## Chem – College: Causes of Periodic Trends

What causes the periodic trends?

VIDEO Explanation for cause of Periodic Trends

To ask about why the periodic trends exist is to ask a very fundamental question about how the structure of the atom and the structure of how the universe itself is put together. To understand we start off with what is called Coulomb’s Law. Coulomb’s Law is an equation that describes one of the few forces that holds our universe together (similar to gravity). Coulomb’s Law can tell us about more than one possible scenario but in this case we are going to use it to talk about how the proton (positive charge) interacts with the electron (negative charge).

Coulomb’s Law Equation:

 F = Q q1 q2 r2

F is the force, how strong / tight / close the electron is held to the proton. Big Q is a constant (ignore it) The q1 is the amount of protons. The q2 is the amount of electrons. The r is the distance between the proton and electron and is squared in the equation. Coulomb’s Law essentially tells us that protons and electrons are held together in an atom by a combination of the distance between the protons and electrons and how many protons and electrons they have.

If you really want to understand the periodic trends and this fundamental law of nature (Coulomb’s Law), then you need to focus on 2 things:

1) The distance between protons and the electrons is the most important factor of the periodic trends. Particularly the distance of the outer most electrons to the protons. The shorter the distance between the outermost electrons and the nucleus, the more tightly it is bound to the atom. Therefore, shorter distance means higher electronegativity, higher ionization energy, and smaller atomic radius.

2) How many protons and neutrons an atom has is a less important factor of the periodic trends than the distance. The more protons and electrons an element has means higher electronegativity, higher ionization energy, and smaller atomic radius.

Examples: Use the logic of Coulomb’s Law to determine the differences in the periodic trend. Use the periodic table if you need it. VIDEO Determining Periodic Trend by Coulomb’s Law Examples Video 1.

1) Which element has a HIGHER electronegativity and why? Nitrogen or Sulfur.

Answer: Nitrogen because its outer electrons are closer to the nucleus.

2) Which element has a SMALLER atomic radius? Aluminum or Silicon.

Answer: Silicon because it has 1 more proton than Aluminum but their outer most electrons are about the same distance from the nucleus.

3) Which element has a LOWER ionization energy? Germanium or Arsenic.

Answer: Germanium because it has 1 less proton than Arsenic but their outer most electrons are about the same distance from the nucleus.

4) Which element has a LARGER atomic radius and why? Phosphorus or Selenium.

Answer: Selenium because its outer electrons are further from the nucleus.

PRACTIC PROBLEMS: Use the logic of Coulomb’s Law to determine the differences in the periodic trend. Use the periodic table if you need it.

1) Which element has a HIGHER electronegativity and why? Xenon or Bromine.

Answer: Bromine because its outer electrons are closer to the nucleus.

2) Which element has a SMALLER atomic radius? Magnesium or Sodium.

Answer: Magnesium because it has 1 more proton than Sodium but their outer most electrons are about the same distance from the nucleus.

3) Which element has a HIGHER ionization energy? Chlorine or Oxygen.

Answer: Oxygen because its outer electrons are closer to the nucleus.

4) Which element has a LARGER atomic radius? Rubidium or Strontium.

Answer: Rubidium because it has 1 less proton than Strontium but their outer most electrons are about the same distance from the nucleus.  