The atomic mass is the mass of an atom or molecule. To calculate the average mass of elements and molecules and to solve stoichiometry problems, atomic mass is used. It is also called atomic weight. In this article, we will study how to find atomic mass.
Atomic Mass
Atomic mass, also known as atomic weight, is a measure of the mass of an atom. This value is typically expressed in atomic mass units (amu), with one atomic mass unit being defined as 1/12th of the mass of a C-12 atom. The atomic mass can be found on the periodic table and is typically given as a decimal number.
2. Methods of Determination of Atomic Mass
The atomic mass of an element can be found in several ways:
By looking it up in a periodic table - Most periodic tables list the atomic mass of each element, usually to the nearest whole number.
By calculating the weighted average of isotopes - If you know the relative abundance and atomic mass of each isotope of an element, you can calculate its weighted average atomic mass.
By using mass spectrometry - Mass spectrometry is a technique that measures the masses of ions to determine the composition of a sample. This can be used to determine the exact atomic mass of an element, including the masses of its isotopes and their relative abundances. Overall, the atomic mass of an element is a weighted average of the masses of all its isotopes, taking into account their relative abundances in nature.
Overall, the atomic mass of an element is a weighted average of the masses of all its isotopes, taking into account their relative abundances in nature.
3. Calculation of Atomic Mass
1 Determine the Atomic Mass of an Atom
The total masses of protons and neutrons account for nearly all of an atom's mass, the atomic mass of an atom can be determined by adding the total number of protons and neutrons of that particular isotope. The number of protons in an atom gives the atomic number of that atom. For example, boron has an atomic number of 5, therefore, the total number of protons in a boron atom is 5. The total number of neutrons must generally be specified when describing which isotope the atom belongs to. To find/calculate the atomic mass of an element, you need to know the atomic masses of its isotopes and their relative abundances. Here's the formula for the weighted average atomic mass:
Sample Calculation: Q:
Calculate the atomic mass of Carbon with 7 neutrons. Solution: Atomic number of carbon = 6 = Number of protons. Number of neutrons = 7 Therefore, atomic mass = 6 + 7 = 13 amu. Q: Calculate the atomic mass of Chlorine with 20 neutrons. Solution: Atomic number of chlorine = 17 = Number of protons. Number of neutrons = 20 Therefore, atomic mass = 17 + 20 = 37 amu.
2 Determining the weighted
Average atomic mass of isotopes Weighted average atomic mass = (mass of isotope 1 x fractional abundance of isotope 1) + (mass of isotope 2 x fractional abundance of isotope 2) + .. For example, if an element has two isotopes with the following atomic masses and abundances: Isotope 1: atomic mass = 100 amu, fractional abundance = 0.9 Isotope 2: atomic mass = 110 amu, fractional abundance = 0.1 The weighted average atomic mass would be: Weighted average atomic mass = (100 amu x 0.9) + (110 amu x 0.1) = 90 amu + 11 amu = 101 amu So the atomic mass of this element would be approximately 101 amu.
Note:
The abundances should be expressed as fractions, not as percentages.
Sample Calculation Q:
The element oxygen has 3 isotopes with the following atomic masses and abundances: Isotope 1: Atomic mass = 15.995 amu, percentage abundance = 99.759% Isotope 2: Atomic mass = 16.995 amu, percentage abundance = 0.037% Isotope 3: Atomic mass = 17.99 amu, percentage abundance = 0.204% Solution: The weighted average atomic mass would be: Weighted average atomic mass = (15.95 amu x 99.759%) +(16.995 x 0.037%) + (17.99 x 0.204%) = 15.911 + 0.00628 + 0.036 = 15.953 ≅ 16 Therefore, the weighted atomic mass of oxygen is 16 amu.
3. Determine a Natural Sample's Atomic Mass
The periodic table can easily be used to determine the atomic mass of a given natural sample. The element symbol or atomic number must be known in order to locate the element on the periodic table. Once the necessary data is obtained, it can be compared to the periodic table, which will provide the atomic mass of the natural sample in atomic mass units in decimal figures. Also Read: What is Molar Mass?
Key Takeaways
Atomic mass is also known as atomic weight.
The relative abundance of isotopes in a naturally occurring element is used to calculate the average mass of an element.
The atomic mass is the sum of the masses of its protons, neutrons, and electrons; an electron's mass is so small in comparison to the other particles that its mass is simply that of the nucleus (protons and neutrons).
FAQs
1. What is the atomic mass?
The atomic mass can be defined as the mass of one atom of a specific element, expressed in atomic mass units (amu). It is a weighted average of the masses of all naturally occurring isotopes of a particular element, taking into account their relative abundances.
2. How can you determine the atomic mass of an element?
The determination of the atomic mass of an element can be done through a variety of methods, including mass spectrometry, spectroscopy, and isotopic analysis. In mass spectrometry, the sample is ionized and then the resulting ions are separated based on their mass-to-charge ratio. This allows for the determination of the relative abundance of each isotope, which can be used to calculate the atomic mass.
3. How is an atomic number and atomic mass related?
The atomic number of an element is the total number of protons present in the nucleus of an atom of that element, while the atomic mass is the total number of protons and neutrons in the nucleus. While there is a relationship between the atomic number and the atomic mass, they are not the same thing.
4. Is atomic mass the same as molecular mass?
No, atomic mass and molecular mass are different concepts. Atomic mass refers to the mass of one atom of an element, while molecular mass refers to the total mass of all the atoms in a molecule. The molecular mass can be calculated by adding up the atomic masses of all the atoms in the molecule.
5. Why is atomic mass expressed in atomic mass units?
The atomic mass is expressed in atomic mass units (amu) because it provides a convenient and standardized way to compare the masses of different atoms. One amu is defined as exactly 1/12 of the mass of a carbon-12 atom. By expressing the atomic mass in amu, it is possible to compare the masses of atoms from different elements on a common scale.
Conclusion
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