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Periodic Trends – One By One

In this section, we will discuss the periodic properties and trends. It is very important from the MCAT point of view to understand the trends of the periodic table.

Atomic Size

As mentioned earlier, the periodic table is very versatile. The periodic table can give you the relative atomic sizes of atoms, and elemental ions. The two trends regarding the atomic radii are given below.



  1. From left to right along a period, the atomic radius decreases, as the atomic number increases.
  2. Along a group from top to bottom, the atomic radius increases.


One reason for such a trend is attributed to the principal quantum number. As the principal quantum number increases, the size of the orbital increases. Another reason for this trend is attributed to the nuclear shielding by the electron cloud that is between the nucleus and the outermost shells, thereby decreasing the influence of the effective nuclear charge.
Problem 4-1
Arrange the following elements in terms of increasing atomic radius:
Mg, K, Cl, Ba 
  1. Cl, Mg, K, Cs
  2. Cl, K, Mg, Cs
  3. Cs, K, Mg, Cl
  4. Mg, K, Cl, Ba

The answer is choice A.
Mg and Cl are in the same period (Period 3).
But Cl is in Group VIIA and Mg is in Group IIA. So Mg is larger than Cl.
The next is K which is in period 4, and thus bigger than both Mg and Cl.
Finally, Cs which is in period 6 has the largest atomic radius.
So in the increasing order of atomic size is: Cl<Mg<K<Cs.

Ionic Radius

Often you will get questions on arranging ions and atoms according to their sizes. Some of the trends that you should keep in mind regarding ionic radii are listed below:


  1. Negatively charged ions have bigger ionic radii than the corresponding neutral atoms.
  2. Positively charged ions have smaller ionic radii than the corresponding neutral atoms.

Ionization Energy (IE)

Ionization energy is the minimum amount of energy required to remove an electron from an atom. The amount of energy required to remove the first electron is called the first ionization energy (IE1). The second ionization energy (IE2) refers to the amount of energy required to remove the second electron. The second ionization energy is always greater than the first ionization energy, since it is more difficult to remove a second electron from an already positive ion, compared to the removal from an electrically neutral atom. The third ionization energy is greater than the second ionization energy, and so on.
First IE < Second IE < Third IE < Fourth IE < Fifth IE < Sixth IE < . . .

The general trend of ionization energy is summarized as follows:

  1. Generally, along a period from left to right, the ionization energy increases with increasing atomic number.
  2. The ionization energy decreases from top to bottom along a group as the atomic size increases.



Electron Affinity

Electron affinity is the change in amount (energy either released or absorbed) of energy for the process of adding an electron to an atom (neutral) in its gaseous state, resulting in an ion of –1 charge. The general trend of electron affinity is given below: 



The electron affinity increases or in other words, the negativeness of the electron affinity increases from left to right along a period.




The relative tendency of an atom to attract the bonding electrons to itself is called electronegativity. The popularly used electronegativity scale is based on a system called Pauling's scale, according to which fluorine (the most electronegative element) has an electronegativity value of 4.0. Nonmetals are the most electronegative elements. The general trend of the electronegativity is as follows:
  1. Generally, the electronegativity increases from left to right along a period.
  2. The electronegativity decreases down a group from top to bottom.



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