Student Resources

Figure 2.17 Atomic Radii of Select Elements Across the Periodic Table
Overall:

• 
  Atomic radius is determined as half the distance between the nuclei of two identical atoms bonded together.
  
• 
  The atomic radius of atoms generally decreases from left to right across a period.
  
• 
  The atomic radius of atoms generally increases from top to bottom within a group.
  

Electronegativity

Another important periodic trend to be aware of is how electronegativity differences can be identified. Electronegativity is the measure of an atom’s tendency to attract a bonding pair of electrons. There are various numerical scales for rating electronegativity. Figure 4.3 shows one of the most popular—the Pauling scale. The Pauling scale assigns fluorine, the most electronegative atom, a 4.0 while less electronegative atoms have smaller grades. We will see in chapters 3 and 4 that electronegativity plays an important role in chemical bonding. The trends for electronegativity in the periodic table are that electronegativity increases as you go across a period, and increases as you go up a group, with fluorine being the most electronegative atom. Noble gases are given an electronegativity rating of 0 due to their inherent stability, which keeps them from forming bonds with other atoms.
 

Figure 2.18 Electronegativities of Various Elements. The Pauling Scale for electronegativities has the value for fluorine atoms set at 4.0, the highest value.

Ionization Energy

Another periodic trend that you will be expected to know is the trend for ionization energy. Ionization energy is defined as the amount of energy required to remove the most loosely bound electron of an atom. How tightly bound the electrons of an atom are will affect the amount of energy required to remove one of the valence electrons. Electrons that are closer to the nucleus are going to be more tightly held than those that are further away and will require more energy to pull them off of the atom. For this reason, we see that ionization energy decreases as you go down a family group and the atoms get larger. This same concept can be applied to atoms across a period. We will see that the highest ionization energy will be found on the right side of the period where the atoms are the smallest, and the lowest ionization energy on the left where the atoms have a larger radii. In general, ionization energy decreases as you go down a family group, and increases as you go across a period from left to right (Figure 2.19).