Periodic table average atomic mass

The average atomic mass shown in the periodic table is not equal to the mass number. 

These three isotopes are why you see carbon s atomic mass on the periodic table written as 12.01. If you do a quick bit of deductive reasoning, you can probably determine that carbon-12 is far and away the most common of the three isotopes due to the average atomic mass being closest to 12. 

The total mass of an atom is called its atomic mass. This is the sum of the masses of all the atom s components (electrons, protons, and neutrons). Because electrons are so much less massive than protons and neutrons, their contribution to atomic mass is negligible. As we explore further in Section 9.2, a special unit has been developed for atomic masses. This is the atomic mass unit, amu, where 1 atomic mass unit is equal to 1.661 X 10-24 gram, which is slightly less than the mass of a single proton. As shown in Figure 3.21, the atomic masses listed in the periodic table are in atomic mass units. As is explored in the Calculation Corner on page 95, the atomic mass of an element as presented in the periodic table is actually the average atomic mass of its various isotopes. 

The data needed for 12C are found in Tables 2-1 and 21-1. We cannot use average atomic masses from the periodic table for these calculations because our calculations call for the masses of single isotopes H, 12C, etc.). 

Mass number is equal to the proton number plus the neutron number. The average atomic mass appears on the periodic table and is the mass of each naturally occurring isotope of an element weighted by the fractional abundance of the isotope. 

The average atomic mass of an element is the average of the masses of all the element s isotopes. It takes into account the abundance of each isotope within the element. The average atomic mass is the mass that is given for each element in the periodic table. 

Thus chemists need to know an element s isotopic abundance and the mass of each isotope to calculate the average atomic mass. How do chemists determine the isotopic abundance associated with each element How do they find the mass of each isotope They use a mass spectrometer, a powerful instrument that generates a magnetic field to obtain data about the mass and abundance of atoms and molecules. You will learn more about the mass spectrometer in Tools Techniques on page 166. You can use the data obtained with a mass spectrometer to calculate the average atomic mass given in the periodic table. 

Looking at the periodic table confirms that the average atomic mass of lithium is 6.94 u. The upcoming Sample Problem gives another example of how to calculate average atomic mass. 

In this case, the abundance of each isotope is close to 50%. An average atomic mass of about 108 u seems right, because it is between 106.9 u and 108.9 u. Checking the periodic table reveals that the average atomic mass of silver is indeed 107.9 u. 

In some periodic tables, the average atomic mass is referred to as the atomic weight of an element. This terminology, while technically incorrect, is still in use and is generally accepted. 

From the periodic table, the average atomic mass of boron is B = 10.81 u. 

In section 5.1, you learned how to use isotopic abundances and isotopic masses to find the average atomic mass of an element. You can use the average atomic mass, found in the periodic table, to describe the average mass of an atom in a large sample. 

How can you use this relationship to relate mass and moles The periodic table tells us the average mass of a single atom in atomic mass units (u). For example, zinc has an average atomic mass of 65.39 u. One mole of an element has a mass expressed in grams numerically equivalent to the element s average atomic mass expressed in atomic mass units. One mole of zinc atoms has a mass of 65.39 g. This relationship allows chemists to use a balance to count atoms. You can use the periodic table to determine the mass of one mole of an element. 

The periodic table lists the average atomic mass of chlorine as 35.45 u. Are there any chlorine atoms that have a mass of 35.45 u Explain your answer. 

The periodic table provides information about each element, as shown in the key for Figure 4. This periodic table lists the atomic number, symbol, name, average atomic mass, and electron configuration in shorthand form for each element. 

Use a periodic table or isotopic composition data to determine the average atomic masses of elements. 

You remember that isotopes are atoms that have different numbers of neutrons than other atoms of the same element do. So, isotopes have different atomic masses. The periodic table reports average atomic mass, a weighted average of the atomic mass of an element s isotopes. A weighted average takes into account the relative importance of each number in the average. Thus, if there is more of one isotope in a typical sample, it affects the average atomic mass more than an isotope that is less abundant does. 

Table 26-6 shows abundance of the two isotopes of silver found in nature. The more abundant isotope has an atomic mass of a little less than 107, but the average atomic mass of silver on the periodic table is about 107.9. Explain why it is higher. (Chapter 5)... 

A Lewis dot diagram is a convenient, shorthand method to represent an element and its valence electrons. You have used the periodic table already as a source of information about the symbols, names, atomic numbers, and average atomic masses of elements. In Chapter 3, you will learn that the arrangement of elements on the periodic table yields even more infor mation about the electronic structures of atoms and how those structures can help you to predict many of the properties of elements. 

You know from Chapter 2 that average atomic masses of the elements are given on the periodic table. For example, the average mass of one iron atom is 55.8 u, where u means atomic mass units. The atomic mass unit is defined so that the atomic mass of an atom of the most common carbon isotope is exactly 12 u, and the mass of 1 mol of the most common isotope of carbon atoms is exactly 12 g. The mass of 1 mol of a pure substance is called its molar mass. For example, the molar mass of iron is 55.847 g, and the molar mass of platinum is 195.08 g. Relative masses of elements are demonstrated in Figure 12.4. The molar mass is the mass in grams of the average atomic mass. 

This is the weighted-average atomic mass of chlorine that appears on the periodic table. 

The average atomic mass of each element (expressed in amu) is also known as its atomic weight. Although the term average atomic mass is more proper and the simpler term atomic mass is frequently used, the term atomic weight is most common. The atomic weights of the elements are listed both in the periodic table and in the table of elements, which are found inside the front cover of this text. 

The mass of 60.4% of the atoms of an element is 68.9257 amu. There is only one other natural isotope of that element, and its atomic mass is 70.9249 amu. Calculate the average atomic mass of the element. Using the periodic table and/or the table of the elements, write its symbol and name. 

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