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It"s necessary to have the ability to distinguish atoms of one aspect from atoms of another element. Aspects are pure building materials that comprise all other matter, so each one is given a unique name. The names of elements are additionally represented by unique one- or two-letter symbols, such as \(\ceH\) for hydrogen, \(\ceC\) because that carbon, or \(\ceHe\) because that helium. However, it would much more powerful if these names could be used to identify the numbers of protons and also neutrons in the atoms. That"s wherein atomic number and also mass numberare useful.
Scientists distinguish in between different facets by count the number of protons in the cell core (Table \(\PageIndex1\)). If one atom has actually only one proton, we recognize that it"s a hydrogen atom. One atom v two proton is always a helium atom. If researchers count 4 protons in one atom, they understand it"s a beryllium atom. An atom with 3 protons is a lithium atom, one atom with 5 protons is a boron atom, an atom with 6 protons is a carbon atom . . . The list goes on.
Since an atom that one facet can be differentiated from an atom of one more element by the variety of protons in the nucleus, scientists are always interested in this number, and how this number differs between different elements. The number of protons in an atom is referred to as its atomic number (\(Z\)). This number is very important since it is unique for atoms of a offered element. All atoms of an element have the same number of protons, and also every element has a different number of protons in its atoms. For example, every helium atoms have two protons, and no other aspects have atoms with two protons.
Of course, because neutral atoms need to have one electron for every proton, one element"s atom number also tells girlfriend how many electrons room in a neutral atom of that element. Because that example, hydrogen has actually an atomic variety of 1. This method that an atom the hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, on the other hand, has actually an atomic number of 79, which way that one atom of gold has actually 79 protons, and, if it"s neutral, 79 electrons together well.
The mass number (\(A\)) of an atom is the total number of protons and neutrons in that nucleus. The fixed of the atom is a unit called the atom mass unit \(\left( \textamu \right)\). One atom mass unit is the fixed of a proton, or around \(1.67 \times 10^-27\) kilograms, which is one extremely little mass. A neutron has actually just a tiny bit more mass than a proton, however its fixed is regularly assumed to be one atom mass unit as well. Due to the fact that electrons have actually virtually no mass, just around all the fixed of one atom is in that is protons and neutrons. Therefore, the total variety of protons and also neutrons in an atom identify its mass in atom mass devices (Table \(\PageIndex1\)).
Consider helium again. Most helium atoms have actually two neutrons in addition to two protons. As such the mass of many helium atom is 4 atomic mass devices (\(2 \: \textamu\) for the protons + \(2 \: \textamu\) because that the neutrons). However, some helium atoms have an ext or less than 2 neutrons. Atoms through the same variety of protons however different numbers of neutrons are dubbed isotopes. Since the variety of neutrons deserve to vary because that a offered element, the mass number of various atoms the an element may additionally vary. For example, part helium atoms have three neutrons rather of 2 (these are dubbed isotopes and also are discussed in detail later on).
Why carry out you think the the "mass number" includes protons and neutrons, yet not electrons? You know that most of the massive of an atom is focused in the nucleus. The fixed of an atom counts on the number of protons and also neutrons. Girlfriend have already learned the the fixed of an electron is very, very small compared to the massive of one of two people a proton or a spirit (like the massive of a penny contrasted to the massive of a bowling ball). Count the number of protons and neutrons speak scientists about the total mass of an atom.
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\<\textmass number \: A = \left( \textnumber that protons \right) + \left( \textnumber that neutrons \right)\>
An atom"s fixed number is an extremely easy come calculate, provided that you know the number of protons and neutrons in an atom.
What is the mass number of an atom that helium that contains 2 neutrons?
\(\left( \textnumber that protons \right) = 2\) (Remember the an atom of helium always has 2 protons.)
\(\left( \textnumber of neutrons \right) = 2\)
\(\textmass number = \left( \textnumber that protons \right) + \left( \textnumber the neutrons \right)\)