Atoms are the fundamental particles that form all matter. They can gain or lose electrons, becoming charged particles called ions. The periodic table organizes elements and reveals patterns in their properties. One such pattern is that both atomic and ionic sizes generally increase as you move down a vertical column, known as a group.
Defining Atomic and Ionic Radii
Atomic radius refers to the measurement indicating the size of a neutral atom. It is typically calculated as half the distance between the nuclei of two identical atoms when they are bonded together. This measurement effectively represents the atom’s overall size, from its central nucleus to its outermost electron boundary.
Ionic radius, in contrast, measures the size of an ion. An ion is an atom that has gained or lost one or more electrons, resulting in a net electrical charge. When an atom loses electrons, it forms a positively charged ion called a cation. Conversely, when an atom gains electrons, it forms a negatively charged ion known as an anion. Ionic radius is determined by measuring the distance from the nucleus to the outermost electron shell of these charged atoms.
Why Atomic Size Increases Down a Group
The increase in atomic size as you move down a group on the periodic table is directly related to the arrangement of electrons within an atom. Each element in a group, as you descend, adds a new main electron shell, or energy level, further away from the nucleus.
These additional electron shells mean that the outermost electrons are progressively located at greater distances from the positively charged nucleus. The inner electrons within these new shells act to “shield” or “screen” the outermost electrons from the full attractive pull of the nucleus. This phenomenon, known as electron shielding, occurs because the negative charge of the inner electrons repels the outer electrons, lessening the nuclear attraction they experience.
As more inner shells are added, the shielding effect becomes more pronounced, reducing the effective nuclear charge felt by the valence electrons. The effective nuclear charge is the net positive charge experienced by an electron, considering both the attraction from the nucleus and the repulsion from other electrons. While the actual number of protons in the nucleus increases down a group, the increased shielding from the growing number of inner electrons largely counteracts this stronger nuclear charge.
The combination of increased distance from the nucleus and greater electron shielding leads to a weaker attraction between the nucleus and the outermost electrons. This reduced pull allows the electron cloud to expand, resulting in a larger atomic size for elements lower down in a group.
Why Ionic Size Increases Down a Group
The principles explaining the increase in atomic size also apply to ionic size down a group. As you move down a group, elements form ions that possess more electron shells compared to ions of elements above them. This addition of electron shells contributes to a larger ionic radius.
For cations, which are formed when an atom loses valence electrons, the ionic radius is smaller than that of the neutral parent atom. However, the trend of increasing size down a group still holds for cations. For example, a sodium ion (Na+) is smaller than a potassium ion (K+), which is in turn smaller than a rubidium ion (Rb+), because each successive ion has an additional electron shell.
Anions are formed when an atom gains electrons. The trend of increasing ionic size down a group persists for anions. For instance, a fluoride ion (F-) is smaller than a chloride ion (Cl-), which is smaller than a bromide ion (Br-), due to the presence of more electron shells and increased shielding in the larger ions.
The consistent factor driving the increase in ionic radius down a group, for both cations and anions, is the addition of new electron shells. Each new shell places the outermost electrons further from the nucleus, and the increased number of inner electrons provides greater shielding from the nuclear charge. These effects ensure an ion’s size will be larger than the ion of the element directly above it in the same group.