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The arrangement of the periodic table



The Periodic table is a visual representation of the elements. The way the elements are arranged reveals information about their properties.

Select the elements in the table below to learn more about their properties and history.


The periodic table arranges elements in order of increasing atomic number. The rows and columns in the periodic table are known as periods and groups, respectively.


The elements in each group have similar chemical properties due to the similarity in their valence electron configurations.

Example: Group \(\mathrm{1}\) elements

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

All of these elements have one \(s\) electron in their outermost (valence) shell. Group \(\mathrm{I}\) elements, except hydrogen, are soft metals and demonstrate high reactivity with water and oxygen.

Valence electrons and core electrons

Electrons located in the outermost shell (largest and highest-energy shell) of an atom are known as valence electrons. The rest of the electrons in the atom are known as core electrons. Valence electrons are accessible, thus participate in the chemical reactions. Core electrons are tightly bound to the nucleus and inaccessible.

Classification of the elements

Groups of elements

Group Name Examples Chemical properties
\(\mathrm{I}\)A Alkali metals \(\ce{Li}\), \(\ce{Na}\), \(\ce{K}\) Soft and shiny metals, highly reactive with water
\(\mathrm{II}\)A Alkaline earth metals \(\ce{Be,\,Mg,\,Ca}\) Soft and shiny metals, moderately reactive with water
\(\mathrm{VII}\)A Halogens \(\ce{F,\,Cl,\,Br}\) Generally reactive elements, exist as gases at room temperature
\(\mathrm{\mathrm{VIII}}\)A Noble gases \(\ce{He,\,Ne,\,Ar}\) Exist as gases, unreactive elements

Metals and non-metals

Based on selected physical properties, elements are classified into metals and nonmetals. Metals are located on the left side of the periodic table, and nonmetals are located on the right.

Physical properties of metals - conductors of electricity and heat, exist as solids at room temperature (except Mercury), ductile, malleable, shiny appearance (metallic lustre), high density and high melting point.

Elements know as metalloids exhibit both metallic and nonmetallic properties. Metalloids are located between metals and nonmetals in the periodic table.

s,p,d,f blocks

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

Classification based on electron configurations of the elements

  • The noble gases - The last electron located in the \(p\) subshell except \(\ce{He}\). All noble gases, except \(\ce{He}\) have electron configuration ending with \(p^{6}\) (completes the \(p\) subshell).
  • The representative elements - The elements located in \(s\) block and the first five columns of \(p\) block belongs to the representative elements group. The last electron of these elements fills an \(s\) subshell partially or entirely or fills a \(p\) subshell partially.
  • The transition elements - The elements located in the \(d\) block.
  • The inner transition elements - The elements located in the \(f\) block.

Classification of the periodic table based on electron configuration of the elements.

Periodic table trends

Atomic radii

Atomic radii increase from the top to the bottom of a group of the periodic table. As \(n\) increases when proceeding down a group, valence orbitals become larger, leading to an increase in atomic radii.

In contrast, from left to right of a period, atomic radii decrease. The effective nuclear charge (Z) increases when the atomic number (Z) increases across the period. As a result of the higher Z, orbitals become smaller.

Ionisation energy

The first ionisation energy is the minimum energy required to remove an electron from a neutral atom. The second and third ionisation energies are the energies necessary to remove more electrons from the atom.

First ionisation energy increases when moving from left to right across a period as larger Z makes electrons bound tightly. When proceeding down on a group, the first ionisation energy decreases as an electron in a higher energy level is easier to remove. The trend in the first ionisation energy is inverse of the atomic radii.

Electron affinity

Electron affinity is the amount of energy change when an electron is added to a gaseous phase atom to form an anion. The electron affinity becomes more negative when moving across a period from left to right.


Electronegativity is the power of an atom in a molecule to attract electrons. The larger the value, the larger the electron attracting ability. Atoms with higher electronegativity form anions, whereas atoms with smaller electronegativity form cations. Electronegativity decreases from top to bottom and increases from left to right of the periodic table.

Periodic trends

    Source: Periodic table data comes from Periodic-Table-JSON, Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0). Interactive Periodic table based on a version by Adrian Roselli.