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Nvk3l mass finder4/11/2023 ![]() Mass number = Atomic number Number of neutrons Knowing the number of neutrons and the atomic number or the number of protons or electrons of an atom, we can determine the mass number of an element. Nameįig: Atoms of the First Six Elements Calculating Mass Number Similarly, lithium has three protons and four neutrons and a mass number of \(7\). As the number of protons must equal the number of electrons, there are two electrons in the helium atom. Hence, \(2 2 = 4\), which gives the mass number of the helium atom. Let us consider the element helium its atomic number is \(2\) hence it has two protons in its nucleus. The atomic mass, now known as the mass number, is the sum of the protons and neutrons in the nucleus. Rutherford, in its experiment, proved that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The neutron’s discovery provided the missing part of the picture. It was then found that the atomic weight for an element was often twice the number of protons (or more). Initially, all atomic weights were measured with respect to hydrogen, which has an atomic weight of one.Īfter the discovery of protons, scientists found out that the weight of an atom was mostly due to protons-electrons were known to contribute almost nothing to the atomic weight of the element. However, after discovering the nucleus, elements in the periodic table were arranged according to increasing atomic numbers. ![]() The early Periodic table developed by Mendeleev was based on Atomic weights of elements. Mass Number: In the early \(1800\)s, as a part of his research on atoms, John Dalton determined the atomic weights of elements. ![]()
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