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Chapter 2: Problem 103

An isotope of argon (atomic number \(=18\) ) has a mass number of 40 . How many neutrons are in this isotope? A. 18 B. 40 C. 22 D. 58

Short Answer

Expert verified
C. 22

Step by step solution

01

Understand Atomic Number

The atomic number of an element represents the number of protons in the nucleus. For argon, the atomic number is 18.
02

Understand Mass Number

The mass number of an isotope is the sum of the number of protons and neutrons. In this case, the mass number of argon is 40.
03

Calculate the Number of Neutrons

To find the number of neutrons, subtract the atomic number (number of protons) from the mass number. Neutrons = Mass number - Atomic number Neutrons = 40 - 18 Neutrons = 22
04

Match with Choices

Compare the calculated number of neutrons (22) with the given choices. C. 22 is the correct answer.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Isotopes
Isotopes are variations of the same element, where the number of protons remains the same but the number of neutrons varies. This variation causes isotopes of an element to have different mass numbers. For example, the element argon (with an atomic number of 18) has multiple isotopes, such as argon-36, argon-38, and argon-40. Here, the numbers 36, 38, and 40 are the mass numbers of different isotopes. Each isotope has 18 protons but a varying number of neutrons, which alters their mass but not their chemical properties. Isotopes are important in fields like nuclear medicine, where certain isotopes are used for imaging and treatment.
Mass Number
The mass number is the total count of protons and neutrons in an atom's nucleus. It is represented as the sum of these two types of particles. The mass number is crucial because it helps distinguish between different isotopes of an element. For instance, in the example of argon-40 given in the exercise, the mass number is 40. This number is not the same as the atomic mass found on the periodic table, which is usually a weighted average of all isotopes of an element. Therefore, when calculating properties specific to a single isotope, always refer to the mass number, not the atomic mass.
Atomic Number
The atomic number of an element is the number of protons found in the nucleus of an atom. This number is unique to each element and defines its chemical identity. For example, argon’s atomic number is 18, meaning it has 18 protons. The atomic number is important because it determines the element’s position on the periodic table, giving us lots of information about its chemical behavior. Understanding the atomic number helps in identifying the element, predicting its reactivity, and explaining its place in groups or periods of the periodic table.
Neutron Calculation
To find the number of neutrons in an isotope, you need to subtract the atomic number (number of protons) from the mass number. This is because the mass number is the combined count of protons and neutrons. For example, in the isotope argon-40:

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