Question

Though all the elements in a given period (horizontal row) of the periodic table have their valence electrons in the same types of orbitals, the sizes of the atoms decrease from left to right within a period. Explain why.

Short answer

As we move from left to right within a period in the periodic table, the atomic number increases, leading to more protons in the nucleus and electrons in the atom. This stronger net nuclear attraction on the valence electrons pulls them closer to the nucleus, reducing the atomic size. Additionally, there is minimal electron shielding since valence electrons are in the same energy level. These factors contribute to the decreasing atomic size across a period.

Step-by-step solution

Understand the periodic table and periods

In the periodic table, elements are arranged according to their atomic numbers. A period is a horizontal row in the table, and it indicates the energy level of the outermost electrons or valence electrons. Elements within the same period share the same number of electron shells. As we go from left to right within a period, the atomic number increases, meaning that there are more protons and electrons in the atoms.

Examine the effect of atomic number on atomic size

As we move from left to right across a period, the atomic number increases, resulting in a higher number of protons in the nucleus and more electrons in the atom. The increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. Consequently, the increase in protons leads to a decrease in the atomic size within a period.

Consider the effect of electron shielding

Electron shielding refers to the ability of inner-shell electrons to reduce the effective nuclear charge experienced by the outer-shell electrons. In a period, the valence electrons are in the same energy level, and there is only minimal additional shielding from the inner electrons as we move from left to right. The increased nuclear charge outweighs the shielding effect, resulting in a net pull of the electrons closer to the nucleus and a smaller atomic size.

Combine these factors to explain the trend in atomic size within a period

In conclusion, as we move from left to right within a period, the increasing atomic number results in more protons in the nucleus and electrons in the atom. This leads to a stronger net nuclear attraction on the valence electrons, pulling them closer to the nucleus. Additionally, there is minimal electron shielding as the valence electrons are in the same energy level. Combining these factors explains why the atomic size decreases from left to right across a period in the periodic table.

Key concepts

Periodic Table Periods

The periodic table is a systematic arrangement of the elements structured in horizontal rows called periods. These periods are key to understanding the properties of elements, including their atomic size. As one progresses from left to right in a period, the elements have their valence electrons in the same principal energy level, or shell, but the number of protons in the nucleus increases. This increases the positive charge that influences the electrons. As the protons attract the electrons more strongly, the overall size of the atom tends to decrease.

Understanding the role of periods is crucial when studying variations in atomic size because it shows the layers of electron depth and how the outermost layer is affected by the increasing number of protons across a period.

Valence Electrons

Valence electrons are the electrons situated in the outermost shell of an atom and are vital in determining an atom's chemical properties. These electrons are responsible for the bonding with other atoms. In the context of atomic size, the quantity of valence electrons does not directly influence the size; rather it's their interaction with the nucleus that matters.

As we move across a period, the same number of electron shells are filled with valence electrons while the effective nuclear charge increases. Consequently, these valence electrons are drawn closer to the nucleus due to the increased attraction, which in turn reduces the atomic size, making valence electrons key to understanding the trend in atomic size across a period.

Effective Nuclear Charge

Effective nuclear charge (ENC) is the net positive charge experienced by an electron in an atom. This concept helps us comprehend why there is a decrease in atomic size across a period. While the actual number of protons (the actual nuclear charge) in the nucleus increases with each succeeding element in a period, the ENC also increases due to minimal additional electron shielding.

This means that even though there are more electrons, the inner-shell electrons do not efficiently shield the outermost electrons from the nucleus's pull. Therefore, the increased ENC draws the valence electrons closer, reducing the atom's radius. Hence, the ENC plays a pivotal role in the contraction of atomic size observed across a period on the periodic table.

Electron Shielding

Electron shielding is the phenomenon where inner-shell electrons act as a 'shield' for the outer electrons, reducing the full impact of the nuclear charge on these outer electrons. This effect can influence the size of an atom significantly.

During a traverse across a period, there is only a minor increase in shielding as the valence electrons are in the same energy level subject to the identical shielding effect by the same inner electrons. Because the inner electrons cannot effectively buffer the increasing positive charge due to more protons, the full brunt of the increased nuclear charge is felt by the valence electrons, causing the atomic size to decrease across a period. Hence, understanding electron shielding is essential in explaining why the increased nuclear charge leads to a smaller atomic size from left to right in the periodic table.