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Chapter 4: Problem 40

What is the difference between the mass number and the atomic mass of an element?

Short Answer

Expert verified
The mass number is a whole number for each isotope, while the atomic mass is a weighted average taking into account all isotopes.

Step by step solution

01

Define Mass Number

The mass number of an element is the total count of protons and neutrons in the nucleus of an atom. This is a whole number specific to each isotope of an element.
02

Define Atomic Mass

The atomic mass (or atomic weight) of an element is the average mass of all the isotopes of that element, weighted by their natural abundance. This value is usually found on the periodic table and is not necessarily a whole number.
03

Identify the Difference

The difference between the mass number and the atomic mass is that the mass number is specific to each isotope and is a whole number, while the atomic mass is an average value that takes into account the different isotopes and their abundances.

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Key Concepts

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

mass number definition
The mass number of an element is a critical concept in understanding atomic structure. It represents the total number of protons and neutrons in an atom's nucleus. For example, if an atom has 6 protons and 6 neutrons, its mass number is 12. Note that the mass number is always a whole number. Pro tip: Remember that electrons do not contribute to the mass number as their mass is almost negligible.
atomic mass definition
Atomic mass, also known as atomic weight, is often found on the periodic table and can be a bit trickier to grasp. Unlike the mass number, atomic mass is not a whole number because it represents the weighted average of all isotopes of an element. Isotopes have different numbers of neutrons, so their masses vary. For example, carbon has isotopes with mass numbers of 12 and 14, but the most common has a mass of 12. Atomic mass takes the relative abundance of each isotope into account, resulting in a decimal value.
isotopes
Isotopes are variants of a particular chemical element. They have the same number of protons but different numbers of neutrons in their nuclei. This means that while they share the same atomic number, their mass numbers differ. For instance, carbon-12 and carbon-14 are both isotopes of carbon. Pro tip: To distinguish between isotopes, look at their names or symbols. Carbon-12 (C_{12}) has 6 neutrons, while Carbon-14 (C_{14}) has 8 neutrons.
average atomic mass
The concept of average atomic mass lands us in the realm of weighted averages. It is calculated by summing the products of each isotope's mass and its relative abundance. For instance, if isotope A has a mass of 10 amu and is 50% abundant, and isotope B has a mass of 11 amu and is also 50% abundant, the average atomic mass = 0.5(10) + 0.5(11) = 10.5 amu.
Key takeaway: The average atomic mass can be found on the periodic table and provides a more accurate representation of an element's mass in nature compared to the mass number.
periodic table
The periodic table is an indispensable tool in chemistry. It organizes all known elements by increasing atomic number, which equals the number of protons in an element's nucleus. Elements in the same column (group) have similar chemical properties.
Notably, the periodic table includes essential information about each element, such as the chemical symbol, atomic number, and atomic mass. Handy tip: Use the periodic table to quickly find the average atomic mass and compare elements' properties.

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Most popular questions from this chapter

The following trace elements have been found to be crucial to the functions of the body. Indicate each as a metal, a nonmetal, or a metalloid. (4.2) a. iron b. vanadium c. calcium d. arsenic

For the following atoms, give the number of protons, neutrons, and electrons: (4.3) a. \({ }_{48}^{114} \mathrm{Cd}\) b. \({ }_{43}^{98} \mathrm{Tc}\) c. \({ }_{79}^{199} \mathrm{Au}\) d. \(\frac{222}{86} \mathrm{Rn}\) e. \(\quad 136\) 54 \(\mathrm{Xe}\)

The most prevalent isotope of gold is Au- \(197 .(4.5)\) a. How many protons, neutrons, and electrons are in this isotope? b. What is the atomic symbol of another isotope of gold with 116 neutrons? c. What is the atomic symbol of an atom with an atomic number of 78 and 116 neutrons?

There are two naturally occurring isotopes of thallium: \({ }_{81}^{203} \mathrm{~T} 1\) and \({ }_{81}^{205} \mathrm{~T} 1\). Use the atomic mass of thallium listed on the periodic table to identify the more prevalent isotope.

Write the names for the elements in each of the following formulas of compounds used in medicine: a. salt substitute, \(\mathrm{KCl}\) b. dental cement, \(\mathrm{Zn}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) c. antacid, \(\mathrm{Mg}(\mathrm{OH})_{2}\) d. contrast agent for \(\mathrm{X}\) -ray, \(\mathrm{BaSO}_{4}\)
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