Our world is full of variety; however, there are only a relatively small number of elements on the periodic table. How do only approximately 90 naturally occurring elements make up this world? They bond! When elements combine (or bond), they become a new substance with different properties from original elements.

Here's an example. Sodium is a soft, silvery metal. Sodium can react with chlorine, which is a yellowish, poisonous gas. When the two elements bond, it results in sodium chloride (NaCl). You know this as table salt. Sodium chloride is neither a metal nor a poisonous gas.

Why do elements bond? They bond to reach an electron configuration. In most cases, this is an octet in their outermost valence shell. Chemical bonding is when atoms gain, lose, or share electrons to reach an octet in the outer energy level. What drives this process? How can we know if an atom will gain, lose, or share electrons? First, let’s review electronegativity and ionization energy to understand the behavior of the elements that make up the periodic table.


Electronegativity is the measure of an atom's ability to attract electrons. The distance from the nucleus to its valence electrons affects an atom’s electronegativity. The further away the valence electrons are from the nucleus, the lower the electronegativity. In other words, the more shells an element has, the lower the electronegativity. Valence electrons drive element behavior.

A second factor that affects an atom’s electronegativity is the number of valence electrons. The more valence electrons in the outer valence shell, the greater the electronegativity. According to the octet rule, atoms with smaller atomic numbers will have eight electrons in their valence shell when they combine. The greater the electronegativity an element has, the greater the ability of an atom of that element to attract electrons to itself . Elements with the largest electronegativity will have a large number of valence electrons, but few shells.

Interactive exercise. Assistance may be required.

Interactive exercise. Assistance may be required. Now that you’ve revisited electronegativity, let’s review how it trends on the periodic table.


Interactive exercise. Assistance may be required. Now, look at the periodic table below. Fill in the blank spots in the table by dragging each electronegativity value to the correct location.

Interactive exercise. Assistance may be required.

Ionization energy tells us how much energy is being used to take away an electron from a neutral atom. Elements with fewer than three valence electrons require less energy to remove an electron from their valence shell.

Interactive exercise. Assistance may be required.

When you look across the periodic table, it becomes more challenging to take away electrons. When you look down a family, it becomes easier to take away electrons. This trend is due to the increasing distance of the valence electrons from the nucleus.

Interactive exercise. Assistance may be required. Which area on the periodic table would have the highest ionization energy? (Hint: What area would be hardest to take electrons away from?) Click on the correct answer.