Periodic Trends

Periodic Trends for Electronegativity

Electronegativity is a chemical property that attempts to describe the attraction between a bonding electron and an atom.

• The electronegativity of the elements within a period increases from left to right. When the valence shell of an atom is less than half full, it requires less energy to lose an electron than gain one and thus, it is easier to lose an electron. Conversely, when the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.
• Down a group, the electronegativity decreases from element to element. This is because the atomic number increases down a group and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.
• Important exceptions of the above rules include the noble gases and transition metals. The noble gases possess a complete valence shell and do not usually attract electrons. The transition metals possess a more complicated chemistry that does not generally follow any trends

                                           

Periodic Trends for Ionization Energy

Ionization Energy is the amount of energy required to remove an electron from a neutral atom in its gaseous phase. 

• The ionization energy of the elements within a period generally increases from left to right. This is due to valence shell stability.
• The ionization energy of the elements within a group generally decreases from top to bottom. This is due to electron shielding.
• The noble gases possess very high ionization energies because of their full valence shell as indicated in the graph. Note that Helium has the highest ionization energy of all the elements.

                               

                          

Periodic Trends for Electron Affinity

Electron affinity describes the ability of an atom to accept an electron.

• Electron affinity increases from left to right within a period. This is caused by the decrease in atomic radius.
• Electron affinity decreases from top to bottom within a group. This is caused by the increase in atomic radius.

Periodic Trends for Atomic Radius

For atoms, one-half the distance between the nuclei of two atoms is called the atomic radius.

• Atomic size gradually decreases from left to right across a period of elements. This is because, within a period or family of elements, all electrons are being added to the same shell. But at the same time, protons are being added to the nucleus, making it more positively charged. The effect of increasing proton number is greater than that of the increasing electron number therefore there is a greater nuclear attraction. This means the nucleus attracts the electrons more strongly and therefore, the shell is pulled closer to the nucleus. The outermost electrons are held closer towards the nucleus of the atom. As a result, the atomic radius decreases.
• Going down a group, it can be seen that atomic radius increases. The outermost electrons occupy higher levels due to the higher quantum number (n). As a result, the outermost electrons are further away from the nucleus as the ‘n’ increases. Electron shielding prevents these outer electrons from being attracted by the nucleus, thus they are loosely held and the atomic radius is large.
• Atomic radius decreases from left to right within a period. This is caused by the increase in protons and electrons across a period. One proton has a greater effect than one electron and thus a lot of electrons will get pulled towards the nucleus and thus a smaller radius.
• Atomic radius increases from top to bottom within a group. This is caused by electron shielding.

                                   

Periodic Trends for Melting Point

Generally, the stronger the bonds between the atoms of an element, the higher the energy requirement in breaking that bond.

• Metals generally possess a high melting point.
• Most non-metals possess low melting points.
• The non-metal carbon possesses the highest boiling point of all the elements. The semi-metal boron also possesses a high melting point.

Periodic Trends for Metallic Character

The ease of losing an electron is a measure of an element's metallic character.

• Metallic characteristics decrease from left to right across a period. Metallic characteristics increase down a group. Electron shielding causes the atomic radius to increase thus the outer eletrons ionizes more readily than electrons in smaller atoms.
• Metallic character relates to the ability to lose electrons, and nonmetallic character relates to the ability to gain electrons.

 

Overall, these charts can be simpled into this picture:

Here is a video found on YouTuBe about the periodic trends:

Outside Links

• http://en.wikipedia.org/wiki/Periodic_trends
• http://www.youtube.com/watch?v=d-hEkyYUXSo[1]
• Pinto, Gabriel. "Using Balls of Different Sports To Model the Variation of Atomic Sizes." J. Chem. Educ.1998 75 725.
• Qureshi, Pushkin M.; Kamoonpuri, S. Iqbal M. "Ion solvation: The ionic radii problem." J. Chem. Educ.1991, 68, 109.
• Smith, Derek W. "Atomization enthalpies of metallic elemental substances using the semi-quantitative theory of ionic solids: A simple model for rationalizing periodic trends." J. Chem. Educ. 1993, 70, 368.
• Ionization Energy Trend: http://www.youtube.com/watch?v=ywqg9PorTAw&feature=youtube_gdata
• Other Periodic Table Trends: http://www.youtube.com/watch?v=XMLd-O6PgVs&feature=youtube_gdata

Problems

The following series of problems will review your general understanding of the aforementioned material.

1.) Based on the periodic trends for ionization energy, which do you except to have the highest ionization energy? 

1. A.) Fluorine (F)
2. B.) Nitrogen (N)
3. C.) Helium (He)

2.) Nitrogen has a larger atomic radius than Oxygen. 

1. A.) True 
2. B.) False

3.) Which do you expect to have more metallic character, Lead (Pb) or Tin (Sn)?   
4.) Which element do you expect to have the higher melting point: chlorine (Cl) or bromine (Br)? 
5.) Which element do you expect to be more electronegative, sulfur (S) or selenium (Se)?
6) Why is the electronegativity value of most noble gases equal to zero?
7) Arrange the following atoms according to decreasing effective nuclear charge experienced by their valence electrons: S, Mg, Al, Si
8) Rewrite the following list in order of decreasing electron affinity: Fluorine (F), Phosphorous (P), Sulfur (S), Boron (B).
9) An atom with an atomic radius smaller than that of Sulfur (S) is __________.

1. A.) Oxygen (O)
2. B.) Chlorine (Cl)
3. C.) Calcium (Ca)
4. D.) Lithium (Li)
5. E.) None of the above

10) A nonmetal will have a smaller ionic radius when compared to a metal of the same period.

1. A.) True B.) False

11) Which one of the following has the lowest first ionization energy?

1. A. Element A
2. B. Element B
3. C. Element C
4. D. Element D

Solutions

1. Answer: C.) Helium (He)

Explanation: Helium (He) has the highest ionization energy because, like other noble gases, Helium's valence shell is full. Because of this, Helium is stable and does not readily lose or gain electrons.

2. Answer: A.) True

Explanation: According to periodic trends, atomic radius increases from right to left on the periodic table. Therefore, we would expect Nitrogen to be larger than Oxygen.

3. Answer: Lead (Pb)

Explanation: Lead and Tin share the same column. According to periodic trends, metallic character increases as you go down a column. Lead is underneath Tin therefore we would expect Lead to possess more metallic character.

4. Answer: Bromine (Br)

Explanation: According to periodic trends, in non-metals, melting point increases down a column. Since chlorine and bromine share the same column, we would expect bromine to possess the higher melting point.

5. Answer: Sulfur (S)

Explanation: Note that sulfur and selenium share the same column. Periodic trends tell us that electronegativity increases up a column. This indicates that sulfur is more electronegative than selenium.

6. Answer: Most noble gases have full valence shells.

Explanation: Because of their full valence electron shell, the noble gases are extremely stable and do not readily lose or gain electrons.

7. Answer: S > Si > Al > Mg. 

Explanation: The electrons above a closed shell are shielded by the closed shell. S has 6 electrons above a closed shell, so each one feels the pull of 6 prontons in the nucleus.

8. Answer: Fluorine (F)>Sulfur (S)>Phosphorous (P)>Boron (B)

Explanation: According to periodic trends, the electron affinity generally increases from left to right and from bottom to top.

9. Answer: C.)  Oxygen (O)

Explanation: Periodic trends indicate that atomic radius increases up a group and from left to right across a period. Therefore, oxygen is expect to have a smaller atomic radius than of sulfur.

10. Answer: B.) False

Explanation: The reasoning behind this lies in understanding that a metal usually loses an electron in becoming an ion while a non-metal gains an electron. This results in a smaller ionic radius for the metal ion and a larger ionic radius for the non-metal ion.

11. Element D

Explanation: Element A, B and D have the same number of elentrons in the inner shell, but element D has the least number of eletrons in the outer shell which requires the lowest ionization energy.