Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

The atomic numbers of elements \(\mathrm{X}, \mathrm{Y}, \mathrm{Z}\) are \(19.21\) and 25 respectively. The number of electrons present in the 'M' shells of these elements follow the order (a) \(Z>Y>X\) (b) \(\mathrm{X}>\mathrm{Y}>\mathrm{Z}\) (c) \(\mathrm{Z}>\mathrm{X}>\mathrm{Y}\) (d) \(\mathrm{Y}>\mathrm{Z}>\mathrm{X}\)

Step-by-Step Solution

Answer

(a) \( Z > Y > X \) is the correct order.

Step 1: Understand the Shell Arrangement Rule

Electrons in an atom fill up energy levels or shells in the order K, L, M, etc. Each shell can hold a maximum number of electrons: K can hold 2, L can hold 8, and M can hold 18. Electrons fill the lower energy shells first before filling higher ones.

Step 2: Identify Electron Configuration

For element X with atomic number 19, its electron configuration is arranged as \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 \] This means the M shell contains 8 electrons.For element Y with atomic number 21, the electron configuration \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^1 \] indicates the M shell contains 9 electrons.For element Z with atomic number 25, the configuration \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^5 \]demonstrates there are 13 electrons in the M shell.

Step 3: Compare the Number of Electrons in 'M' Shell

Compare the number of electrons in the 'M' shell: - Element X: 8 electrons - Element Y: 9 electrons - Element Z: 13 electrons The order is Z > Y > X.

Key Concepts

Electron Configuration

Every atom has a specific way in which its electrons are arranged around the nucleus, known as the electron configuration. This arrangement helps determine how an atom will react chemically with others. Atoms fill their electrons into different energy levels or shells: these are identified as K, L, M, and beyond. The filling order is influenced by the principle of minimizing the atom's energy.

For instance, in the electron configuration of element X with atomic number 19, which is potassium, electrons accumulate in shells in the sequence 1s, 2s, 2p, 3s, 3p, and then 4s. The configuration can be detailed as:

• \(1s^2\)
• \(2s^2\)
• \(2p^6\)
• \(3s^2\)
• \(3p^6\)
• \(4s^1\)

This means the M shell, which can hold a maximum of 18 electrons, contains 8 electrons in this instance. Understanding these configurations helps predict crucial properties of elements, such as their valency and conductivity.

Atomic Number

The atomic number is fundamental to identifying an element in the periodic table. It equals the number of protons found in the nucleus of an atom and, importantly, also tells us the number of electrons in a neutral atom.

In our context, element X has an atomic number of 19, meaning it has 19 electrons. Element Y comes with an atomic number of 21, indicating 21 electrons, while element Z, with an atomic number of 25, has 25 electrons.

• The atomic number determines the chemical behavior of an element.
• It's crucial in defining each element's identity and position in the periodic table.
• Atomic number also influences how atoms bond with others, as electron interactions govern chemical bonds.

So, observing the atomic number gives insight into elemental properties and how they relate to others.

Electron Shell

Electron shells are the designated layers or orbits that electrons occupy around the nucleus of an atom. These shells are labeled as K, L, M, N, etc., and each shell can accommodate a specific maximum number of electrons based on its order.

For instance, the M shell, being the third shell, can hold up to 18 electrons in total. This shell is significant when considering the electron configuration of heavier elements. The filling order of these shells is according to increasing energy levels, which means outer shells are filled only after inner shells are complete:

• K shell: 2 electrons maximum
• L shell: 8 electrons maximum
• M shell: 18 electrons maximum

This hierarchical system explains how in our example, element Z utilizes its M shell to hold 13 electrons as it has 25 electrons in total. Such knowledge is essential for comprehending atomic interactions and chemical bonding as it hinges on valence electrons, which reside in the atom's outermost shell.