Question

Assuming that the seventh period is 32 members long, what should be the atomic number of the noble gas following radon (Rn)? Of the alkali metal following francium (Fr)? What would you expect their approximate atomic masses to be?

Short answer

The atomic number of the noble gas following radon (Rn) should be 118, and the alkali metal following francium (Fr) should be 119. Their approximate atomic masses can be expected to be slightly more than that of Radon and Francium respectively, around or slightly above 223 u and 222 u.

Step-by-step solution

Identify the current Noble gas and Alkali Metal

Identify the position of Radon (Rn) and Francium (Fr). Radon, the Noble gas has an atomic number of 86, and Francium, the Alkali metal, begins the seventh period with an atomic number of 87.

Predicting the Next Noble Gas and Alkali Metal

Observe that noble gases appear at the end of each period and alkali metals appear at the beginning. Thus, the next noble gas after Radon should be the 32nd element of the 7th period, which is atomic number 118. The next alkali metal after Francium should be the first element of the 8th period, which is atomic number 119.

Estimating Atomic Masses

Knowing there is a direct relationship between the atomic number and atomic weight, we can expect the atomic weight to increase. However, accurate predictions are challenging due to factors like isotopic abundance and nuclear binding energy. Assuming a standard increase, the atomic weight of the next Noble gas may be slightly more than Radon's (222 u), and the alkali metal following Francium may be slightly more than Francium's (223 u).

Key concepts

Noble Gases

Noble gases are a unique group of elements found in Group 18 of the Periodic Table. These elements are known for their lack of reactivity due to their full valence electron shell. This complete outer shell makes them very stable:

• Helium (He) is the lightest noble gas with an atomic number of 2.
• Radon (Rn), the heaviest naturally occurring noble gas, has an atomic number of 86.

Noble gases are colorless, odorless, and tasteless. They are used in various applications, such as neon lights and in creating a non-reactive environment in welding. The next noble gas after radon is expected to have the atomic number 118, placing it at the end of the seventh period. With atomic numbers increasing as one moves from left to right across a period, noble gases always end each period.

Alkali Metals

Alkali metals belong to Group 1 of the Periodic Table and are known for being highly reactive. These elements have one valence electron that they can easily lose to form positive ions. This makes them very reactive, especially with water. Common characteristics of alkali metals include:

• A soft texture that often allows them to be cut with a knife.
• A low density, with many being less dense than water.

Francium (Fr) is the heaviest naturally occurring alkali metal with an atomic number of 87. Being at the start of the seventh period, the next alkali metal is expected to begin the eighth period with an atomic number of 119. Due to the pattern of reactivity and increasing atomic numbers, alkali metals frequently start new periods.

Atomic Number

The atomic number represents the number of protons found in the nucleus of an atom. It is a critical factor that determines the element's identity and its position on the Periodic Table. Here are some useful insights about atomic number:

• It is always a whole number.
• It increases by one as you move to the next element in the Periodic Table.

For noble gases and alkali metals, knowing the atomic number is crucial. For example, recognizing that radon has an atomic number of 86 helps to predict that the next noble gas will have an atomic number of 118. Similarly, since francium has an atomic number of 87, the next alkali metal will follow with an atomic number of 119.

Atomic Mass

Atomic mass is a measure of the mass of an atom, primarily accounting for the amount of protons and neutrons in its nucleus. It is usually expressed in atomic mass units (u). Key points about atomic mass include:

• It is not a whole number; it often includes decimal points reflecting isotopic masses and relative abundance.
• The atomic mass usually increases with the atomic number.

When predicting the atomic mass of the next noble gas after radon or the next alkali metal after francium, a slight increase is expected due to the addition of not just protons but also neutrons. For instance, as the atomic number increases, so does the general expectation for atomic mass, although exact values require experimental validation.