Question

Little is known about the properties of astatine, At, because of its rarity and high radioactivity. Nevertheless, it is possible for us to make many predictions about its properties. (a) Do you expect the element to be a gas, liquid, or solid at room temperature? Explain. (b) Would you expect At to be a metal, nonmetal, or metalloid? Explain. (c) What is the chemical formula of the compound it forms with $\mathrm{Na}$ ?

Short answer

Astatine (At) is predicted to be a solid at room temperature due to the trend in states of matter among halogens. It is a nonmetal based on its position in the periodic table, particularly within Group 17. When astatine reacts with sodium (Na), it forms the compound NaAt.

Step-by-step solution

(Step 1: Identify the position of astatine in the periodic table)

First, we need to find the position of the element astatine (At) in the periodic table. Astatine has the atomic number 85 and is placed in Group 17, Period 6. It belongs to the halogen family along with elements like fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

(Step 2: Predict the state of astatine at room temperature)

Since astatine belongs to the halogen family, we can analyze the trend of states of matter among halogens at room temperature. Fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature. As we go down Group 17, the elements tend to have higher molecular weight and have increased strength of Van der Waals forces, resulting in a transition from gas to liquid to solid. Being in Period 6, below iodine, it is reasonable to predict that astatine would be a solid at room temperature.

(Step 3: Predict if astatine is a metal, nonmetal, or metalloid)

Elements in Group 17 are generally nonmetals, with a few of them exhibiting metalloid properties in certain conditions. Based on its position in the periodic table, we can conclude that astatine is a nonmetal.

(Step 4: Determine the chemical formula of the compound formed with sodium)

Astatine, as a halogen, has 7 valence electrons and tends to gain one electron to achieve the stable electron configuration of the nearest noble gas. Sodium, which is an alkali metal, has one valence electron and usually loses it to form a stable ion. When sodium (Na) reacts with astatine (At), sodium will lose an electron to form ${\mathrm{Na}}^{+}$, and astatine will gain that electron to form ${\mathrm{At}}^{-}$. The resulting compound will be formed by the electrostatic attraction between the positive sodium ion and the negative astatine ion. The chemical formula of this compound will be NaAt.

Key concepts

Periodic Table

The periodic table is a marvel of scientific organization. It categorizes all known elements based on their atomic number, which is the number of protons in the nucleus, and arranges them into groups and periods. Elements that share similar chemical properties are grouped together in columns called families or groups. For example, the halogen family – which includes astatine – is in Group 17.

The periodic table not only organizes elements but also helps us make predictions about an element's properties. The position of an element within the table can inform us whether it's a metal, nonmetal, or metalloid, as well as hint at its possible states of matter at room temperature. Understanding how to read the periodic table is crucial for chemistry students, as it is the starting point for any analysis of an element's characteristics.

States of Matter

Matter around us exists in three primary states: solid, liquid, and gas. Each state has its unique characteristics. Solids have a fixed shape and volume, liquids have a fixed volume but take the shape of their container, and gases fill the entire space available to them.

The state that a particular element or compound exists in at room temperature can largely be predicted based on its position in the periodic table. For instance, halogens exhibit a trend as we move down the group: starting from gas at the top with fluorine and chlorine, transforming to liquid in the case of bromine, and finally to solid as seen with iodine and possibly astatine. This trend correlates with the increasing molecular weight and the stronger intermolecular forces that arise as a result.

Halogen Family

Halogens are a series of nonmetal elements from Group 17 of the periodic table, including fluorine, chlorine, bromine, iodine, and astatine. These elements are known for being highly reactive, particularly because they have seven valence electrons, just one short of a stable octet.

This high reactivity makes halogens excellent candidates for forming compounds, particularly with metals that readily lose an electron. Halogens typically gain that electron when reacting, which results in the formation of negative ions known as halides. Astatine, being a member of this family, shares these general properties and is expected to form similar types of compounds.

Chemical Formula

Chemical formulas are a way to represent the composition of a compound, showing the types and numbers of atoms involved. For instance, water's chemical formula is H₂O, which means it consists of two hydrogen atoms and one oxygen atom. The chemical formula provides a substantial amount of information about the substance, such as its molecular structure and the ratio of elements within the compound.

In the case of astatine reacting with sodium, the resulting compound is represented by the formula NaAt. This succinct formula indicates that one sodium atom (Na) combines with one astatine atom (At) to form the compound, sodium astatide. Understanding the rules for writing chemical formulas is essential when predicting the products of chemical reactions.