The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.:20 Mass numbers are always whole numbers with no units. 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. Atomic mass of Carbon is 12.0107 u. The atomic mass is the mass of an atom. The atomic mass or relative isotopic mass refers to the mass of a single particle, and therefore is tied to a certain specific isotope of an element. The atomic mass is carried by the atomic nucleus, which occupies only about 10 -12 of the total volume of the atom or The atomic weight is the mass of an atom, typically expressed in atomic mass units (amu). For an isotope, it is the mass of the nucleus, that is the mass of the protons and neutrons, as the mass of the electrons are considered negligible. In their natural state only 21 elements exist as single isotopes, that is a sample has nuclei of only one The atomic weight of silver is a weighted average. Silver is not composed of atoms each of which weighs 107.868. Example #14: Given that the average atomic mass of hydrogen in nature is 1.0079, what does that tell you about the percent composition of H-1 and H-2 in nature? The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The sample problem below demonstrates how to calculate the atomic mass of chlorine. Example 4.20.1 4.20. 1. Use the atomic masses of each of the two isotopes of chlorine along with their respective percent abundances . The properties of these fundamental particles are summarized in Table 2.2.1 2.2. 1. You may notice the sum of an atom’s subatomic particles does not equal the atom’s actual mass: The total mass of six protons, six neutrons, and six electrons is 12.0993 u, slightly larger than the 12.00 u of an actual carbon-12 atom. 原子量 （atomic mass）又称 原子质量 [1] ，符号 ma 或 m ，是單一原子的 質量 ；原子量的單位為 道尔顿 （符号 Da）或 统一原子质量单位 （符號 u），也有人使用 amu，但通常不計單位。. 而1 道尔顿 的定義為一个 碳12 原子靜止質量的 [2] 。. 原子核 的 质子 和 中子 Most naturally-occurring elements consist of mixtures of isotopes, so you can't just add the masses of the protons and neutrons in a single atom. 1. The easy way. Look up the atomic mass in the Periodic Table. For example, the atomic mass of Cl is 35.45 u. 2. The harder way. Take the weighted average of the atomic masses of the element's isotopes. The average Atomic Mass of this isotope is 17.999 or 18 (approx.). The average Atomic Mass of various elements are determined by multiplying the Atomic Mass of each isotope by its fractional abundance and adding the value obtained. For example, chlorine contains two types of atoms having relative Masses of 35u and 37u. Atomic weight is the weight of atoms of an element. The science behind this measurement is extremely complex, and it has changed quite a bit since the early 1800s, when the concept was first proposed. It is also important to distinguish between weight and mass: atomic weight, also called relative atomic mass, reflects the average weight of a

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