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Ch.2 - Atoms, Molecules, and Ions
Chapter 2, Problem 94a

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.

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Identify the atomic number of argon, which is 18, as it has 18 protons.
Determine the mass number for each isotope by adding the number of protons and neutrons: Mass number = Protons + Neutrons.
For the first isotope with 18 neutrons: Mass number = 18 (protons) + 18 (neutrons) = 36.
For the second isotope with 20 neutrons: Mass number = 18 (protons) + 20 (neutrons) = 38.
For the third isotope with 22 neutrons: Mass number = 18 (protons) + 22 (neutrons) = 40.
Write the full chemical symbols for each isotope using the format: \( ^{\text{Mass Number}}_{\text{Atomic Number}}\text{Element Symbol} \).
The full chemical symbols are: \( ^{36}_{18}\text{Ar} \), \( ^{38}_{18}\text{Ar} \), and \( ^{40}_{18}\text{Ar} \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons in their nuclei. This difference in neutron count results in varying atomic masses for the isotopes. For example, argon has isotopes with 18, 20, and 22 neutrons, leading to different atomic masses while retaining the same chemical properties.
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Chemical Symbol

A chemical symbol is a one- or two-letter notation used to represent an element on the periodic table. The first letter is always capitalized, while any subsequent letters are lowercase. For isotopes, the chemical symbol is often accompanied by a superscript indicating the total number of protons and neutrons (mass number) and a subscript indicating the number of protons (atomic number).
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Lewis Dot Symbols

Mass Number

The mass number of an isotope is the total count of protons and neutrons in its nucleus. It is represented as a superscript in the isotope's chemical symbol. For instance, the isotopes of argon can be denoted as ¹⁸Ar, ²⁰Ar, and ²²Ar, where the numbers 18, 20, and 22 represent the mass numbers corresponding to the different neutron counts.
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Related Practice
Textbook Question

Identify the element represented by each of the following symbols and give the number of protons and neutrons in each: (a) 7433X

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Textbook Question

Identify the element represented by each of the following symbols and give the number of protons and neutrons in each: (b) 12753X

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Textbook Question

The nucleus of 6Li is a powerful absorber of neutrons. It exists in the naturally occurring metal to the extent of 7.5%. In the era of nuclear deterrence, large quantities of lithium were processed to remove 6Li for use in hydrogen bomb production. The lithium metal remaining after removal of 6Li was sold on the market. (b) The atomic masses of 6Li and 7Li are 6.015122 and 7.016004 u, respectively. A sample of lithium depleted in the lighter isotope was found on analysis to contain 1.442% 6Li. What is the average atomic weight of this sample of the metal?

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Textbook Question

The element chromium (Cr) consists of four naturally occurring isotopes with atomic masses 49.9460, 51.9405, 52.9407, and 53.9389 u. The relative abundances of these four isotopes are 4.3, 83.8, 9.5, and 2.4%, respectively. From these data, calculate the atomic weight of chromium.

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Textbook Question

Copper (Cu) consists of two naturally occurring isotopes with masses of 62.9296 and 64.9278 u. (a) How many protons and neutrons are in the nucleus of each isotope? Write the complete atomic symbol for each, showing the atomic number and mass number. (b) The average atomic mass of Cu is 63.55 u. Calculate the abundance of each isotope.

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Open Question
Using a suitable reference such as the CRC Handbook of Chemistry and Physics or http://www.webelements.com, look up the following information for nickel: (b) the atomic masses (in u), (c) the natural abundances of the five most abundant isotopes.