Problem 78 - Chemistry Central Science

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) What is the chemical formula of the compound it forms with Na?

Step-by-Step Solution

Answer

Astatine is expected to be a solid at room temperature since it belongs to the halogen group and exhibits a trend of increasing boiling and melting points down the group. The chemical formula of the compound formed between astatine and sodium is NaAt.

Step 1: (a) Predicting the state of Astatine at room temperature

Astatine (At) can be found in Group 17 (VIIA) of the periodic table, which is the group of halogens. Other elements in this group are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Based on their position in the periodic table, these elements show a gradual change in physical state: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature. Since astatine is positioned below iodine in the periodic table, and the elements exhibit a trend of increasing boiling and melting points down the group, we can predict that astatine will most likely be a solid at room temperature. Answer: Astatine is expected to be a solid at room temperature.

Step 2: (b) Determining the chemical formula of the compound formed with Sodium

In order to find the chemical formula of the compound formed when astatine reacts with sodium (Na), we first need to determine the valency or charge of each of the ions involved in the reaction. Sodium (Na) is an alkali metal and is in Group 1 (IA) of the periodic table. Elements in this group form +1 cations (positively charged ions) when they react. Therefore, sodium will have a charge of +1 as Na^+. Astatine (At) belongs to the group of halogens, which commonly form -1 anions (negatively charged ions) when they react. Therefore, astatine will have a charge of -1 as At^−. Now, to find the chemical formula of the compound formed, we combine the positive and negative ions in such a way that their charges are neutralized: Na^+ + At^- → NaAt Answer: The chemical formula of the compound formed between astatine and sodium is NaAt.

Key Concepts

Understanding Periodic Table Trends

When exploring elements like astatine on the periodic table, recognizing the trends across rows (periods) and down columns (groups) is crucial.

One key trend is the change in physical states from gases to liquids to solids as we move down a group. This occurs due to increases in atomic size, molecular weight, and intermolecular forces among atoms. For the halogens in Group 17, this means starting with gaseous fluorine and chlorine, moving to liquid bromine, and arriving at solid iodine, and by extension, solid astatine, at room temperature.

Electronegativity, another trend, decreases down a group. Halogens are highly electronegative, with this trait diminishing from fluorine to astatine. Ionic and atomic radii generally increase down a group, influencing the nature of chemical bonds formed.

Understanding these trends is fundamental to predicting an element's properties, thus fostering a deeper grasp of seemingly elusive elements like astatine within the broader landscape of chemistry.

Investigating the Halogen Group

The halogen group, consisting of fluorine, chlorine, bromine, iodine, and astatine, exhibits fascinating chemical characteristics due to its position in Group 17 of the periodic table.

Halogens are non-metals, renowned for their high reactivity, especially toward alkali and alkaline earth metals. This property is a result of their quest to achieve a noble gas electron configuration by gaining one additional electron, completing their outer shell.

Halogens form diatomic molecules (e.g., F₂, Cl₂), and as oxidizing agents, their strength diminishes down the group—fluorine is the strongest, whereas astatine is the weakest. Students may also be interested to know that the higher atomic weight of halogens going down the group reflects in their state of matter at room temperature: from gaseous state in the lighter halogens to solid in the heavy astatine.

Deciphering Chemical Formulas

Chemical formulas represent the types and numbers of atoms in molecules. In the case of NaAt, the formula reflects a binary compound composed of sodium (Na) and astatine (At).

To write the correct chemical formula, we balance the charges of the reacting ions. Sodium, from Group 1 of the periodic table, forms ions with a +1 charge, while halogens typically form -1 ions. A 1:1 ratio of Na⁺ to At⁻ ions results in a neutral compound, hence the formula NaAt.

Through chemical formulas, we visualize the composition and ratios of elements in a compound. Therefore, NaAt tells us that each sodium ion pairs with an astatine ion, embodying the practice of bonding that balances positive and negative charges to yield stable compounds.