The atom does not have a definite boundary. In order to determine the exact radius, r, of an atom, it is necessary to think about this in terms of the chances of finding an electron at a distance r from the nucleus.
Take a look at the image to the right, the bluish-grey colour gradient represents the electron cloud that surrounds the atom. The nucleus resides at the center of the atom. The colour gradation represents the probabilities or chances of finding the electron around the nucleus. The darker the electron cloud, the higher the probability of locating electrons in that volume of space.
As one moves outward from the nucleus, the distance, r, increases. The chances of locating electrons at a greater distance from the nucleus will become less. Therefore, the atom size is based on the average distance between the outer electrons and the nucleus.
The arrows in the image show the direction of INCREASE in atomic size.
- Within a group of elements, the size of the atoms tends to increase from top to bottom. As you move down the group, the number of electrons for each element increases, and new higher energy levels are occupied by these electrons. Higher energy levels are farther away from the nucleus.
- Within a period of elements, from left to right, the size of atoms tends to decrease. As you move from left to right across a period, the number of electrons for each an element increases, but all the electrons are occupying the same energy level. However, the number of proton of each element within the period increases. The increased in positive nuclear charge enables the nucleus to draw the electron cloud closer to itself, and hence, decreasing the atomic size.
BCIT courses: CHEM 0011