Atomic radius represents the distance from the nucleus of an atom to its outer shell.
Understanding Atomic Radius
As a result of electrons not being confined in truly organized orbitals, the radial size of the elements can vary greatly. By comparing identical atoms bonded together we get a more accurate description of atomic radius.
Atomic radius is defined as half the distance between the nuclei in a molecule of two identical elements.
Trends in Atomic Radius
The general trend is that as we move from left to right across a period and as we move up any group the atomic radius decreases.
PRACTICE: Which of the following correctly lists the five atoms in order of decreasing size (largest to smallest)?
Ca, K, Ne, O, P
a) P > Ne > Ca > K >O
b) O > Ca > K > P > Ne
c) F > O > P > Ca > K
d) K > Ca > P > O > Ne
e) Ca > P > O > Ne > K
STEP 1: Locate each of the elements given within the question.
STEP 2: Since we are going from the largest to smallest atomic radius start on the left side where elements are larger and move to the right.
STEP 3: Going from left to right across the period and up any groups gives the answer from largest to smallest as:
K > Ca > P > O > Ne
This gives Choice D as the correct answer.
Exceptions to Atomic Radius (Transition Metals)
Exceptions to atomic radii occur when examining the transition metals of the periodic table.
Under the lanthanide contraction we have a decrease in the atomic radii as we transition from the 3d to 4d transition metals. Conversely, a transition from 4d to 5d transition metals sees the atomic radii held relatively constant.
Exceptions to Atomic Radius (Main Group Elements)
There are also a few exceptions to atomic radii when examining some of the main group elements.
When comparing oxygen to nitrogen, tellurium to antimony or polonium to bismuth we see an unexpected increase in atomic radii. This can be attributed to an increase in electron-electron repulsions, which causes a slight increase in the atomic radius.
The Other Periodic Trends
The number of electrons and size of ions and atoms have a big impact on their chemical properties. These chemical properties can be explained through other periodic trends such as ionic radius, electronegativity, ionization energy, effective nuclear charge and metallic character.