Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The element with \(\mathrm{Z}=120\) (not yet discovered) will be an/a:(a) Inner- transition metal (b) Alkaline earth metal (c) Alkali metal (d) Transition metal

Step-by-Step Solution

Answer

(b) Alkaline earth metal

Step 1: Understanding Atomic Number

The element with atomic number \( Z = 120 \) is not yet discovered. We need to predict its properties based on its atomic number. Atomic number 120 places the element in the 8th period of the periodic table.

Step 2: Determine the Group

Since each period in the table starts with alkali metals and progresses to noble gases, atomic number 120 implies the element belongs to Group 2 (IIA) because 118 is the last noble gas, and 119 would begin the next alkali metal, making 120 an alkaline earth metal.

Step 3: Verify the Grouping Rules

Group 2 elements are known as alkaline earth metals, which have two electrons in their outer shell. This matches the periodic trend and rules, giving the element properties similar to known alkaline earth metals like Beryllium and Magnesium.

Key Concepts

Alkaline Earth Metals

Alkaline earth metals are a group of elements in Group 2 of the periodic table. This group includes beryllium, magnesium, calcium, strontium, barium, and radium. These elements are characterized by their two valence electrons in the outermost energy level. This electron configuration makes them highly reactive, but less so than the alkali metals in Group 1.
Here are some important properties of alkaline earth metals:

• They tend to form +2 cations because they lose two electrons easily during chemical reactions.
• They are shiny and have a silvery-white appearance.
• They have higher melting points compared to alkali metals.

Alkaline earth metals react with water to form alkaline hydroxides and release hydrogen gas. However, only magnesium and calcium react this way at room temperature, while others require heating. Being divalent, they are less reactive than the alkali metals but still sufficiently reactive for most applications in industry and chemistry.

Atomic Number

The atomic number of an element is a fundamental property that signifies the number of protons in the nucleus of an atom. This number not only determines the element's identity but also its position on the periodic table.
Let’s understand how it works:

• Identity of Elements: Each element has a unique atomic number, which means no two elements have the same number of protons. For example, hydrogen, with an atomic number of 1, always has one proton.
• Predicting Element Properties: By knowing an element's atomic number, we can predict its basic properties, such as metallic or non-metallic nature, reactivity, and more.

As explained in the solution, the atomic number 120 predicts the element will be in the eighth period of the periodic table, marking it as an alkaline earth metal by position. The periodic table organizes elements based on increasing atomic number, leading to a systematic repetition of chemical properties in columns and groups.

Periodic Trends

Periodic trends refer to the patterns observed in the properties of elements as you move across periods and down groups in the periodic table. These trends arise due to the regular and predictable changes in atomic structure.
Here are three significant trends:

• Atomic Radius: Generally increases as you move down a group and decreases across a period from left to right. This is because each period adds a new electron shell, enlarging the atomic radius, while increasing nuclear charge across a period pulls electrons closer.
• Electronegativity: Tends to decrease down a group and increase across a period. The more an atom attracts electrons in chemical bonds, the higher its electronegativity.
• Ionization Energy: Typically decreases down a group and increases across a period. This energy is required to remove an electron from an atom, and is higher for atoms holding their electrons tightly, as seen across periods.

Understanding these trends helps chemists predict how new elements like element 120 might behave chemically. The new element's position among alkaline earth metals suggests it will follow similar trends, such as a relatively large atomic radius and low electronegativity, potentially reacting readily by losing two electrons.