Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

Following statements regarding the periodic trends of chemical reactivity of the alkali metals and the halogens are given. Which one of these statements gives the correct picture? (a) the reactivity decreases in the alkali metals but increases in the halogens with increase in atomic number down the group (b) in both the alkali metals and the halogens the chemical reactivity decreases with increase in atomic number down the group (c) chemical reactivity increases with increase in atomic number down the group in both the alkali metals and halogens (d) in alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group

Step-by-Step Solution

Answer

Option (d) is correct: reactivity increases in alkali metals and decreases in halogens down the group.

Step 1: Understanding Alkali Metals Reactivity

Alkali metals, which include lithium, sodium, potassium, rubidium, cesium, and francium, are highly reactive elements found in group 1 of the periodic table. As you move down the group from lithium to francium, the atomic size increases, and the outermost electron is located further from the nucleus. This results in a weaker attraction between the nucleus and the outermost electron, making it easier to lose this electron. Hence, the reactivity of alkali metals increases as you move down the group.

Step 2: Understanding Halogens Reactivity

Halogens include fluorine, chlorine, bromine, iodine, and astatine, and are found in group 17 of the periodic table. These elements react by gaining one electron to achieve a stable electronic configuration. As you move down the group from fluorine to astatine, the atomic size increases, making it more difficult for these elements to attract extra electrons because the valence shell is farther from the nucleus and more shielded by inner electrons. Therefore, the reactivity of halogens decreases as you move down the group.

Step 3: Analyzing the Statements

Now that we understand the trends for alkali metals and halogens, let's evaluate the options: (a) states that reactivity decreases in alkali metals but increases in halogens down the group, which contradicts our findings about both groups. (b) states that reactivity decreases for both groups as atomic number increases, which is only true for halogens. (c) states that reactivity increases for both groups, which only applies to alkali metals. (d) states that reactivity increases in alkali metals and decreases in halogens as you move down the group, aligning with the trends we identified.

Key Concepts

Alkali Metals

Alkali metals are a fascinating group of elements found in Group 1 of the periodic table, which includes lithium, sodium, potassium, rubidium, cesium, and francium. These metals are known for their high reactivity, meaning they can easily participate in chemical reactions. This reactivity is largely due to the atomic structure of alkali metals.

Each alkali metal atom has a single electron in its outer shell, known as the valence electron. As you move down the group from lithium to francium, the atomic size increases. This means the outer electron is situated further from the nucleus. The increased distance and the added inner electron layers make the attraction between the nucleus and the valence electron weaker.

• The weaker attraction means it’s easier for these metals to lose their valence electron.
• This results in increased reactivity as you move down the group.
• Reactivity in these metals relates directly to their characteristic behaviors, such as a tendency to form +1 ions.

Therefore, the overall trend in the alkali metals is that reactivity increases as you go down the periodic table.

Halogens

Halogens are another group of elements located in Group 17 of the periodic table, encompassing fluorine, chlorine, bromine, iodine, and astatine. These nonmetals are highly reactive as well, but they react differently compared to alkali metals.

The reactivity of halogens is tied to their need to gain one electron to achieve a full outer electron shell, leading to a stable electronic configuration. As you descend the group from fluorine to astatine, several changes occur:

• The atomic size increases, causing the outer shell to be further from the nucleus.
• More electron shells are added, increasing the shielding effect and causing decreased attraction between the nucleus and the outer electrons.
• This makes it tougher for these elements to attract additional electrons, so their reactivity decreases.

Thus, for halogens, the general trend observed is a decrease in reactivity as you move down the group.

Reactivity Trends

Understanding reactivity trends in the periodic table is crucial for predicting chemical behavior. Reactivity varies between different groups due to differences in electron configurations and atomic structure.

For alkali metals in Group 1, reactivity trends are characterized by an increase as you move down the group. This is due to the ease with which these metals lose their single outermost electron.

• The further the electron is from the nucleus, the less it is held tightly, raising the metal's reactivity.
• This trend makes francium one of the most reactive alkali metals.

On the other hand, halogens in Group 17 exhibit a different trend, where reactivity decreases as you go down the group.

• The increasing atomic size and shielding effect make it harder for these elements to capture additional electrons as you move from fluorine to astatine.
• Fluorine is known as the most reactive halogen due to its small size and strong nuclear charge.

Thus, recognizing these trends is vital for understanding chemical reactions and predictability in various chemical processes.

Periodic Table Groups

The periodic table of elements is a tabular demonstration of chemical elements arranged by atomic number, electron configuration, and recurring chemical properties. The key to understanding chemical behavior lies in the structure of the table, which organizes elements into rows called periods and columns called groups.

Groups are vertical columns in the periodic table and are extremely important for recognizing patterns of reactivity:

• Each group contains elements with similar properties and valency due to their similar electron configurations.
• For example, all elements in Group 1 (alkali metals) have a single electron in their outermost shell.
• Likewise, Group 17 contains the halogens, known for needing only one additional electron to possess a full outer shell.

This systematic arrangement allows scientists and students alike to predict properties of elements and understand their reactivity trends effectively, making it a cornerstone of chemistry.