The structure of the periodic table

The periodic table is more than just a list of elements; it is a structured map that reveals patterns and relationships among them. By examining the arrangement of elements, we gain valuable insights into their properties and behaviours.

The periodic table arranges elements in order of increasing atomic number. The rows in the periodic table are known as periods. As an example, \(\ce{C}\), \(\ce{N}\) and \(\ce{O}\) are in period \(2\). The columns in the periodic table are called groups. \(\ce{H}\), \(\ce{Li}\) and \(\ce{Na}\) are group \(1\) elements.

You can select elements in the periodic table to learn more about their properties and history.

Interactive periodic table based on a version by Adrian Roselli.

Classification of the elements by properties

Elements can be classified based on their physical and chemical properties.

Physical properties

Based primarily on selected physical properties, elements are classified as metals and non-metals. Metals are located on the left side of the periodic table, and non-metals are located on the right. Elements know as metalloids exhibit both metallic and non-metallic properties. Metalloids are located between metals and non-metals in the periodic table.

The physical properties of metals, non-metals and metalloids are shown in the table below.

Name	Physical properties	Examples
Metals	good conductors of electricity and heat, exist as solids at room temperature (except mercury), ductile, malleable, shiny in appearance (have a metallic lustre), high density, high melting point	\(\ce{Fe},\,\ce{Cu},\,\ce{Au}\)
Non-metals	poor conductors of electricity and heat, can be gases, liquids or solids at room temperature, vary in appearance	\(\ce{O},\,\ce{C},\,\ce{S}\)
Metalloids	have mixed properties that are intermediate between metals and non-metals, can be semiconductors	\(\ce{Si},\,\ce{B},\,\ce{As}\)

Chemical properties

If we focus on chemical properties, we can classify elements as alkali metals, alkaline earth metals, halogens and noble gases. The chemical properties of elements in these classes are shown in the table below.

Name	Chemical properties	Examples
Alkali metals	soft and shiny metals, highly reactive with water	\(\ce{Li}\), \(\ce{Na}\), \(\ce{K}\)
Alkaline earth metals	soft and shiny metals, moderately reactive with water	\(\ce{Be,\,Mg,\,Ca}\)
Halogens	generally reactive elements, exist as gases at room temperature	\(\ce{F,\,Cl,\,Br}\)
Noble gases	exist as gases, unreactive elements	\(\ce{He,\,Ne,\,Ar}\)

Classification of the elements by electron configuration

Elements in the same group have similar chemical properties due to the similarity in their outermost electron shell (valence) configurations. Thus, the electrons in the outermost shell (largest and highest-energy) are known as valence electrons. Valence electrons are accessible to other chemical species. This allows them to participate in chemical reactions. Elements in the same group have the same number of valence electrons.

The rest of the electrons in the atom are known as core electrons. Core electrons are tightly bound to the nucleus and are considered inaccessible.

All of the elements in the table below have one \(s\) electron in their outermost electron shell. Group \(1\) elements, except hydrogen, are soft metals and are highly reactivity with water and oxygen.

Element	Electron configuration
\(\ce{_{3}Li}\)	\(1s^{2}\,2s^{1}\)
\(\ce{_{11}Na}\)	\(1s^{2}\,2s^{2}\,2p^{6}\,3s^{1}\)
\(\ce{_{19}K}\)	\(1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\,4s^{1}\)
\(\ce{_{37}Rb}\)	\(1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{6}\,3d^{10}\,4s^{2}\,4p^{6}\,5s^{1}\)

\(s\)-, \(p\)-, \(d\)-, and \(f\)-blocks

The elements in the periodic table are categorised into \(s\)-, \(p\)-, \(d\)- and \(f\)-blocks based on the location of the last electron in the valence shell. The \(s\)- block contains elements that have their last electron in \(s\) subshell. Similarly, \(p\)-, \(d\)-, and \(f\)-blocks have elements with their last electron located in \(p\), \(d\), and \(f\) subshells, respectively. The \(s\)-, \(p\)-, \(d\)- and \(f\)-blocks are shown in the figure below.

Based on the location of the last electron in an element, elements can also be classified as a noble gas, main group (or representative) element, transition element or inner transition element. This is summarised in the table and figure below.

Name	Description
Noble gases	The last electron completes the \(p\) subshell, except \(\ce{He}\)
Main group elements	The last electron partially fills or completes the \(s\) subshell, or partially fills a \(p\) subshell; all of the elements in the \(s\) block and the first five columns of the \(p\)-block
Transition elements	The last electron partially fills or completes the \(d\) subshell; all elements in the \(d\)-block
Inner transition elements	The last electron partially fills or completes the \(f\) subshell; all elements in the \(f\)-block

Worked example – interpreting the periodic table

Determine the number of valence electrons in \(\ce{P}\), \(\ce{F}\), \(\ce{Ca}\) and \(\ce{Fe}\).

Step 1: Recall how the periodic table is organised, in relation to the number of valence electrons.

The periodic table is organised such that elements in the same group have the same number of valence electrons. Thus, the group numbers of these elements can be used to determine the number of valence electrons.

Step 2: For groups \(1-10\), group number is equal to the number of valence electrons. For groups \(12-18\), subtract \(10\) out of the group number to get the number of valence electrons. For group \(11\), we would expect the number of valence electrons to be \(1\) due to the \(s^{1}\) electron configuration. However, there are a few exceptions in this case.

Element	Group number	Valence electrons
\(\ce{P}\)	\(15\)	\(15-10=5\)
\(\ce{F}\)	\(17\)	\(17-10=7\)
\(\ce{Ca}\)	\(2\)	\(2\)
\(\ce{Fe}\)	\(8\)	\(8\)

Your turn – the structure of the periodic table

Test yourself on your understanding of the structure of the periodic table.

Periodic table data, by Periodic-Table-JSON, licensed under CC BY-SA 3.0