Problem 94 - Chemistry: The Central Science in SI Units, Global Edition

Question

The element argon has three naturally occurring isotopes, with \(18,20,\) and 22 neutrons in the nucleus, respectively. (a) Write the full chemical symbols for these three isotopes. (b) Describe the similarities and differences between the three kinds of atoms of argon.

Step-by-Step Solution

Answer

The full chemical symbols for the three naturally occurring isotopes of argon are \(_{18}^{36}\textrm{Ar}\), \(_{18}^{38}\textrm{Ar}\), and \(_{18}^{40}\textrm{Ar}\). All three isotopes have the same atomic number (18 protons) and chemical behavior due to having the same number of electrons (18 electrons). However, they differ in their neutron numbers (18, 20, and 22 neutrons, respectively) and mass numbers (36, 38, and 40, respectively), resulting in slightly different physical properties such as densities or atomic masses.

Step 1: Find the atomic number of argon

To write the chemical symbols for the isotopes of argon, we need to find the atomic number of argon, denoted by 'Z', which represents the number of protons in the nucleus. You can find this information on the periodic table. In the case of argon, the atomic number is 18.

Step 2: Write the full chemical symbols for the three isotopes of argon

The chemical symbol for argon is 'Ar'. For isotopes, we write their mass number (the total of protons and neutrons) on the top left corner and the atomic number on the bottom left corner of the chemical symbol. The mass number (A) is the sum of protons (Z) and neutrons (N). Since argon has 18 protons, we can calculate the mass number for each isotope using their neutron numbers. Isotope 1 (18 neutrons): \(A = Z + N = 18 + 18 = 36\), which gives us the chemical symbol: \(_{18}^{36}\textrm{Ar}\) Isotope 2 (20 neutrons): \(A = Z + N = 18 + 20 = 38\), which gives us the chemical symbol: \(_{18}^{38}\textrm{Ar}\) Isotope 3 (22 neutrons): \(A = Z + N = 18 + 22 = 40\), which gives us the chemical symbol: \(_{18}^{40}\textrm{Ar}\)

Step 3: Describe the similarities and differences between the three kinds of argon atoms

Now let's discuss the similarities and differences between these three isotopes of argon: Similarities: 1. All three isotopes have the same atomic number (18 protons), which means they are all argon atoms. 2. The number of electrons in all three isotopes is the same (18 electrons), resulting in the same chemical behavior and properties. Differences: 1. The isotopes have different numbers of neutrons: 18, 20, and 22 neutrons, respectively. 2. They have distinct mass numbers: 36, 38, and 40, respectively. Consequently, the isotopes have slightly different physical properties, such as densities or atomic masses. In summary, the full chemical symbols for the three naturally occurring isotopes of argon are \(_{18}^{36}\textrm{Ar}\), \(_{18}^{38}\textrm{Ar}\), and \(_{18}^{40}\textrm{Ar}\). Their similarities include the same atomic number and chemical behavior, while their differences lie in their neutron numbers and mass numbers, resulting in distinctive physical properties.

Key Concepts

Argon

Argon is a fascinating and abundant noble gas found in the Earth's atmosphere, making up about 0.93% of it. This element is colorless, tasteless, and odorless. It's part of the group of elements known as noble gases, which are generally inert and don't readily form chemical compounds. Because of its non-reactive nature, argon is used widely in situations where materials must be protected from the harmful effects of oxygen. This includes applications like welding and in incandescent light bulbs.

Beyond its common uses, argon is unique for its presence in every breath we take, contributing to our planet's protective atmospheric layer. Even though this gas doesn't play a role in human biological processes, it is crucial in many industrial and scientific applications.

Atomic Number

The atomic number is a fundamental concept in understanding elements on the periodic table. It represents the number of protons in an atom's nucleus. For every chemical element, this number is unique and serves as an identifier. In the case of argon, the atomic number is 18. This means that every argon atom has 18 protons in its nucleus.

The atomic number not only determines the element itself, but it also defines the element's position on the periodic table and its chemical properties. In neutral atoms, the number of electrons equals the atomic number, dictating how the element will interact with others. For instance, argon's 18 electrons configure in such a way that they resist further bonding, making it a stable, inert gas.

Neutrons

Neutrons are neutral particles found within the nucleus of an atom alongside protons. They play a pivotal role in stabilizing atomic nuclei, which would otherwise repel due to the positive charges of protons. The number of neutrons in an atom can vary, resulting in different isotopes of the same element.

For argon, the existence of its isotopes depends on the number of neutrons. As seen in naturally occurring argon isotopes:

• Argon-36: 18 neutrons
• Argon-38: 20 neutrons
• Argon-40: 22 neutrons

Despite the differences in the number of neutrons, all these isotopes maintain 18 protons, keeping them as argon. These variations in neutrons mean the isotopes have different mass numbers, affecting their physical properties without altering their chemical behavior.

Chemical Symbols

Chemical symbols provide a concise way to represent elements and their isotopes. Generally, a chemical symbol consists of one or two letters derived from the element's name. For argon, this symbol is 'Ar'. However, when dealing with isotopes, an extended notation is used to include more information.

The isotope notation involves writing both the atomic number and the mass number. For example:

• Argon-36: \(_{18}^{36}\textrm{Ar}\)
• Argon-38: \(_{18}^{38}\textrm{Ar}\)
• Argon-40: \(_{18}^{40}\textrm{Ar}\)

These notations are integral for distinguishing isotopes based on the number of neutrons. While the atomic number remains unchanged in all isotopes of argon, the mass number informs us about the total protons and neutrons, offering insight into the isotope's stability and mass.