Everyone would have learnt about the periodic table of elements at some point in their lives, most likely at school in science lessons. If one is not keen on science, it would have been a passing experience to fulfil the mandatory association with it. However, it would still be one of those things that stick in the memory because it is so unusual; a genius in its composition and riveting in its information.
The periodic table is a collection of essential chemical information, a classification system, relating to approximately 100 elements which are arranged “periodically”, that is, according to properties that repeat in regular, predictable patterns. They are organized in a series of 18 columns and 7 rows and each row is called a period. This specific arrangement of the elements makes the table very useful simply because if one knows the location of an element in the table, one can predict its properties. Each column is called a group, or family, and there are nine groups: Alkali metals, Alkaline Earth metals, Transition metals, Other metals, Metalloids, Non-metals, Halogens, Noble Gases and Rare Earth elements. Elements in the same group have similar physical characteristics. For example, all of the elements in group 1 (the Alkali Metals at the far left of the table) react easily with other elements. However elements in a period (a row) do not share properties and the properties of the elements actually change as you move from left to right across a row. Elements can be solid, liquid or gas at room temperature and some periodic tables show this through a color code or other key.
Dmitri Mendeleyev, a scientist from Siberia, in Russia in 1869, devised the first periodic table. He found that he could arrange the 65 known elements of his time in a grid, or table, so that each element was governed by two main aspects: a higher atomic weight than the one on its left and similar chemical properties to other elements in the same column. He realized that his creation lay at the very heart of chemistry and even noted gaps – spaces where elements should be – but had not yet been discovered!
The Structure of Atoms
Elements are arranged in the periodic table according to atomic number, from left to right and top to bottom. The atomic number of an element is equal to the number of protons found in an atom of that element. For example, an atom of Carbon has six protons in its nucleus and its atomic number is 6. The elements are also arranged according to atomic mass which is close in number to the sum of its protons and neutrons. Carbon, with six protons and, on average, six neutrons in its nucleus, has an atomic mass of 12.0107. With the lighter elements, the atomic mass is almost double the element’s atomic number. As one moves up to the heavier elements, the number of neutrons relative to protons increases, causing the mass to be increasingly more than double the atomic number.
To understand why the table is organised in this way one needs to also understand the structure of atoms. An atom is the smallest particle of an element and it is made up of a certain number of protons (positive charges), an equal number of electrons (negative charges), and approximately the same number of neutrons (neutral charges). The only exception is hydrogen, which can have zero neutrons. Protons and neutrons form the nucleus of an atom, and electrons zoom endlessly around the nucleus in constant motion. They orbit the nucleus at specific levels, or shells. However, this movement is not completely random. As the atomic number increases, so does the number of electrons around the nucleus. Electron configurations depend upon the energy state and magnetic spin of each electron. These qualities place electrons into particular subshells within each shell. The electron configuration shown in a periodic table indicates how many electrons are found in each shell, from innermost to outermost, and each element has one more electron than the element before it.
For example, the electron configuration for Chromium goes over four energy levels in this order: 2,8,13,1. The first shell, for example, includes only one subshell at the lowest-level energy state and can hold no more than two electrons. The second shell, with two subshells that contain four levels of energy states, can hold no more than eight electrons, and so on. The rows of the periodic table correspond to the number of shells needed to hold the electrons. As a result, within a group, all the elements have the same number of valence electrons— electrons in the outermost shell. For example, the elements in group 18 (the far right column) have full shells, making them much less chemically reactive compared to other groups. They are known as the Noble Gases (Helium, Neon, Argon, Krypton, xenon and Radon). All this means that single electrons in an outer shell can easily be taken away from the atom with very little energy. This makes atoms of elements in the left-hand column very reactive (and good conductors of heat and electricity). while making it very difficult to add or remove electrons from an atom that has eight electrons in its outer shell because the atoms of these elements, on the far right, are non-reactive.
Dmitri Mendeleyev organised the table in such a way that elements with similar characteristics fell into the same columns. Doing so naturally created rows within the table. What scientists later discovered was that the elements in each successive row contained an additional electron shell – a significant aspect. For example, the atoms of hydrogen and helium in the first row each had one electron shell; atoms of elements listed in the second row had two electron shells, and so on to elements in the final row, whose atoms each have seven shells. They also found that elements sharing the same group (e.g. the Alkali metals in group 1) all have the same number of outer electrons, leading to similar chemical properties.
From this, scientists learned what caused elements to have different characteristics. Each element’s physical characteristics are determined, mainly, by the number of valence electrons. As with the number of protons, the number of electrons increases by one as you move across the table from left to right, top to bottom. Atoms of elements in the left-hand column have one electron in their outer shell, while atoms of elements in the right-hand column have eight electrons in their outer shell.
Other relationships also became obvious, including patterns in atomic radius, electronegativity, and ionization energy. As the atomic number increases from left to right across a period, nuclear charge increases, which also increases the strength of attraction between a nucleus and its electrons. Thus the nucleus holds onto its electrons more tightly, and the atomic radius—the measure of an atom’s size—generally decreases. Electronegativity is the measure of an atom’s ability to attract electrons in chemical bonds. Elements with low electronegativity have outer shells that are almost empty; elements with high electronegativity have outer shells that are mostly full.
Ionization energy is the amount of energy needed to remove an outer electron from an atom. Within a period, ionization energy generally increases with atomic number: a higher nuclear charge results in a stronger attraction of electrons. However, ionization energy tends to decrease down a group because as the distance between the nucleus and valence shell electrons increases, the outer electrons become easier to remove.
Information of a typical Element
Using Chromium as an example of the elements on the table, one can learn a lot of information with just figures, colours and positions:
Atomic Number: 24
Atomic Mass: 51.9961 amu
Melting Point: 1857.0 °C (2130.15 K, 3374.6 °F)
Boiling Point: 2672.0 °C (2945.15 K, 4841.6 °F)
Number of Protons/Electrons: 24
Number of Neutrons: 28
Classification Group: Transition Metal
Crystal Structure: Cubic
Density @ 293 K: 7.19 g/cm3
Number of Energy Levels: 4
First Energy Level: 2
Second Energy Level: 8
Third Energy Level: 13
Fourth Energy Level: 1