Question

Give the number of neutrons in each naturally occurring isotope of argon: argon- \(36,\) argon \(-38,\) argon- 40 .

Short answer

Argon-36 has 18 neutrons, argon-38 has 20 neutrons, and argon-40 has 22 neutrons.

Step-by-step solution

Understand the Composition of an Isotope

Isotopes of an element have the same number of protons but different numbers of neutrons. The number of protons in argon is constant for all its isotopes as argon has an atomic number of 18, meaning it has 18 protons.

Identify the Mass Number

The mass number of an isotope is shown in its name. For instance, argon-36 has a mass number of 36. Mass number is the sum of protons and neutrons in the nucleus of an atom.

Calculate Number of Neutrons for Argon-36

For argon-36, subtract the number of protons from the mass number to find the number of neutrons: \[ \text{Number of Neutrons} = \text{Mass Number} - \text{Number of Protons} = 36 - 18 = 18.\]

Calculate Number of Neutrons for Argon-38

For argon-38, use the same calculation: \[ \text{Number of Neutrons} = \text{Mass Number} - \text{Number of Protons} = 38 - 18 = 20.\]

Calculate Number of Neutrons for Argon-40

For argon-40, the calculation is: \[ \text{Number of Neutrons} = \text{Mass Number} - \text{Number of Protons} = 40 - 18 = 22.\]

Compile the Results

Now we know the number of neutrons for each argon isotope: argon-36 has 18 neutrons, argon-38 has 20 neutrons, and argon-40 has 22 neutrons.

Key concepts

Atomic Number

The atomic number is a fundamental property of an element. It tells us the number of protons in an atom's nucleus. For argon, the atomic number is 18, which means every argon atom has 18 protons. This number is crucial because it defines the element's identity. No matter the isotope, argon will always have 18 protons.

It's important to understand that the atomic number is unique for each element, distinguishing it from others on the periodic table. Since protons are positively charged, the atomic number also indicates the number of electrons in a neutral atom. Thus, for neutral argon atoms, there are also 18 electrons, balancing the positive charge of the protons. This balance is the key to an atom's chemical behavior.

• Atomic number = Number of protons
• Defines element identity
• Equals number of electrons in a neutral atom

Mass Number

Every isotope of an element is characterized by its mass number. This is the combination of the number of protons and neutrons in an atom's nucleus. For example, in argon-36, the mass number is 36. This number can change among isotopes because while the protons stay the same, the number of neutrons can vary.

To find the mass number for an isotope like argon-38 or argon-40, you simply refer to the number that comes after the element's name: 38 or 40 in these cases. Calculating the mass number involves this straightforward approach:

• Mass number = Number of protons + Number of neutrons

This calculation helps us understand different isotopes of the same element and how they contribute to the natural abundance of elements we observe.

Neutrons

Neutrons are uncharged particles in an atom's nucleus. They play a vital role in the stability and identity of isotopes. By influencing the mass without changing the element's chemical properties, neutrons allow for the existence of different isotopes. For argon, the number of neutrons can change, giving us isotopes like argon-36, argon-38, and argon-40.

Calculating the number of neutrons is simple once you know the atomic number and mass number:

• Number of neutrons = Mass number - Atomic number

For instance, argon-40 has a mass number of 40 and an atomic number of 18. Subtracting these gives 22 neutrons. Understanding this relationship helps explain isotope variability and how they all contribute to an element's atomic mass.