Table of Contents
- What is Atomic Mass?
- How to Find the Atomic Mass of an Atom
- Importance of Atomic Mass
- Lesson Summary
Atoms are the smallest units of elements, or the simplest substances that cannot be broken down further chemically. Both atoms and elements are organized onto a display called the periodic table of elements.
Atoms are made of protons, neutrons, and electrons. The nucleus, or center of the atom that makes up most of the mass of the atom, is made up of protons and neutrons. Protons and neutrons are both hadron particles that have a positive charge or a neutral charge, respectively, and each are considered to have a mass of 1 atomic mass unit (amu). Amu's are units of measurement for atomic mass. It is not a coincidence that a proton and neutron are both equal to 1 amu. Atomic mass units were defined based on the masses of protons and neutrons by determining that one carbon atom has only six protons and usually six neutrons in its nucleus and therefore should be about 12 amu. This is much easier than saying one proton or neutron is equal to 1/167000000000000000000000th of a gram. Electrons are even smaller, and the mass of a proton is about 1,836 times greater than the mass of an electron.
Atomic mass is the total mass of particles of matter in an atom, i.e., the masses of protons, neutrons, and electrons in an atom added together. However, electrons are so small that they are negligible when finding the mass of an atom. Therefore, the atomic mass of an individual atom can be said to equal the total number of protons and neutrons. For example, fluorine atoms typically have 9 protons and 10 neutrons, and therefore have an atomic mass of 19 amu. When describing atoms and elements, there are three other numbers that may be present in a periodic table: atomic number, mass number, and average atomic mass.
The atomic number of an element is equal to the amount of protons that are contained in one atom of that element. Each element is made up of atoms that all have the same number of protons. For example, all carbon atoms have 6 protons, so carbon's atomic number is '6.'
Mass number is the sum of the number of protons and neutrons in the nucleus of an atom. So, if an atom of carbon has 6 protons and 6 neutrons in its nucleus, its mass number will be exactly 12. When looking at the periodic table of elements, one can calculate the number of neutrons in at atom of a given element by using the following formula:
Number of neutrons = mass number - atomic number
So, carbon has an atomic number of 6 and a mass number of 12. Plugging those numbers into the formula allows one to determine that carbon atoms usually have 6 neutrons.
When looking at a periodic table of elements, there is a number under each element that has decimal places. This is the average atomic mass of that element. Atoms of the same element always have the same number of protons but can vary in their number of neutrons. So, each element will have different isotopes, or atoms of the same element that have different numbers of neutrons. When looking at carbon on the periodic table, it says carbon has an average atomic mass of 12.011 amu. This number is determined by the fact that in any sample of carbon there are some atoms of carbon that have masses of 13 amu and 14 amu because carbon-13 atoms have 7 neutrons and carbon-14 atoms have 8 neutrons.
Here are three examples of atomic mass and average atomic mass:
One can find the atomic mass of an atom in a couple of ways. One way is to refer to the periodic table of elements. Usually, the atomic mass of an atom can be seen under an element's symbol and atomic number; it is commonly only found in the form of the average atomic mass or the mass number.
Another way to find the atomic mass of an atom is to add up the total mass of protons, neutrons, and electrons. But, because electrons are 1/1865th the mass of a proton or neutron, it is okay to simply add up the number of protons and the number of neutrons to get atomic mass. Adding up the masses of an atom's electrons when determining the mass is mostly only relevant when measuring the exact atomic mass differences between ions of the same element. Ions are atoms that are missing or have additional electrons when comparing them to the number of protons in the atom. To find the atomic mass of an element, one can look at the average atomic mass of an atom on the periodic table of elements.
Atoms are too small to observe by shining light on them and seeing them under something like a microscope. Therefore, one of the only ways we can observe atoms and how they differ is through measuring their masses. Any sample or piece of something can be put into a mass spectrometer which will separate all the atoms by taking advantage of their different masses, and then provide the elemental composition of the sample.
In more detail, a mass spectrometer first vaporizes a sample so that all the atoms float freely, then ionizes the sample or changes the charge of each atom by adding or taking away electrons. Next, the now charged atoms are sent through a chamber that influences the sample with an electromagnet. Because each atom is charged the electromagnet will affect and move each atom. Additionally, because the sample might be made up of different atoms of different masses, each atom will be influenced by the electromagnet based on their masses and inertia - so that each atom ends up in a stream of other atoms of the same mass. Once the atoms are separated into streams, they are captured and detected by a system that then measures the mass of the stream and calculates what percentage of the sample is made of which elements and isotopes. Being able to measure the atomic mass of atoms and therefore identify the atoms that make up a substance or sample has become extremely useful among many disciplines in science. By identifying the atoms that make up a sample, scientists can consequently use a mass spectrometer to date samples by using a method called radiometric dating. One of the most accurate radiometric dating methods is uranium-lead dating.
Some atoms of certain atomic masses are unstable and will decay over time into another atom and element. For example, uranium-238 atoms decay into lead-206 atoms at a rather definitive rate. Uranium is a radioactive element. Radioactive elements undergo decay and have something called half-life. Uranium-238 has a half-life of 4.47 billion years, which means half of any sample of uranium-238 will decay and become lead-206 after 4.47 billion years. Scientists can use the half-life of uranium, and its ratio to lead in a sample to determine the age of the sample.
Overall, the atomic mass of an atom is determined by adding up the total mass of the subatomic particles in the atom, i.e., the masses of the protons, neutrons, and electrons that make up the atom. Because the masses of protons and neutrons are so similar, they have each been labeled as being equal to 1 atomic mass unit (amu). Electrons have so little mass that the atomic mass of an atom can mostly be calculated by adding up the total number of protons and neutrons in an atom, which are found at the center or nucleus of the atom. Atoms are the most basic units of elements, which are substances that cannot chemically be broken down further; and are made up of atoms that all have the same number of protons. Elements can however be made of atoms called isotopes, or atoms of the same element that differ in their number of neutrons. Atoms and elements are organized onto an informative display called the periodic table of elements. The elements are organized onto the periodic table by their atomic number, or number of protons, and display further information like the average atomic mass of atoms in an element and/or the mass number, and information regarding electrons and their orbitals. The average atomic mass is how to find the atomic mass of an element. Because elements have different isotopes, the average atomic mass of an element's atoms may have decimal places or fractions of an atomic mass unit. Mass number is simply the number of protons plus the number of neutrons in an atom when looking at a periodic table.
The importance of atomic mass is highlighted by the invention and use of the mass spectrometer. The mass spectrometer is a technological system used to find the elemental and atomic composition of a sample of matter in question. The mass spectrometer has been of great use when making important scientific discoveries, recognizing the impact of humanity on itself, and when taking routine scientific measurements.
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Atomic mass can be found by adding up the total masses of subatomic particles in an atom. Because both protons and neutrons have an atomic mass of 1 amu, and electrons have masses of such little consequence, atomic mass can be calculated by adding up the total number of protons and neutrons in an atom. Additionally, atomic mass can be found on the period table of elements.
Atomic mass is the total mass of the subatomic particles in an atom and is measured in atomic mass units (amu).
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