Question

Using only the periodic table, arrange each set of atoms in order from largest to smallest: (a) K, Li, Cs; (b) Pb, Sn, Si; (c) F, O, N.

Short answer

(a) Cs > K > Li; (b) Pb > Sn > Si; (c) N > O > F

Step-by-step solution

(a) Arrange K, Li, Cs in order from largest to smallest

To arrange K, Li, and Cs in order from largest to smallest, we will look at their positions in the periodic table. We can find K (Potassium) in Group 1 and Period 4, Li (Lithium) in Group 1 and Period 2, and Cs (Cesium) in Group 1 and Period 6. Since all three elements are in the same group, we can compare their sizes based on their periods. The atomic radii increase as we move down a group, so Cs is the largest, followed by K, and finally Li. Thus, the order is Cs > K > Li.

(b) Arrange Pb, Sn, Si in order from largest to smallest

To arrange Pb, Sn, and Si in order from largest to smallest, we will look at their positions in the periodic table. We can find Pb (Lead) in Group 14 and Period 6, Sn (Tin) in Group 14 and Period 5, and Si (Silicon) in Group 14 and Period 3. Since all three elements are in the same group, we can compare their sizes based on their periods, similar to part (a). The atomic radii increase as we move down a group, so Pb is the largest, followed by Sn, and finally Si. Thus, the order is Pb > Sn > Si.

(c) Arrange F, O, N in order from largest to smallest

To arrange F, O, and N in order from largest to smallest, we will look at their positions in the periodic table. We can find F (Fluorine) in Group 17 and Period 2, O (Oxygen) in Group 16 and Period 2, and N (Nitrogen) in Group 15 and Period 2. Since all three elements are in the same period, we can compare their sizes based on their groups. The atomic radii decrease as we go across a period from left to right, so N is the largest, followed by O, and finally F. Thus, the order is N > O > F.

Key concepts

Atomic Size Trends

Understanding the atomic size trends in the periodic table is fundamental for comparing the sizes of different atoms. Atomic size, often measured as the atomic radius, typically varies in two main directions in the periodic table: across periods (from left to right) and down groups (from top to bottom).

As you move from left to right across a period, the atomic size decreases. This happens because additional protons in the nucleus increase the nuclear charge, pulling the electron cloud closer and leading to a smaller radius for each subsequent element. However, going down a group in the periodic table, the atomic size increases. Here, additional electron shells are added, which increases the distance between the nucleus and the outermost electrons, resulting in larger atomic radii.

This general trend helps us to make predictions about the relative sizes of atoms based on their position in the periodic table, which is particularly useful when elements are within the same group or period.

Group and Period Properties

The properties of elements within groups and periods of the periodic table are not random; they change in a predictable way. Groups (the vertical columns) contain elements with the same number of electrons in their outermost shell, which gives them similar chemical properties. However, as one moves down a group, the number of electron shells increases, thus incrementally increasing the atomic radius despite the similar electron configuration.

In periods (the horizontal rows), the situation is quite different. As we move across a period from left to right, we add one proton to the nucleus and one electron to the outer shell with each subsequent element. The increasing nuclear charge outweighs the addition of electrons, causing the electrons to be drawn in closer to the nucleus and leading to a decrease in atomic size. This regular variation in atomic and ionic size across periods and down groups is a reflection of the underlying electronic structure of the atoms.

Atomic Radius Comparison

Comparing atomic radii is crucial when predicting how atoms will interact and bond with each other. The periodic table serves as a map for such comparisons, with a clear set of rules on how atomic size varies among elements. By understanding the trends of atomic size, it becomes easier to arrange elements by their relative sizes.

For instance, within a group such as the alkali metals (Group 1), the atomic radius increases as you go from Lithium (Li) at the top to Cesium (Cs) at the bottom, due to the additional electron shells that are present in the heavier alkali metals. Conversely, within a period, the atomic radius decreases from left to right, such as within Period 2 where Nitrogen (N) is larger than Oxygen (O), which in turn is larger than Fluorine (F).

Thus, comparing atomic radii is straightforward when the elements are in the same group or period. This allows for the arrangement of elements in ascending or descending order of size, as shown in the exercise. By considering group and period trends concurrently, such comparisons can often be made even without explicit numerical values for the atomic radii, illustrating the predictive power of periodic trends.