Problem 61 - Chemistry and Chemical Reactivity

Question

(a) Explain why the sizes of atoms change when proceeding across a period of the periodic table. (b) Explain why the sizes of transition metal atoms change very little across a period.

Step-by-Step Solution

Answer

Atomic size decreases across a period due to increasing nuclear charge; transition metals show little size change because d electrons add inner subshell rather than outer shell electrons.

Step 1: Understanding Atomic Size Across a Period

The atomic size, or atomic radius, generally decreases across a period from left to right. This is because as you move across a period, the number of protons in the nucleus increases, which in turn increases the positive charge of the nucleus. This stronger positive charge pulls the electrons closer to the nucleus, reducing the size of the atom. Also, as electrons are added to the same principal energy level, they experience an increased nuclear charge without additional shielding effect, leading to a smaller atomic radius.

Step 2: Transition Metals and Minimal Size Change

Transition metals experience little change in atomic size across a period. In these elements, electrons are added to an inner d subshell rather than the outer shell. This configuration means that even though the nuclear charge is increasing, the electrons added do not significantly contribute to the shielding of the outermost electrons. Thus, the increase in nuclear charge slightly outweighs the little additional shielding provided by d electrons, resulting in only minimal change in atomic size across a period.

Key Concepts

Periodic Table

The periodic table is a systematic arrangement of elements based on their atomic number, electron configurations, and recurring chemical properties.
It is organized in rows called periods and columns known as groups. Across a period, the element's properties gradually change.

When you move from left to right across a period, elements gain protons, electrons, and generally become smaller in atomic size.

• This occurs because more protons increase the nuclear charge, which pulls electrons closer to the nucleus.
• Additionally, elements within the same period have electrons added to the same energy level, which provides consistency in how the atoms respond to the increased nuclear charge.

The periodic table helps predict the behavior of elements and understand trends like changes in atomic size.

Transition Metals

Transition metals are found in the d-block, situated in the middle of the periodic table.
They are known for their ability to form various oxidation states and to create complex ions.

One unique property of transition metals is their relatively stable atomic size across a period. This contrasts with the non-transition elements.

• In transition metals, the additional electrons enter an inner d subshell instead of the outermost shell, contributing to little change in atomic size.
• While their nuclear charge increases, it doesn't significantly affect their atomic size due to the nature of electron addition.

Understanding these metals is crucial for fields like chemistry and materials science due to their utility and unique characteristics.

Nuclear Charge

Nuclear charge refers to the total positive charge of the nucleus, primarily determined by the number of protons within it.
The greater the nuclear charge, the stronger the attractive force exerted on electrons.

As you progress across a period on the periodic table, the nuclear charge steadily increases.

• This stronger attraction causes shrinking of the atomic radius since electrons are pulled closer to the nucleus.
• Though both nuclear charge is increased and new electrons are added to equal energy levels, the effect on atomic size varies by element.

In transition metals, despite an increased nuclear charge, the addition of electrons to an inner d subshell means size remains almost constant.

Shielding Effect

The shielding effect in atoms occurs when inner electrons partially block the attraction between the nucleus and the outer electrons.
This effect is critical in explaining why nuclear charge doesn't always translate directly into size reduction.

In non-transition elements, shielding is minimal since added electrons do not increase inner electron layers.

• In transition metals, the situation is unique because electrons fill an inner d subshell, offering minimal additional shielding.
• Thus, outer electrons are not greatly shielded, and the nuclear charge's influence on atomic size remains significant but not as pronounced.

The balance of nuclear charge and shielding influences many atomic properties, impacting trends across the periodic table.