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