Periodic Table Trends

It can be amazing to observe the trends of the Periodic Table!  Thank you, Mendeleev!

Coulomb's Law

is a law of physics describing the electrostatic interaction between electrically charged particles.

So all trends are due to Coulomb's Law or the number of Principle Energy Levels

Atomic Radii

Definition: Half the Distance between nuclei of identical atoms that are bonded together; Units: Angstroms

Period Trend- Decreases left to right because the number of electrons and protons increase without adding an energy level so the distance decreases due to the attraction of the electron to the proton. (Electrons are more attracted to the nucleus, increasing the effective nuclear charge)

Group Trend- Increases as you go down because an energy level is added so it MUST get bigger!.

Hint: Francium has the largest atomic radii

Reactivity of Elements due to Octet Rule

The Octet Rule takes into account that elements will react to achieve a Noble Gas Electron Configuration.  Since all noble gases contain 8 valence electrons, elements will bond by losing, gaining, or sharing valence electrons to reach a total of 8 valence electrons.  Any atom that loses or gains electrons is categorized as an ion!

Metals reach Noble Gas Configuration by losing electrons.  For example, Alkali Metals have one valence electron that they will lose to achieve noble gas configuration.  If Sodium loses one electron during bonding, it will have a similar electron configuration as the Noble Gas Neon.  Metals that have lost electrons become more positive (weird concept but think about losing negativity to become positive).  This is called a Cation!

Nonmetals attain a Noble Gas Electron Configuration by gaining electrons.  For example, Chlorine will gain one electron during bonding to have the same electron configuration as Argon.  Nonmetals that gain electrons in bonding become more negative are called Anions.


  • Since metals lose electrons, the most metallic element is Fr.  Since it is so large, it is easy for Francium to lose its one valence electron that is located at n=1.   As you move up and right from Fr, metallic properties decrease. Cations are smaller than the neutral atom (loses outermost energy level to form ion)
  • Since nonmetals gain electrons, the most nonmetallic element is F. If the goal of a nonmetal is to gain electrons, the smaller the atom size (least principle energy levels), the easier it is to gain an electron because of the nuclear charge being positive.  As you move down and left from Fluorine, nonmetallic properties decrease.  Anions are larger than the neutral atom because additional electrons cause a greater repulsion without any additional effective nuclear charge (no more protons added).

What Reacts?

The easiest way to react is to combine metals that need to lose one valence electron with a nonmetal that needs to gain one valence electron.  This is why Group 1 and 17 frequently react.  The same is true for Group 2 and 16.

Ionic Radii

Definition: Size of the ion; Cations become smaller since metals lose an energy level to reach Noble Gas Configuration, Anions become larger since there are more electrons in the valence shell to repel.

Trends through Metals: Decreases as you move to the right within the metals because there are more protons to attract the same number of electrons closer to the nucleus.  Increases as you move down the periodic table because it adds an energy level.  

Hint: Francium is the most metallic

Trends through Nonmetals:  Starting with group 14, they decrease to the right because electrons are gained increasing the attraction to the nucleus.  Ionic radii increases moving down the periodic table because an energy level is added.

Hint: Fluorine is the most nonmetallic  

Ionization Energy

Definition:Energy required to remove electrons; higher the value, the harder it is to remove the electron; increases are you try to remove more electrons

Period Trend:Increases to the right because it is harder to take an electron from a nonmetal since they want to GAIN an electron.

Group Trend: Decreases as you move down because it is easier to take away an electron the further from the nucleus the valence shell is

Other ideas:

  • Second Ionization Energy- Energy required to remove a second electron.  This is always higher than the first ionization energy.
  • Third Ionization Energy- Energy required to remove a third electron. This is always higher than the first and second ionization energy.  
  • BIG JUMPS in ionization energies means that the lower of the two energies was at a noble gas configuration.  It is harder to remove any additional electrons.
  • HINT: Fluorine has the HIGHEST Ionization energy except the Noble Gases


Definition: Ability to attract electrons from the other atom is a compound; Higher possible value is 4

Group Trend:Increases to the right since nonmetals only need to gain electrons to reach noble gas configuration.

Period Trend: Decreases moving down the periodic table since the valence shell is further from the nucleus.

HINT: Fluorine is the most electronegative


Electron Affinity

Definition:  energy change when an electron is acquired by a neutral atom;  A higher negative value is favored

Energy Released = negative value= favored reaction= energy released = energy is a product  


Energy required = positive value = not favored reaction = energy is a reactant

Periodic Trend: More negative value (greater energy release = good) as you move to the right.  

Group Trend: More negative values are higher on the periodic table

HINT:  Fluorine has the Highest Electron Affinity

Just Think Why...
Electron to Proton Attraction
Similar Charge Repulsion
Adding Energy Levels

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