The periodic table was devised by Mendeleev in response to observed regularities in the chemistry of elements before there was any understanding of their electronic basis. The periodic table remains the most important framework for understanding the comparative chemistry of elements and many major trends can be understood from the atomic trends.
The periodic table used today are arranged in periods and groups. There are seven periods representing the principle quantum numbers n=1 to n=7 and each period is filled sequentially. The electrons in the outermost shell are called valence electrons. They are involved in chemical bonding and determine the chemical reactivity and properties of the elements.
Trends in the Periodic TableBack to Top
The periodic table of the elements is a visual representation of the atomic structure of the known chemical elements. Periodic trends become evident when the chemical and physical properties of the elements vary according to their position on the periodic table.
Types of periodic trends that can be determined by an elements position on the periodic table include atomic radii, ionic radii, density, ionization energy, electronegativity and electron affinity. Each of these properties contributes to the chemical reactivity of the element.
Periodic table with periodic trends chart divisions indicating valence shells and summary of atomic property trends are depicted in the following diagram.
Electronegativity Trends in the Periodic TableBack to Top
"The tendency of an atom to participate in bonding with another atom is related to a property of the atom known as electronegativity." Covalent bonds results from the unequal sharing of bonding pairs of electrons. Electrons are shared unequally when there are differences in the electronegativity values of the two atoms concerned.
There is a clear trend in electronegativity in the periodic table. Non metals are elements that gain electrons to complete their outer shells. They have greater electronegativity values than metals, which lose electrons to achieve full outer shells. Electronegativity values generally decrease as atomic number increases down a group. They increase as atomic number increases across a period.
The trend in electronegative values of the elements in the periodic table shown below.
Ionization Energy Trend on Periodic TableBack to Top
"Ionization energy is defined as the energy required to remove an electron from an atom." Energy is always required to remove electrons. Francium in the lower left corner of the periodic table has the lowest ionization energy while helium has the highest ionization energy. In general ionization energy decreases from top to bottom of any group in the periodic table and increases from left to right across a period in the table.
Removing an electron from an atom always takes energy, so the ionization energy is always endothermic. The removal of second electron requires even more energy, the removal of a third electron requires the third ionization energy and so on.
The ionization energies for several metals are listed in the following table.
First (electron) ionization energy
Second (electron) ionization energy
Third (electron) ionization energy
Ionization Energy Trends in the Periodic Table is shown below.
Reactivity Trends in the Periodic TableBack to Top
Elements present in the groups on the left-hand side of the periodic table tend to react by losing electrons to form positive ions. As we go down the group atoms tend to lose the electrons more easily. In other words on moving down the group metals it gets easier for the elements to react, they become more reactive. Another way of mentioning it is
- Reactivity increases as we move down the group on the left-hand side of the periodic table
- Reactivity decreases from left to right across a period on the left hand side of the periodic table
The reactivity and metallic character are closely related. the reactivity of metals increases with increase in metallic or electropositive character. On the other hand reactivity of non metals increases with increase in electronegative character. Reactivity of metals are shown by considering the period 3 elements in the periodic table as shown below.