove an electron from the atom. Thus as we move along a row, its ionization energy gets smaller.
The periodic table offers a huge amount of information that both beginner and experienced chemists use to understand the metallic properties, the electron arrangement, the quantum numbers, the electronegativity, the ionic behavior, the expected compounds resulting from a reaction between various elements, and more.
The periodic table is considered to be the outcome of a stroke of genius. It is so much more than just an arrangement of elements – the table sorts the elements according to their chemical families and chemical behaviors, so that its organizing principles enable predicting the behavior of an unfamiliar element according to our knowledge of the adjacent elements. Knowing how to read the periodic table enables expanding our chemical knowledge and thus better understanding the material world in and around us.