Trends of the Periodic Table

Trends of the periodic table are a foundational concept of organic chemistry. You likely first learned trends of the periodic table in general chemistry, but let’s refresh now. Specifically, we will discuss “what is electronegativity?” and the atomic size trend.


What is electronegativity?

What is electronegativity? Electronegativity is how much an atom desires electrons. If an atom is highly electronegative, it will try to take electrons from its less electronegative neighbors. Electronegativity increases as you go to the right and up on the periodic table.

Trends of the periodic table: What is electronegativity? How much an atom wants electrons. It increases going up and to the right.

Therefore, fluorine (shown on the periodic table above) is the most electronegative atom on the periodic table. When an electronegative atom like fluorine is next to a less electronegative atom, the more electronegative atom tends to hog or take some of the electrons. The result of this hogging is called induction, which occurs when partial charges appear on atoms as a result of a highly electronegative atom taking electrons.

What is electronegativity? When an atom really wants electrons so it steals it from neighboring atoms.


Keep it Simple

To remember the order of electronegativity of atoms, use the mnemonic “FONClBrISCH.” The atoms in the mnemonic are listed in decreasing order of electronegativity so F>O>N>Cl>Br>I>S>C≈H. This mnemonic is good for quickly identifying the answer to electronegativity problems, particularly when the answer is more ambiguous such as when elements are not in either the same column or row of the periodic table.


Atomic Size Trend

Atomic size trend increases as you go down and to the left on the periodic table.

Trends of the periodic table: Atomic size trend generally increases as you go down and to the left.

As you go to the right, the atomic size trend decreases because you are adding one more proton to the nucleus (the positively-charged center of the atom) each time you move one element to the right. More protons (and therefore more positive charge) in the nucleus produces a greater pull on the surrounding negatively-charged electrons causing them to reduce their radius.

As you go down the periodic table, the atomic size trend increases as you are adding many more protons (generally 8 or 18 more protons each time you move 1 element down a column). The atom therefore get larger simply because so many more protons (and therefore, more neutrons and electrons) are being added into the atoms orbitals.