Periodic trends

Question

Explain the periodic trends for atomic radius, ionization energy, and electronegativity, and give an element whose ionization energy does not follow the trend and explain.

Answer 1

Atomic radius:  atomic radius increases within a group going DOWN and within a period it decreases going from left to right.

Explanation - going down within a group, you are increasing the number energy levels and thus increasing the atomic radius. As you go across within a period, from left to right, you have the same number of energy levels BUT you have more protons in the nucleus, and this pulls the outer orbitals closer. Thus, going from left to right, the atomic radii decrease, or going from right to left, it increases.

 

Electronegativity:  This increases within a group going from bottom to top, and it increases within a period going from left to right.  

Explanation - going down within a group, EN decreases because atomic number increases going down the group and thus the distance between valence electrons and nucleus increase (increased atomic radius)  Going left to right within a period, EN increases because it is easier for valence shell that is less than 1/2 full to lose an electron than to gain one, and vice versa.

 

Ionization energy:  This increases going left to right within a period and going up within a group, just like EN. 

Explanation: Within a period, it increases going left to right due to valence shell stability and it decreases going down within a group due to electron shielding.

The exceptions are the noble gases, such as Helium.  These have 8 valence shell electrons and do not want to gain or lose any.