Question

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

Short answer

Using the periodic table and considering the atomic size trends, we can arrange the given sets of atoms in order from largest to smallest as follows: (a) Cs > K > Li; (b) Pb > Sn > Si; (c) N > O > F.

Step-by-step solution

(a) Arrange K, Li, Cs in Order of Size

All three elements, K (Potassium), Li (Lithium), and Cs (Cesium), belong to Group 1 (alkali metals) in the periodic table. As we move down the group, the atomic size increases. So, the order of these elements, from largest to smallest, is Cs > K > Li.

(b) Arrange Pb, Sn, Si in Order of Size

Pb (Lead), Sn (Tin), and Si (Silicon) belong to Group 14 in the periodic table. As we move down the group, the atomic size increases. Therefore, the order of these elements, from largest to smallest, is Pb > Sn > Si.

(c) Arrange F, O, N in Order of Size

F (Fluorine), O (Oxygen), and N (Nitrogen) are all elements in the same period, Period 2. As you move from left to right across a period, the atomic size decreases. Thus, the order of these elements, from largest to smallest, is N > O > F.

Key concepts

Atomic Size

Atomic size, often referred to as atomic radius, is the distance from the center of an atom’s nucleus to the outer edge of its electron cloud. Understanding atomic size gives insight into how atoms will interact with each other in chemical reactions.

Several factors influence atomic size:

• Nuclear Charge: The number of protons in an atom's nucleus affects its size. More protons increase the positive charge, pulling electrons closer which can decrease size.
• Electron Shielding: Inner electrons can shield outer electrons from the nucleus's pull, sometimes causing them to be held less tightly, increasing atomic size.

Atomic size plays a critical role in determining an element's properties and reactivity. By understanding the periodic trends associated with atomic size, we can make informed predictions about how atoms will behave in various chemical contexts.

Group Trends

In the periodic table, elements are grouped into columns known as groups or families. One of the key trends within a group is how atomic size changes as you move down the group.

Here is how atomic size behaves within groups:

• Atomic size increases moving down a group. This is because additional electron shells are added as you progress downwards, which increases the distance between the nucleus and the electron cloud.
• Each step down adds a new layer of electrons, enhancing the effect of electron shielding, thus eclipsing the increased nuclear charge.

For example, within the alkali metals group (Group 1), lithium is smaller than potassium, which in turn is smaller than cesium. This is consistently observed across other groups, indicating a clear and predictable pattern.

Recognizing group trends helps in predicting how an element might react, which is crucial for various chemical applications.

Period Trends

While navigating across a period in the periodic table, the atomic size trend behaves differently compared to within a group. Periodic trends provide valuable insights into how atomic properties evolve as one moves from left to right across a row.

Key aspects of period trends include:

• Atomic size decreases as you move from left to right across a period. This is due to the increase in nuclear charge (more protons) within the same electron shell, drawing electrons closer to the nucleus and reducing atomic radius.
• No additional electron shells are added, which keeps electron shielding constant while the nucleus's pull strengthens, resulting in smaller atoms.

For instance, in Period 2, nitrogen is larger than oxygen, which is larger than fluorine. This reflects the increasing nuclear pull shrinking atomic size across the period.

Understanding period trends is essential for foreseeing chemical bonding and interactions, aiding significantly in the study of chemistry.