Question

Argon has three naturally occurring isotopes, \({ }^{36} \mathrm{Ar},{ }^{38} \mathrm{Ar}\) and \({ }^{40}\) Ar. What is the mass number of each isotope? How many protons, neutrons, and electrons are present in each?

Short answer

The isotopes \(^ {36} \text{Ar}\), \(^ {38} \text{Ar}\), and \(^ {40} \text{Ar}\) each have 18 protons, 18, 20, and 22 neutrons respectively, and 18 electrons.

Step-by-step solution

- Identify the Mass Numbers

The mass number of an isotope is the sum of the number of protons and neutrons in its nucleus. For \(^ {36} \text{Ar}\), the mass number is 36. For \(^ {38} \text{Ar}\), the mass number is 38. For \(^ {40} \text{Ar}\), the mass number is 40.

- Determine the Number of Protons

All isotopes of an element have the same number of protons. Since these are isotopes of argon (Ar), they all have 18 protons because the atomic number of argon is 18.

- Determine the Number of Neutrons

The number of neutrons can be found by subtracting the number of protons from the mass number. For \(^ {36} \text{Ar}\): 36 - 18 = 18 neutrons. For \(^ {38} \text{Ar}\): 38 - 18 = 20 neutrons. For \(^ {40} \text{Ar}\): 40 - 18 = 22 neutrons.

- Determine the Number of Electrons

In a neutral atom, the number of electrons is equal to the number of protons. Since all these argon isotopes are neutral: they each have 18 electrons.

Key concepts

mass number

The mass number of an isotope is crucial for understanding its composition. It represents the total number of protons and neutrons in the nucleus of an atom.
For example, when looking at the argon isotopes \({ }^{36} \text{Ar}\), \({ }^{38} \text{Ar}\), and \({ }^{40} \text{Ar}\), the mass number is simply the number after the element symbol.
This means the mass numbers of these isotopes are 36, 38, and 40, respectively.
Remember, the mass number allows us to distinguish between different isotopes of the same element since the proton number remains constant, but the neutron number can vary.

protons

Protons determine the element itself and are found in the nucleus of an atom. Each element has a unique number of protons.
The atomic number of an element equals the number of protons. For argon (Ar), this number is 18.
This means that every argon isotope, regardless of the mass number, contains 18 protons.
It's important to note that the number of protons always remains the same for all isotopes of a given element.

neutrons

Neutrons also reside in the nucleus and contribute to the mass number. The number of neutrons can vary between isotopes.
To determine the number of neutrons in an isotope, subtract the number of protons from the mass number:

• For \({ }^{36} \text{Ar}\): 36 - 18 = 18 neutrons.
• For \({ }^{38} \text{Ar}\): 38 - 18 = 20 neutrons.
• For \({ }^{40} \text{Ar}\): 40 - 18 = 22 neutrons.

Understanding neutron counts helps in identifying the specific isotope and its characteristics.

electrons

Electrons are negatively charged particles that orbit the nucleus. In a neutral atom, the number of electrons equals the number of protons.
Since all isotopes of argon are considered neutral in this context, each of these isotopes has 18 electrons.
This ensures that the overall charge of the isotope balances out to zero.
It's essential to remember that the electron count matches the proton count in neutral atoms, regardless of isotopic differences.