Scientists organize elements into the periodic table, a chart that reveals recurring patterns in their properties. One such pattern is observed in atomic size, which generally decreases as one moves from left to right across a row, or period. This consistent trend in atomic dimensions provides insights into the forces that govern the structure of atoms.
What is Atomic Size?
Atomic size refers to the radius of an atom, representing the distance from the center of its nucleus to its outermost electron shell. This measurement is not a fixed, rigid boundary because the location of electrons around the nucleus is probabilistic, forming an electron cloud rather than a solid sphere. Therefore, atomic size is often considered an average distance or an effective radius. Atomic radii are typically measured in picometers (pm), a unit of length equal to one trillionth of a meter (10^-12 meters).
Factors Influencing Atomic Size
The size of an atom is determined by a balance of forces within its structure. Electrons occupy specific energy levels, or shells, around the nucleus; higher energy levels are generally further from the nucleus. More occupied electron shells mean the outermost electrons are situated at a greater distance, contributing to a larger atomic size.
The nuclear charge, which is the positive charge exerted by the protons in the nucleus, is another factor. This positive charge attracts the negatively charged electrons, pulling them closer to the nucleus. A greater number of protons in the nucleus results in a stronger attractive force on the electrons.
Inner-shell electrons also play a role through the shielding effect. These inner electrons reduce the effective positive pull of the nucleus on the outermost, or valence, electrons. They essentially “shield” the valence electrons from the full nuclear charge, lessening the attraction felt by the outer electrons and allowing them to move further away from the nucleus.
The Period Trend Explained
As one moves across a period from left to right in the periodic table, the atomic size consistently decreases. This trend occurs because electrons are added to the same outermost electron shell, meaning the outer electrons remain within the same principal energy level.
Simultaneously, with each successive element across a period, the number of protons in the nucleus increases by one. This increase in protons leads to a stronger positive nuclear charge.
Despite the addition of electrons to the same outer shell, the shielding effect from inner electrons remains relatively constant across a period. The inner core electron count does not change, and electrons in the same shell do not effectively shield each other from the increasing nuclear charge. The minimal increase in electron-electron repulsion within the same shell is outweighed by the growing nuclear attraction.
The dominant effect is the increasing nuclear charge pulling the electron cloud closer to the nucleus. With constant electron shells and a largely unchanged shielding effect, the stronger attraction from the nucleus causes the outermost electrons to be drawn inward. This stronger pull results in a reduction of the overall atomic radius, explaining why atomic size decreases across a period.