Question

For transition elements, describe and explain the observed trends in atomic radius as you move: a. across a period in the periodic table b. down a column in the periodic table

Short answer

Across a period, atomic radius decreases due to increased nuclear charge pulling electrons closer. Down a column, atomic radius increases owing to additional electron shells being added.

Step-by-step solution

Understand the Atomic Radius Trend Across a Period

Atomic radius generally decreases as you move across a period in the periodic table. This is because with each additional element across a period, an electron is added to the same energy level while at the same time protons are being added to the nucleus. The increased number of protons results in a greater nuclear charge which draws the electrons closer, reducing the atomic radius.

Explain the Atomic Radius Trend Down a Column

As you move down a column in the periodic table, the atomic radius increases. When moving down a group, each element has an additional energy level compared to the one above it, making the atom larger. Therefore, despite the increasing positive charge in the nucleus, the effect of the additional electron shell is greater, hence the atomic radius increases.

Key concepts

Transition Elements

Transition elements, which are found in the middle of the periodic table, are unique in their atomic structure and behaviors. They constitute groups 3 through 12 and are characterized by the filling of the d-orbital. Understanding the atomic radius trends among these elements sheds light on more complex properties such as catalytic activity and color of compounds.

As we move across a period from left to right within the transition elements, electrons are being added to the same d-orbital, and the number of protons in the nucleus increases. This increase in nuclear charge effectively pulls electrons closer to the nucleus, often slightly decreasing the atomic radius despite the addition of electrons. This trend isn't as pronounced in transition elements as it is in main-group elements due to the shielding effect of the inner d-electrons diminishing the increase in effective nuclear charge. The co-called 'lanthanide contraction' is also a notable phenomenon associated with elements that follow the lanthanide series, often leading to smaller atomic sizes than expected.

Periodic Table

The periodic table is a systematic arrangement of the elements based on their atomic number, electron configuration, and recurring chemical properties. Elements are ordered from the lightest to heaviest, and are organized into periods (rows) and groups or families (columns).

Particularly, the periodic table allows us to predict the behavior of elements. As we discuss atomic radius trends across a period, the increasing nuclear charge effectively pulls on the electron cloud, reducing the atomic radius. On the other hand, as we move down a group, an additional electron shell is added, which normally results in an increased distance between the outermost electrons and the nucleus, thereby increasing the atomic radius. These trends are crucial for predicting how atoms will interact with one another to form chemical bonds.

Nuclear Charge

Nuclear charge is the total charge of the nucleus, which is due to the protons it contains, and is a key factor affecting the atomic radius of an element. Since protons are positively charged, the more protons there are, the higher the nuclear charge. As the atomic number—representing the number of protons in an atom—increases across a period, so does the nuclear charge.

However, the effect of increasing nuclear charge is moderated by the number of electron shells and the resulting electron-electron repulsions. This is where the concept of effective nuclear charge comes into play, as it's the net positive charge experienced by an electron in a multi-electron atom. Transition elements showcase a more subtle effect of nuclear charge on atomic radius compared to main-group elements, because of the additional electron-electron repulsion within the incomplete d-subshells.

Electron Shells

Electron shells, or energy levels, play a paramount role in dictating the size of an atom. These shells can be thought of as the 'orbit' in which electrons reside around the nucleus. The further out these shells are, the larger the atomic radius becomes. Each shell accommodates electrons in subshells and orbitals, with specific limits on how many electrons each can hold.

Moving down a group in the periodic table, each subsequent element has one more electron shell than the previous. This additional shell means electrons can be found further from the nucleus, thus increasing the atomic radius. The force of attraction that the nucleus exerts on these outer electrons weakens with increasing distance, which also contributes to the increase in atomic radius down a group. In transition elements, the additional electrons enter d-orbitals, which provide a shielding effect that somewhat offsets the increased nuclear charge.