Periodic Table

Everything around us is matter. Some of it is pure and some is impure, and it exists in the form of elements, compounds and mixtures. An element is the simplest pure substance. It cannot be broken down into anything simpler by ordinary chemical methods. The oxygen in the air, the iron in a nail, the carbon in charcoal and the copper in an electric wire are all elements.

Till now, 118 elements have been discovered. Of these, 92 are natural, which means they are found in nature, and 26 are artificial, which means scientists have made them in laboratories.

Each element is made up of its own type of atom. All the atoms of gold are alike, and they are different from the atoms of oxygen or iron. This is why every element has its own physical properties, such as colour, state, hardness and melting point. Every element also has its own chemical properties, such as the way it reacts with water, air or acids.

Imagine having to study 118 elements one at a time, each with its own long list of properties. They would be very hard to remember, and no pattern would be visible. When elements are compared, however, some of them turn out to behave very much alike. Sodium and potassium are both soft, shiny metals that can be cut with a knife, and both react strongly with water. Chlorine and fluorine are both poisonous gases, and both combine with sodium to form salts.

Scientists used this likeness to sort the elements. The elements having similar properties are kept in one group, and the elements having different properties are kept in different groups. This sorting is called the classification of elements, and its basis is the properties of the elements.

Classification makes the study of elements easier in three ways:

• Instead of learning 118 separate elements, we study a smaller number of groups whose members behave alike.
• If we know the properties of one element in a group, we can predict the properties of the other members of that group.
• The relationship between the properties of different elements becomes easy to see.

The simplest way to sort elements is to divide them into metals and non-metals. Metals such as iron, silver, sodium and aluminium are usually shiny, conduct heat and electricity well, and can be beaten into sheets. Non-metals such as oxygen, nitrogen, chlorine, carbon and sulphur are usually dull and do not conduct electricity, and many of them are gases at room temperature. Graphite, a form of carbon, is an exception because it does conduct electricity.

This sorting is useful, but it is rough. Do all metals have the same properties? They do not. Sodium is so soft that it can be cut with a knife, and when it is dropped into water it reacts so violently that it darts about and may even catch fire. Iron, on the other hand, is hard and only rusts slowly in moist air. Mercury is a metal, yet it is a liquid at room temperature. Two elements can both be metals and still behave very differently.

So scientists asked a deeper question. Is there any relation between the atomic structure of an element and its properties? There is. The properties of an element depend on the structure of its atoms, especially on the electrons in the outermost shell. Elements whose atoms are built in a similar way show similar properties. A good classification must therefore go further than metals and non-metals, and place truly similar elements side by side.

The final result of classifying the elements is a single table in which every element has its own fixed place.

In a periodic table, each element sits in its own box, usually marked with the element's symbol. The position of that box tells you which elements it resembles, because its similar partners sit in the same column.

The Russian scientist Dmitri Mendeleev made a careful scientific study of the elements known in his time. He examined their atomic weights, the compounds they formed and the way they reacted, and he took atomic weight as the key. On the basis of atomic weight he created a periodic table for the classification of elements. From his study he also formulated a law, known as Mendeleev's periodic law.

Put simply, as the atomic weight of the elements increases, their properties keep coming back in a fixed pattern. The properties depend on atomic weight, and that is why the law calls them functions of atomic weight.

Something is periodic when it repeats after a regular interval. The days of the week are periodic, because after Saturday, Sunday comes round again. The seasons are periodic too, returning in the same order every year.

Mendeleev's law says the properties of elements behave in the same way. When the elements are arranged in the increasing order of their atomic weights, their physical and chemical properties recur periodically. In other words, as you move along the list, similar properties appear again and again at regular intervals.

Here are some elements arranged in increasing order of atomic weight: hydrogen 1, lithium 7, beryllium 9, boron 11, carbon 12, nitrogen 14, oxygen 16, fluorine 19, sodium 23, magnesium 24, aluminium 27, silicon 28, phosphorus 31, sulphur 32, chlorine 35.5 and potassium 39.

Lithium, sodium and potassium are all soft, highly reactive metals that react with water. Counting along the list, sodium comes seven places after lithium, and potassium comes seven places after sodium. In the same way, fluorine and chlorine are similar non-metals, and chlorine comes seven places after fluorine. Properties returning at regular intervals like this is exactly what periodic means.

If all the elements are written in one long line, similar elements end up far apart. Mendeleev solved this by cutting the list into rows so that similar elements fall one below another. Elements with similar properties then lie in the same vertical column, and the elements with different properties that come between them lie in a horizontal row.

A monthly calendar is built the same way. The dates run in order along each row, and each row is one week. Every Sunday falls in the same column. A column holds days that are alike, while a row holds a run of different days in order.

So the increasing order of atomic weight gives the order of the elements, placing similar elements in one column makes the groups, and the rows that result are the periods. In this way a classification table for elements is made, and it is known as the periodic table.