# Periodic Trends

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• Happy Monday everyone. For today's lesson, I have moved to this part of the classroom﻿. Many of you are asking why and there is a purpose to this and you will find out in today's lesson. The first thing we are going to discuss is atomic and ionic radius trends. We have talked about atoms this chapter but they are a very important part in science. To find these trends we will use the periodic table.
• Atomic radius is estimated on the periodic table in a simple way. As you go more to the right across a period, the atomic radius decreases because you are adding more electrons. When you move the left on a period, the atomic radius increases because there are less electrons. It all depends on the charges distance from the nucleus of the atom. Some substances have a larger and smaller atomic radius than others. Something such as cadmium has a larger atomic radius than sulfur. The substance Sb on the table has the largest however.
• The other radius that we will talk about is ionic radius. This is very similar to ﻿atomic radius in some ways. In a neutral atom many elements exist as anions and cations. If the atom loses its outermost electron, the ionic radius is larger than the atomic radius because it has lost an energy shell. For example if electrons from a shell of a calcium atom, then the atom becomes a cation that has a smaller radius than the atom. Or the other way. If electrons are added to the shell of a carbon atom then it forms a cation that has a smaller radius.
• This next thing ties into those concepts as well. Ionization energy trends can be located on the periodic table. Ionization energy is best described as energy needed to remove an electron from an atom or ion in a gas phase. When going the the right on the table, the energy increases. In ionization energy trends, the energy tends to decrease towards the bottom of a group in the periodic table. This is mostly because the electrons are getting farther from the  nucleus.﻿ Among the elements in the main group, the general trend in the first ionization energy moving across a period is not followed by group 2 and 13. The best explanation for this is that the ionization energy increases because the stability of a half-full p subshell is increased.
• The last two concepts we will discuss today are the trends in electronegativitly and electron affinity. The main concept once again similar to the previous trends. Starting with electronegativity, as you move downward on the periodic table, the electronagativity decreases because the increased number of energy levels puts the outer electrons far away from the nucleus. Electronegativities of the elements Be, Mg, Ca, and Sr however follow a specific trend within their group. Based on this the one with the least attraction for an electron would be Mg.
• ﻿Electron affinity on the other hand is rather simple. As you move move downward and across in a group, the affinity becomes less negative. The size of the orbital increases and the affinity for the electron is less. Now you may have noticed that I moved from the left to the right side of the room ¨periodically¨ (pun intended) throughout the lesson. That is because it is a common theme in discovering trends of substances within the periodic table. BING! That is the bell, do not forget to grab the assignment on your way out the door folks!
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