Question

One bit of evidence that the quantum mechanical model is "correct" lies in the magnetic properties of matter. Atoms with unpaired electrons are attracted by magnetic fields and thus are said to exhibit paramagnetism. The degree to which this effect is observed is directly related to the number of unpaired electrons present in the atom. Consider the ground-state electron configurations for Li, N, Ni, Te, Ba, and Hg. Which of these atoms would be expected to be paramagnetic, and how many unpaired electrons are present in each paramagnetic atom?

Short answer

The paramagnetic atoms among the given elements are Li, N, and Ni. Li has one unpaired electron (1s² 2s¹), N has three unpaired electrons (1s² 2s² 2p³), and Ni has two unpaired electrons (1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸). The other elements (Te, Ba, and Hg) have all their electrons paired.

Step-by-step solution

Determine electron configurations of given elements.

To do this, use the periodic table and the Aufbau principle. The order in which electron subshells are filled is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, and so on. Consider the electron configurations of the given elements: Li (Atomic number: 3): 1s^2 2s^1 N (Atomic number: 7): 1s^2 2s^2 2p^3 Ni (Atomic number: 28): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^8 Te (Atomic number: 52): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^4 Ba (Atomic number: 56): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 Hg (Atomic number: 80): 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 4f^14 5d^10

Identify paramagnetic atoms and count unpaired electrons.

An atom is paramagnetic if it has unpaired electrons in its electron configuration. For Li: 1s^2 2s^1 -> One unpaired electron For N: 1s^2 2s^2 2p^3 -> Three unpaired electrons For Ni: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^8 -> Two unpaired electrons For Te: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^4 -> All electrons are paired. For Ba: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 -> All electrons are paired. For Hg: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^6 6s^2 4f^14 5d^10 -> All electrons are paired. From the above analysis, Li, N, and Ni are paramagnetic, with 1, 3, and 2 unpaired electrons, respectively.

Key concepts

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom's orbitals. These orbitals belong to various energy levels and subshells, designated by the principal quantum number, designated as \(n\), and the subshells \(s, p, d,\) and \(f\). The configuration follows a specific order, filling from lower to higher energy levels. This is depicted with notations like \(1s^2 2s^2 2p^6\), where the superscript indicates the number of electrons in that subshell.
Understanding electron configurations helps in predicting an atom's chemical behavior. It explains why elements exhibit specific bonding patterns, and their reactivity among other properties. For instance, nitrogen (N) with the configuration \(1s^2 2s^2 2p^3\) has three electrons in its 2p subshell, making it likely to form bonds with three other atoms to complete its octet. In contrast, mercury (Hg) has all of its subshells filled, making it less reactive.

Aufbau Principle

The Aufbau principle is crucial in determining electron configurations. The German word "Aufbau" means "building up." It suggests that electrons fill atomic orbitals in order from the lowest energy level to higher ones. This order is visually represented by the periodic table and depicted by the sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, etc.

The principle relies on the energy hierarchy, where electrons seek the least energy configuration possible. For example, lithium (Li), requiring only three electrons, follows the configuration \(1s^2 2s^1\). In this case, the first electron fills the 1s orbital, while the remaining electron occupies the 2s orbital, aligning with this principle.

• The sequence is sometimes adapted due to exceptions involving d and f orbitals. But, for most elements, the Aufbau principle remains applicable.

Unpaired Electrons

Unpaired electrons are those valence electrons occupying an atomic orbital alone. Unlike paired electrons that fill an orbital with their opposite spin counterparts, unpaired electrons contribute to magnetism and reactivity.
In atoms like nitrogen (N), with the configuration \(1s^2 2s^2 2p^3\), the three electrons in the p-shell remain unpaired. These unpaired electrons cause a paramagnetic behavior, meaning the atom is attracted to magnetic fields.

Here's why unpaired electrons matter:

• They determine whether an atom is paramagnetic or diamagnetic. Paramagnetic atoms have at least one unpaired electron while diamagnetic atoms have all paired up.
• They play a vital role in chemical bonding. Unpaired electrons often participate in covalent bonding, as they seek stability by pairing up with electrons from other atoms.

Understanding unpaired electrons is essential in chemistry, as they provide insights into the magnetic and reactive properties of an element.