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) \(\mathrm{ll}\) (c) 23 (d) 6

Step-by-Step Solution

Answer

Bromine has a total of 17 electrons in its p-orbitals in its ground state.

Step 1: Identify the Element

The atomic number indicates the number of protons in an element and for a neutral atom, it also equals the number of electrons. The element with atomic number 35 is bromine (Br).

Step 2: Know the Electron Configuration Basis

Electron configuration describes the distribution of electrons among the orbitals. The filled p-orbitals can be represented as: 2p^6, 3p^6, and 4p (for partially filled).

Step 3: Distribute the Electrons

Bromine has 35 electrons. Distribute electrons in orbitals using the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and 4p. Remember the s subshells hold 2 electrons, the p subshells hold a maximum of 6, and the d subshells hold 10.

Step 4: Assign Electrons According to Energy Level

Start filling in electrons: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^10, 4p^5. Note that p-orbitals are present in the 2nd, 3rd, and 4th shells.

Step 5: Calculate Total Electrons in p-orbitals

Sum the electrons in the p-orbitals: 2p has 6 electrons, 3p has 6 electrons, and 4p has 5 electrons. Thus, the total number of electrons in p-orbitals is 6 + 6 + 5 = 17.

Key Concepts

Atomic Number

When we talk about an element's atomic number, we are referring to the number of protons in the nucleus of an atom. The atomic number is fundamental because it not only defines the element but also determines its position in the periodic table. For neutral atoms, the atomic number is equivalent to the number of electrons orbiting the nucleus.

Let's explore **why this is important**:

• The atomic number helps identify the element. For example, an atomic number of 35 corresponds to the element bromine (Br).
• It allows us to understand the type and number of particles present in the atom, which is crucial for deducing electron configurations.
• Knowing the atomic number can help predict the chemical behavior of an element, as elements with similar atomic numbers often have similar properties.

P-orbitals

In atomic theory, **orbitals** are regions around an atom's nucleus where electrons are likely to be found. The **p-orbitals** are one such set of orbitals and are shaped like dumbbells. These orbitals are important because they determine the chemical bonding and properties of an element.

Here are some key points about p-orbitals:

• **P-orbitals start appearing in the second energy level (n=2).** This means they are present from the second shell onwards.
• Each p-orbital can hold a maximum of 6 electrons (2 per orbital), and there are 3 p-orbitals in each energy level: designated as px, py, and pz.
• The electron configuration of elements often ends at the p-orbitals because they have higher energy levels compared to s-orbitals.
• Understanding the number of electrons in p-orbitals is significant for determining the electron configuration and reactivity of elements such as bromine.

Bromine

Bromine is a fascinating element found on the periodic table with the symbol "Br" and atomic number 35. It's located in group 17, known as the halogens.

Here's why bromine is interesting and important:

• **Bromine is a halogen.** This means it is highly reactive, especially with alkali metals and alkaline earth metals, forming salts.
• In its natural state, bromine is a red-brown liquid at room temperature, one of only two elements that are liquid in their elemental form (mercury being the other).
• Its electron configuration is essential in chemistry, as it tells us that bromine has a complex arrangement of electrons in various orbitals, resulting in its reactivity and bonding characteristics.
• The electron configuration of bromine is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵, which was crucial in calculating that bromine has a total of 17 electrons in its p-orbitals.