Problem 73 - Chemistry The Molecular Science

Question

What is the electron configuration for (a) a bromine atom? (b) a bromide ion?

Step-by-Step Solution

Answer

(a) Br: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵ (b) Br⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶

Step 1: Understanding Electron Configuration

Electron configuration refers to the arrangement of electrons in an atom's energy levels or shells. It follows the order dictated by the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

Step 2: Write the Configuration for Bromine Atom

Bromine (Br) has an atomic number of 35. Electrons fill the orbitals in the order of increasing energy. The electron configuration for a bromine atom is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.

Step 3: Check the Shell Order

Ensure you follow the correct order of filling electron shells using the order based on increasing principal energy levels and sub-levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Bromine ends in 4p with five electrons (4p⁵).

Step 4: Write the Configuration for Bromide Ion

A bromide ion (Br⁻) has gained one additional electron compared to a neutral bromine atom resulting in a configuration similar to that of krypton: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶.

Step 5: Confirm the Stable Electron Configuration

The bromide ion has a full outer shell with 36 electrons, matching the noble gas configuration (krypton), showing stability in acquiring an additional electron.

Key Concepts

Bromine Atom

A bromine atom is an element on the periodic table with the chemical symbol Br and an atomic number of 35. In terms of atomic structure, it consists of 35 protons, 35 electrons, and a varying number of neutrons depending on its isotope. The electrons are arranged in orbitals according to specific rules, which determine how they fill different energy levels or shells.
The electron configuration of a bromine atom is determined by filling up electron orbitals in order, based on their energy levels. For bromine, the electron configuration is:

• 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.

This notation shows that bromine has two electrons in the first shell, eight in the second shell, eighteen in the third shell, and seven in the fourth shell. The last few electrons occupy the 4p orbital, which is characteristic of the 4p elements in the periodic table, demonstrating its position as a halogen in group 17 (VIIA). The electron arrangement highlights bromine's reactive nature, as it seeks to complete its outer shell.

Bromide Ion

In a bromide ion, the neutral bromine atom gains an electron to become negatively charged. This results in a chemical species represented as Br⁻. The extra electron is added to the outermost shell, completing the electron configuration.

• The electron configuration for a bromide ion is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶.

Compared to a neutral bromine atom, the bromide ion has one more electron, making it isoelectronic with krypton, a noble gas. This indicates a stable, full outer shell that characterizes ions formed by halogens. The process of gaining an electron to form a bromide ion leads to enhanced chemical stability due to the complete electron configuration resembling that of a noble gas. This makes bromide ions less reactive than their neutral atom counterparts, as they have achieved their most stable form.

Aufbau Principle

The Aufbau Principle is a fundamental concept in chemistry used to determine the electron configuration of atoms and ions. It is based on the idea that electrons fill atomic orbitals starting from the lowest energy level to the highest. This principle dictates the order of filling:

• 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.

According to the Aufbau Principle, each orbital can hold a maximum of two electrons, and electrons will fill these orbitals in a manner that minimizes the energy of the atom. For example, in the case of a bromine atom, the electrons are filled in the sequence to reach 4p⁵, while for a bromide ion, the configuration extends to 4p⁶, matching krypton's stable arrangement.
This principle is essential as it helps in understanding the arrangement of electrons in an atom, predicting chemical behavior, and explaining the formation of ions, like the bromide ion, where an extra electron is added to achieve noble gas configuration.