The Periodic Table

As scientists discovered more and more elements, they wanted to organise them in some sensible order, so that elements with similar properties could be grouped together. Putting elements in a logical arrangement would make their properties easier to learn and predict. The orderly arrangement of elements that resulted is called the periodic table, but it took several attempts by different scientists before a good arrangement was found. The earliest of these attempts were made by Dobereiner and Newlands.

The first notable attempt was by Johann Dobereiner, who arranged certain elements into groups of three, called triads. In a Dobereiner's triad, three elements with similar properties were grouped together, and he noticed that the atomic mass of the middle element was roughly the average of the other two. For example, in the triad of lithium, sodium, and potassium, the atomic mass of sodium is about the average of those of lithium and potassium. This showed there was some pattern linking elements' properties and masses.

However, Dobereiner's triads had a serious limitation: only a few elements could be arranged into such triads. Most of the known elements did not fit into any triad, so this system could not be used to organise all the elements. It was a clever early idea but too limited to be a general classification.

The next attempt was by John Newlands, who arranged the known elements in order of increasing atomic mass and noticed that every eighth element had properties similar to the first, like the repeating notes in a musical scale. He called this the Law of Octaves (an octave being a group of eight). This was an improvement, but it also had limitations: it worked only for the lighter elements and broke down for the heavier ones, and Newlands had to force some elements into the same slot, which did not really fit. So although Dobereiner's triads and Newlands' octaves both revealed that element properties repeat in a pattern, neither could organise all the elements successfully, paving the way for better arrangements by Mendeleev and others.

The first truly successful arrangement of the elements was made by the Russian scientist Dmitri Mendeleev. He created the Mendeleev's periodic table, in which he arranged the known elements in order of their increasing atomic mass. His great insight was to place elements with similar properties in the same vertical column (group). Mendeleev based his arrangement on the periodic law that the properties of elements are a periodic (repeating) function of their atomic masses.

Mendeleev's table was a remarkable achievement for several reasons. He did not simply follow atomic mass blindly; where necessary, he left gaps (blank spaces) in his table for elements that had not yet been discovered, rather than forcing the wrong elements into a column. Even more impressively, he predicted the properties of these undiscovered elements based on their position in the table. When these elements (such as gallium and germanium) were later discovered, their properties closely matched Mendeleev's predictions — a stunning success that showed his table was based on a real pattern in nature.

Mendeleev's periodic table thus had major strengths: it grouped similar elements together correctly, it left room for elements yet to be found, and it successfully predicted the existence and properties of those elements. This made it far more useful and powerful than the earlier attempts by Dobereiner and Newlands. For this reason, Mendeleev is often called the "father of the periodic table".

However, Mendeleev's table also had some limitations. The position of hydrogen was uncertain, since its properties resembled more than one group. The arrangement by atomic mass led to a few anomalies, where elements seemed to be in the wrong order — in a few cases an element of higher atomic mass had to be placed before one of lower mass to keep similar elements together. Also, isotopes (atoms of the same element with different masses) posed a puzzle, since they would need different positions if mass alone were used. These problems were later solved by arranging elements by atomic number instead of atomic mass, leading to the modern periodic table.

The problems with Mendeleev's table (such as the anomalies in the atomic-mass order) were solved when the scientist Henry Moseley discovered that the atomic number, not the atomic mass, is the more fundamental property of an element. This led to the Modern Periodic Law, which states that the properties of elements are a periodic function of their atomic numbers. In other words, when elements are arranged in order of increasing atomic number, elements with similar properties recur (appear again) at regular intervals.

Based on this law, the modern periodic table arranges all the elements in order of increasing atomic number. Arranging by atomic number (rather than atomic mass) removed the anomalies of Mendeleev's table, because the atomic number is always a whole number that increases steadily, and it also solved the problem of isotopes (since isotopes have the same atomic number, they occupy the same place). The modern periodic table is the version we use today.

The modern periodic table is organised into horizontal rows and vertical columns. The horizontal rows are called periods, and there are 7 periods (numbered 1 to 7). The vertical columns are called groups, and there are 18 groups (numbered 1 to 18). Elements in the same group have similar chemical properties (because they have the same number of valence electrons), while elements across a period show a gradual change in properties. This neat arrangement makes the table extremely useful for studying and predicting the behaviour of elements.

The table can also be divided into blocks based on which type of shell the outermost electrons are filling: these are called the s, p, d, and f blocks. The s and p blocks contain the "main group" elements; the d block contains the transition metals; and the f block contains a special set of elements usually shown separately at the bottom of the table. So the modern periodic table — built on the modern periodic law of increasing atomic number, with 7 periods, 18 groups, and s, p, d, f blocks — is a powerful and orderly way to organise all the known elements, allowing us to understand and predict their properties.

One of the most useful features of the periodic table is that several properties of the elements change in a regular, predictable way as we move across a period or down a group. These regular patterns are called periodic trends. Because the trends are predictable, we can use the position of an element in the table to estimate its properties without measuring them. Four important trends are atomic radius, ionisation energy, electronegativity, and metallic character.

The atomic radius is a measure of the size of an atom. Across a period (left to right), the atomic radius decreases, because the increasing positive charge in the nucleus pulls the electrons in more tightly. Down a group (top to bottom), the atomic radius increases, because each element down the group has an extra shell of electrons, making the atom larger. So atoms get smaller across a period and larger down a group.

The ionisation energy is the energy needed to remove an electron from an atom. Across a period, the ionisation energy increases, because the smaller atoms hold their electrons more tightly, so more energy is needed to remove one. Down a group, the ionisation energy decreases, because the outer electrons are farther from the nucleus and more loosely held, so they are easier to remove. Electronegativity — the tendency of an atom to attract shared electrons in a bond — follows the same pattern as ionisation energy: it increases across a period and decreases down a group.

Finally, the metallic character — how strongly an element shows metallic properties — follows the opposite trend. Metallic character decreases across a period (elements become less metallic and more non-metallic from left to right) and increases down a group (elements become more metallic going down). This is why metals are found on the left side of the table and non-metals on the right side. To summarise the trends across a period: atomic radius and metallic character decrease, while ionisation energy and electronegativity increase; down a group, the reverse happens. These periodic trends make the table a powerful tool for predicting how elements behave.

Based on their properties and their positions in the periodic table, elements can be divided into three broad classes: metals, non-metals, and metalloids. This classification is very useful, because elements in each class share many common properties and behave in similar ways. The periodic table makes this division clear, since each class occupies a particular region of the table.

Metals are elements that are usually hard, shiny (lustrous), good conductors of heat and electricity, malleable (can be hammered into sheets), and ductile (can be drawn into wires); most are solids at room temperature with high melting points. Examples include iron, copper, aluminium, gold, and sodium. Metals are found on the left side and centre of the periodic table, and they make up the majority of the elements. Metals tend to lose electrons in reactions.

Non-metals are elements that are generally the opposite of metals: they are usually not shiny (dull), poor conductors of heat and electricity (with a few exceptions), and when solid they are often brittle (break easily rather than bending); many are gases or low-melting solids. Examples include oxygen, hydrogen, carbon, sulphur, and chlorine. Non-metals are found on the right side of the periodic table. Non-metals tend to gain or share electrons in reactions.

Metalloids are a small group of elements that have properties in between those of metals and non-metals — they show some properties of each. Examples include silicon and germanium. Metalloids are found along a diagonal line between the metals and non-metals in the periodic table, forming a kind of "staircase" boundary. They are important in technology — silicon, for instance, is widely used in computer chips and electronics, because it conducts electricity better than a non-metal but not as well as a metal (it is a semiconductor). So the periodic table divides elements into metals (left and centre), non-metals (right), and metalloids (the diagonal between them), each class with its own characteristic properties — a classification that completes our study of the periodic table.