A Level Chemistry Revision: Inorganic Chemistry – Periodicity

by Lucy Bell-Young

Many elements were discovered long before the periodicity of elements was established by Dmitri Mendeleev in 1869. Mendeleev arranged the then-known elements in tabular form and discerned a pattern. With this, he was able to predict the existence of elements that had not yet been discovered.

Since ancient times, philosophers have speculated about the fundamental compositions of all substances that can be combined in different ways to form various substances. Although ancient philosophers were wrong in designating earth, water, air, and fire as the fundamental elements of nature, the concept of fundamental constituents of matter provided the impetus for the scientific pursuit.

Meanwhile, the idea of small, indivisible constituents of matter, which Democritus called “atomos” or atoms, was also introduced by the ancient philosophers, and set the foundations for what we’ve discovered today. The word “atomos” came from the Greek prefix “a”, meaning “not”, and “tomos”, meaning “to cut.” The word “atom” therefore means uncuttable.

What Do the Rows and Columns Represent?

The elements in the periodic table have specific rows and columns relative to each other. Each column is a group of elements with similar chemical properties. For example, the halogens in column 17, also known as Group 7A, are highly reactive gaseous elements at room temperature. They’re also all strong oxidising agents that react with metals to form salts.

Meanwhile, the rows are referred to as the periods of the elements. They represent the same shells, with the valence electrons increasing from left to right. When a shell is completely filled, a new row begins. This pattern repeats several times until all the elements are listed. 

What Are Shells and Subshells?

Each electron shell is an energy level with a fixed distance from the nucleus. Each can be divided into orbitals and suborbitals. They can hold a limited number of electrons, as shown in the illustration below, which shows the first four energy levels. As the energy level increases, the number of electrons it holds also increases. A diagram showing the number of electrons held by increasing energy levels

The different energy levels or shells, in increasing levels, are K, L, M, and N. They can also be labeled as roman numerals: I, II, III, and IV. As you can see in the image above, the first energy level holds a maximum of two electrons, the second energy level, eight. The third level holds 18 electrons at most, and the fourth a max of 32.

The letter labels (K, L, M, and N) refer to X-ray rotations. These energy levels were deduced based on X-ray spectroscopy of the elements. Meanwhile, the orbital names; 1s, 2s, 2p, 3s, 3p, etc. are named after the spectroscopic notations that correspond to the electromagnetic frequencies, or types of light, that each orbital emits.

Based on the maximum number of electrons that each energy level or electron shell can hold, we can deduce a pattern, which can be summarised in a mathematical formula: 


You can determine the nth number of maximum electrons that a shell can hold based on its ordinal number. For example, the third electron shell can hold a maximum of 2(32) electrons, which is 18 electrons. As you move up the electron shells, you can add one subshell (orbital). For example, the first shell only has one subshell, which is the s subshell, while the second shell has s and p subshells.

How Does the Position of Elements Relate to Their Properties?

As previously mentioned, the position of an element in the periodic table is indicative of its properties based on its group and periodicity. As you can see in the illustration below, the periodic table of elements has 18 columns (groups) and has 7 rows (periods). The reactivity of an element and its affinity to combine with other elements depends on its group and period. Elements of the same group or adjacent groups do not combine with each other to form a compound.

For example, in the first column, which is the alkali metals, lithium will not react with sodium to form a compound. However, in a displacement or double displacement reaction, sodium can easily displace lithium, sodium can easily be displaced by potassium, and so on. Therefore, the reactivity of an element relative to other elements in the same group increases as you go down from the upper row to the lower row of the same column.

Conversely, the reactivity of the elements decreases as you go from left to right. Based on this, you can see that when it comes to the most inert or unreactive element, nothing beats helium, which is located at the rightmost and uppermost portion of the periodic table. It belongs to column 18, i.e. the group of the noble gases.

The main reason for the relative reactivity of the elements is their respective valence electrons. If the outermost electron shell has incomplete valence electrons, like the 7 valence electrons of the halogens, they tend to be more reactive because they have a stronger tendency to accept electrons.

What Are the Groups in the Periodic Table?

The periodic table of elements has 18 groups. Each group has similar properties because of several factors, namely:

  • The number of valence electrons
  • The arrangement of the orbitals or electron configuration
  • The radius of the atom
  • Electronegativity

You must remember that electrons are the ones involved in chemical bonding while the atomic numbers, or the number of protons, are the ones that determine the chemical properties of the elements and ultimately the compounds. Meanwhile, the molecular structure of a substance determines its physical properties.

Here’s an overview of all the different groups in the periodic table:

  • Group 1 – Alkali Metals: These metals are very reactive and typically do not occur as pure elements in nature. Although they’re good electrical conductors, they’re not useful as electrical or electronic components because of their reactive nature.
  • Group 2 – Alkaline Earth Metals: Like the alkali metals, some of these elements are very reactive and rarely found as elements in nature. Instead, they’re usually found in compounds or as trace elements. However, some, like beryllium, have direct technological applications. For example, beryllium is mixed with other metals like copper or nickel to form useful alloys that can be used to make gyroscopes, springs, and spot-welding electrodes.
  • Groups 3 to 12 – Transition Metals: These metals are good conductors of electricity and heat. They’re also malleable and stable enough to be used in many applications. For centuries, many of these metals, like iron and copper, have been a significant part in the development of civilisations and technology. Gold is one of the most important metals in terms of economic value to humans.
  • Group 13 – Boron Group: Except for aluminium, the elements in this group are relatively rare and have several technological applications. Aluminium, for example, is a lightweight metal that’s very useful in constructing aircrafts. Boron, on the other hand, is used as a rocket fuel igniter.
  • Group 14 – Carbon Group: In terms of life as we know it, carbon is the most important element. This group is a bit varied, containing elements like silicon, which is a semiconductor and very useful in electronics.
  • Group 16 – Chalcogens: Oxygen is the most important element in this group in terms of biological life. Sulphur and selenium, on the other hand, also have significant roles in biological life, but only in trace amounts.
  • Group 17 – Halogens: These elements are so highly reactive that they’re rarely found as elements in nature. They usually combine with the alkali metals to form salt.
  • Group 18 – Noble Gases: These gases are chemically inert, but are useful in other ways. Helium, for instance, is used in balloons. The other elements, such as xenon and neons, are used in colourful lamps.

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