Problem 115 - Chemistry Matter and Change

Question

What element has the electron configuration \([\) Ar \(] 4 s^{1} 3 \mathrm{d}^{5} ?(\)Chapter 5\()\)

Step-by-Step Solution

Answer

Chromium (Cr) with atomic number 24.

Step 1: Understand Electron Configuration Notation

The given electron configuration is \([\text{Ar}] 4s^{1} 3d^{5}\). This notation indicates the sequence and arrangement of electrons in the orbitals around the nucleus of an atom. \([\text{Ar}]\) denotes the noble gas core configuration of Argon, which has 18 electrons.

Step 2: Calculate the Number of Electrons

To determine the number of electrons, start with those in Argon: 18 electrons. Add the electrons from the remaining configuration \(4s^1 3d^5\): 1 electron from 4s and 5 electrons from 3d. The total is \(18 + 1 + 5 = 24\) electrons.

Step 3: Identify the Element by Atomic Number

Elements are identified by their atomic number, which corresponds to the number of protons (and electrons in a neutral atom). With 24 electrons, the atomic number is 24. Refer to the periodic table to find the element with this atomic number, which is Chromium (Cr).

Key Concepts

Noble Gas Configuration

In chemistry, electron configurations play a vital role in understanding an element's electron arrangement around its nucleus. A "noble gas configuration" provides a streamlined way to write this configuration by referring to the previous noble gas. For example, the noble gas configuration for chromium utilizes argon, denoted as \( [\text{Ar}] \), because argon is the nearest noble gas with 18 electrons. Instead of writing out all 18 electrons, we use the noble gas as a shorthand.
This simplifies how we write complex electron configurations for elements with more electrons. It also helps to identify the beginning of the electron configuration for an element by recognizing well-known and stable electron configurations of noble gases. By starting from the nearest noble gas and adding the remaining electrons following the rules of electron filling order, we can quickly deduce the element's full electron configuration.

Atomic Number

Every element in the periodic table is uniquely identified by its "atomic number." This number is equal to the number of protons in an atom and, for a neutral atom, it also equals the number of electrons. For example, in the exercise, by calculating the total count of electrons from the electron configuration \( [\text{Ar}] 4s^{1} 3d^{5} \), we find there are 24 electrons.
This logically leads us to the atomic number 24, which is unique to chromium.
The atomic number provides crucial information about the element. It determines an element's identity, its position in the periodic table, and influences the chemical properties and behaviors of the element.

Periodic Table

The periodic table is like a giant map for chemists. It's an organized display of all known elements arranged by increasing atomic number. This arrangement helps to highlight periodic trends and similarities in chemical properties among elements.

• The horizontal rows are called periods. Elements in the same period share the number of atomic orbitals.
• The vertical columns are groups or families. Elements within a group often have similar properties and the same number of electrons in their outer orbital.

For chromium specifically, it is found in the 4th period and Group 6 of the periodic table, reflecting its distinctive properties and electron structure. The placement helps to convey that chromium shares similar traits with other elements in its group, like molybdenum and tungsten, particularly in terms of electron configuration.

Chromium

Chromium, represented by the symbol Cr, is an interesting element due to its electronic configuration and position in the periodic table. With an atomic number of 24, chromium is a transition metal found in Group 6. Its electron configuration, \( [\text{Ar}] 4s^{1} 3d^{5} \), reveals its status as a less common exception in electron-filling rules, where half-filled d subshells confer extra stability.
Chromium's unique electron arrangement affects its properties, making it a hard, lustrous, and corrosion-resistant metal often used in alloys and chrome plating. Understanding chromium's electron configuration helps on a deeper level to recognize why this metal behaves the way it does chemically, including its ability to form various oxidation states and engage in complex reactions.