Also, relative isotopic mass is not the same as isotopic mass, and relative atomic mass (also called atomic weight) is not the same as atomic mass. :20 Mass numbers are always whole numbers with no units. The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.
In the case of chlorine, chlorine-35 has a mass of around 35 u, and chlorine-37 around 37 u.Īs mentioned above, note that the atomic mass of an atom is not the same as its mass number. Each isotope has its own atomic mass, called its isotopic mass. Both isotopes of chlorine have 17 protons, but chlorine-37 has 20 neutrons, 2 more neutrons than chlorine-35, which has 18. :44 For example, the element chlorine has two common isotopes: chlorine-35 and chlorine-37. An atom of an element with a certain number of neutrons is called an isotope. However, most elements in nature consist of atoms with different numbers of neutrons. The number of protons an atom has determines what element it is. The atomic mass of an atom is usually within 0.1 u of the mass number. :18 Hence, people can get a rough value of an atom's mass in atomic mass units by working out the sum of the number of protons and the number of neutrons in the nucleus, which is called the mass number. Because the masses of protons and neutrons are almost exactly the same, it can be said that both protons and neutrons have a mass of roughly 1 u. Because electrons are so light, people can say that the mass of a carbon-12 atom is made of 6 protons and 6 neutrons. 1 atomic mass unit is defined as 1/12 of the mass of a single carbon-12 atom. Instead, atomic mass is expressed in unified atomic mass units (unit symbol: u). However, because each atom has a very small mass, this is not easy to do. It includes the masses of the 3 subatomic particles that make up an atom: protons, neutrons and electrons.Ītomic mass can be expressed in grams. ( October 2022)Īn atomic mass (symbol: m a) is the mass of a single atom of a chemical element. This needs more sections, which is adressed in the other tag.
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