Problem 32 - A Complete Resource Book in Chemistry for JEE Main

Question

The atomic number of an element is 35 . What is the total number of electrons present in all the p-orbitals of the ground state atom of that element? (a) 17 (b) 11 (c) 23 (d) 6

Step-by-Step Solution

Answer

17 electrons are in the p-orbitals.

Step 1: Identify the Element

The atomic number of the element is given as 35. This corresponds to the element bromine (Br) on the periodic table. Bromine belongs to the group of halogens.

Step 2: Write the Electron Configuration

Bromine has an atomic number of 35, meaning it has 35 electrons. The full electron configuration is: \[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^{10} \, 4p^5 \].

Step 3: Identify p-Orbitals in the Configuration

Examine the electron configuration to identify all the electrons in p-orbitals. In bromine, the electrons in p-orbitals are: \(2p^6\), \(3p^6\), and \(4p^5\).

Step 4: Calculate Total Electrons in p-Orbitals

Add the electrons from each p-orbital: \[ 2p^6 + 3p^6 + 4p^5 = 6 + 6 + 5 = 17 \] electrons.

Key Concepts

Atomic Number

In chemistry, the atomic number is a fundamental property of an element. It indicates the number of protons found in the nucleus of an atom of that element. This number also equals the number of electrons in a neutral atom.
The atomic number not only defines the identity of an element but also its position on the periodic table. For example:

• Hydrogen has an atomic number of 1, which means it has 1 proton in its nucleus and 1 electron surrounding it when neutral.
• Carbon, with an atomic number of 6, has 6 protons and 6 electrons.

Knowing the atomic number is crucial for understanding the basic structure and behavior of an atom, including its chemical reactivity and electron configuration.

p-orbitals

Electrons occupy different energy levels around the nucleus of an atom, and these levels are divided into various sublevels, including s, p, d, and f. The p-orbitals, in particular, are a type of sublevel that can hold a maximum of 6 electrons across their three degenerate orbitals.
These p-orbitals are crucial because they start to appear in the second energy level (n=2) and contribute significantly to the valence shell in many atoms, influencing how elements interact with each other in chemical bonds.

• Each set of p-orbitals (like 2p, 3p) can accommodate 6 electrons.
• Electrons fill these orbitals according to specific rules, such as Hund's Rule, which states that electrons will occupy degenerate orbitals singly before pairing up.

Understanding p-orbitals helps in predicting the chemical bonding and behavior of elements in molecules.

Ground State

The ground state of an atom refers to the lowest energy configuration of its electrons. In this state, all electrons are arranged as close to the nucleus as possible without violating the quantum mechanical rules of electron occupancy, such as the Pauli exclusion principle and Hund's rule.
When chemists refer to an atom being in its ground state, they mean:

• All electrons are in the lowest available energy orbitals.
• The configuration minimizes the energy of the atom, making it more stable.
• Each orbital follows the order of filling dictated by increasing energy levels, typically represented by the Aufbau principle.

A good understanding of ground states is essential, as changes in electron configurations from this state can lead to chemical reactions and the absorption/emission of light.

Bromine

Bromine is a chemical element with the symbol Br and atomic number 35. It's part of the halogen group and is known for its characteristic reddish-brown liquid state at room temperature. Bromine's behavior and compounds are influenced by its electron configuration:

• With an atomic number of 35, bromine has 35 electrons in its neutral state.
• Its electron configuration ends in a partially filled 4p sublevel, which is typical for halogens wanting to gain an extra electron to achieve a more stable configuration similar to noble gases.

Bromine plays a significant role in chemistry and industry, used in a variety of applications, from flame retardants to water purification. Its placement in the periodic table and electron configuration help explain its reactivity and bonding characteristics.