Specific patterns of certain elemental characteristics are present in the periodic table. The tendencies of these patterns increase or decrease as you move along the columns and rows of the table. These patterns are called periodic trends.
One of them is atomic size. The atomic size, or atomic radius, is the distance between the nucleus of an atom to the outermost electron orbital, where the valence electrons are.
Going Up and Down Groups (Columns)
When moving down a group, the atomic size increases. This is because as you go down the period table, new valence shells are added and thus, increasing the radius. Also, the new orbital is further away from the nucleus, meaning the attraction between the positively charged nucleus and the valence electrons decrease. In addition, the electron shells in between the valence shell and nucleus present electron shielding that also minimizes the attraction. As a result, the electrons are not held as tightly towards the nucleus.
Going Left and Right Across Periods (Rows)
When moving left to right across a period, the atomic size decreases. This is because the number of protons increase moving to the right of the row. As the number of protons increase, the nucleus of the atom becomes more positively charged. The increase in positive charge increases the attraction between the nucleus and the electrons of the atom. Thus, the electrons are held more tightly towards the nucleus, decreasing the radius. It is important to note that elements in the same period all have the same number of electrons shells, so electron shielding will not be a factor.
Summary:
Atomic size increases as you go down a column because of the addition of another electron shell and electron shielding.
Atomic size decreases as you go right across a row because of increased protons.
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This article was written for you by Samantha, one of the tutors with TestPrep Academy.
