Trends in Properties of the Elements in the Periodic Table

To know more about the different elements in the periodic table, you need to know the different properties of the elements and their trends.   We need to recall that Periodic Table of Elements, the horizontal rows called periods and the vertical columns called groups or families.

Atomic Size

     Atomic size refers to the how big or small an atom is.  To determine the atomic size of an atom, atomic radius is used, which is a one-half the distance between the two nuclei in two adjacent metal atoms.

     The first figure above shows the atomic radius of iodine diatomic molecule, the atomic radius is measured from the nucleus of the first atom of the iodine to the next atom of the molecule.  The second figure shows how the atomic radius of two adjacent atoms is measured.  

     Usually the atomic size increases from top to bottom and decreases from left to right in a given period.  What could be the possible reason of this trend?  As we move down a group, let's say from H to Fr the atomic size increases that is because of the addition of the energy level.  What about in a given period where elements have the same energy level?  The atomic number increases as we move from left to right in a given period, this means that the number of protons and electrons increases and therefore there is an increase in the effective nuclear charge, this is why the atomic size of the atoms decreases in moving from left to right in a given period.

Below are the radii (in picometers) of representative elements according to their position in the periodic table.

Plotting atomic size versus atomic number will give some repeating patterns.  The elements in group IA are the elements on the highest point of the graph while elements in Group VIIA are the elements in the lowest point.  This only indicates that atomic size decreases in a given a period from left to right.

Ionization Energy

Ionization energy is another property of the elements, which is defined as the minimum energy required to removed an electron from a gaseous atom in its ground state.  It is usually expressed in kJ/mol).  The magnitude of the ionization energy is a measure of how tightly the electron is held in an atom.  The higher the ionization energy, the more difficult it is to removed the valence electrons from an atom.  For many atoms, the amount of energy required to remove the first electron from an atom is called first ionization energy.

The Ionization Energies of First 2 Elements in kJ/mol

What is the trend in ionization energies in going down a group and in moving from left to right in a given period?  Let us see the graph below: 

As we observed based from the graph above, the elements lie on the highest points are the elements in group VIIIA or group O/Noble Gases.  And the elements lie on the lowest point of the graph are the elements in group IA/Alkali metals.  Which means that in moving from left to right in a given period the ionization energy increases.  therefore metals have lower ionization energies than nonmetals.  This is the reason why metals tend to lose electrons forming cations or positive ions and nonmetals tend to gain electrons forming anions or negative ions.  One reason also of the increase of the ionization energy from left to right is the increase of the nuclear attraction between the protons and valence electrons due to the decrease in the atomic size of the atoms.

In going down a group let say in Group IA from Li to Cs, there is a decrease in ionization energy.  This means bigger atoms tend to lose electrons because of the lower nuclear attraction between the protons and the valence electrons.  

Example :  

1.  Which has lower ionization energy Li or Rb?  Na or Ar?

Answer:   Rb has lower ionization energy than Li since Rb is bigger in size than Li.   Na has lower ionization energy than Ar for the same reason.

Electron Affinity

Electron affinity is another property of elements which vary also in a group and within a given period.  It is defined as the energy released when an atom accepts electrons from another atom forming anion.  

Electrons Affinities of Some Representative Elements and Noble Gases in kJ/mol

From the above table, generally the electron affinity increases in moving from left to right in a given period.  This means that  nonmetals have higher electron affinity than metals.  In going down a group the electron affinity is decreased.

Two sign conventions are used for electron affinity, it can be positive or negative.  In most introductory text, the thermodynamic sign is used.  A negative sign indicates that the addition of electrons is an exothermic process.  Historically, however, electron affinity is defined as the energy released when an electron is added to a gaseous atom or ion, the electron by this convention is positive.  

Electronegativity

Electronegativity is another property of an element, it is defined as the ability of an atom to attract an electron to itself in relation to chemical bonding.  The higher the value of electronegativity , the greater is the ability to attract electrons to itself.  The electronegativity of an atom is related to ionization energy and electron affinity, which are also other properties of the elements.  An atom with very negative electron affinity and high ionization energy will attract electrons more easily and resist having its electrons attracted away, and this is called  highly electronegative atom.

Generally, electronegativity increases in moving from left to right in a given period, and decreases in moving down a group, as shown in Table below:

Since nonmetals have higher electronegativity values than metals, they have the ability to attract electrons than metals. That's the reason why nonmetals carries negative charge forming anion and metals form cation.

For general trends in the different properties of elements in the Periodic Table, refer on the table below: