Chapter 8, Problem 8.126QP

Interpretation Introduction

Interpretation: The reason for placement of argon before potassium in spite of its greater atomic mass and the average atomic mass for the given elements should be determined.

Concept Introduction:

Periodic Table: The available chemical elements are arranged considering their atomic number, the electronic configuration and their properties. The elements placed on the left of the table are metals and non-metals are placed on right side of the table.

In periodic table the horizontal rows are called periods and the vertical column are called group.

In periodic table the horizontal rows are called periods and the vertical column are called group. There are seven periods and 18 groups present in the table and some of those groups are given particular name as follows,

$\begin{array}{l}\text{Group-1}\to \text{Alkali metal}\\ \text{Group-2}\to \text{Alkaline metals}\\ \text{Group-16}\to \text{Chalcogens}\\ \text{Group-17}\to \text{Halogens}\\ \text{Group-18}\to \text{Noble gases}\end{array}$

Atomic Number: Atomic number of the element is equal to the number of protons present in the nucleus of the element which is denoted by symbol Z. The superscript presents on the left side of the symbol of the element.

Atomic mass: It is generally expressed in terms of atomic mass unit $\left(\text{amu}\right)$ which denotes the average number of protons and the neutrons present in the element considering the abundance of the isotopes for the element.

Isotopes: Isotopes of the element have same atomic number but different mass number.

Mass number: It is given by the total number of protons and the neutrons present in the nucleus of the element and it is denoted by symbol A. The subscript that lies on the left side of the symbol of the element represents the mass number.

Explanation of Solution

The reason that supports the placement of $\text{Ar}$ before $\text{K}$ and

Analyze the position of argon and potassium in the periodic table.

$\begin{array}{l}\text{Ar}\text{K}\\ \text{Atomic number}1819\\ \text{Atomic mass}\left(\text{amu}\right)\text{39}\text{.948}39.0983\end{array}$

Examining the periodic table shows that argon is placed before the potassium irrespective of high atomic mass of $\text{K}$ the reason that supports the placement is that elements in the periodic table are arranged in their increasing order the atomic number but on the atomic mass of the elements.

The average atomic mass of $\text{Ar}$ and $\text{K}$.

Determine the average atomic mass for the given elements.

$\begin{array}{l}\underset{\_}{\text{Average atomic mass for Ar:}}\\ \text{=}\text{Ar-36}\text{+}\text{Ar-38}\text{+}\text{Ar-40}\\ \text{=}\left(0.00337\times 35.9675\text{amu}\right)+\left(0.00063\times 37.9627\text{amu}\right)+\left(0.9960\times 39.9624\text{amu}\right)\\ =\text{39}\text{.95}\text{amu}\\ \underset{\_}{\text{Average atomic mass for K:}}\\ \text{=}\text{K-39}\text{+}\text{K-40}\text{+}\text{K-41}\\ \text{=}\left(0.93258\times 38.9637\text{amu}\right)+\left(0.000117\times 39.9640\text{amu}\right)+\left(0.0673\times 40.9618\text{amu}\right)\\ =\text{39}\text{.10}\text{amu}\\ \\ \text{Ar-36}\to 0.337\%=0.00337,\text{Ar-38}\to 0.063\%=0.00063\text{and}\text{Ar-40}\to 99.60\%=0.9960\\ \text{K-39}\to 93.258\%=0.93258,\text{K-40}\to 0.0117\%=0.000117\text{and}\text{K-41}\to 6.730\%=0.0673\\ \end{array}$

The average atomic mass for the given elements is obtained by addition of individual atomic mass of the isotopes multiplied with their percent abundance of the respective isotope.

Therefore, the average atomic mass for argon is $\text{39}\text{.95}\text{amu}$ and the average atomic mass for potassium is $\text{39}\text{.10}\text{amu}$

Conclusion

The reason that support the position of argon before potassium and the average atomic mass of the element is determined by using the atomic number, atomic mass and the abundance of the isotopes for the element.