Learning Objectivesspecify atomic number. Define mass number. Identify the variety of protons, neutrons, and also electrons in one atom.
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It"s vital to have the ability to distinguish atoms of one facet from atoms of an additional element. Aspects are pure building material that make up all various other matter, so each one is provided a distinct name. The names of elements are also represented by unique one- or two-letter symbols, such together \(\ceH\) for hydrogen, \(\ceC\) because that carbon, or \(\ceHe\) because that helium. However, that would an ext powerful if this names can be provided to determine the number of protons and also neutrons in the atoms. That"s wherein atomic number and mass numberare useful.
Scientists distinguish between different elements by counting the number of protons in the cell nucleus (Table \(\PageIndex1\)). If an atom has only one proton, we recognize that it"s a hydrogen atom. An atom v two proton is constantly a helium atom. If scientists count 4 protons in one atom, they recognize 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 perform goes on.
Since one atom the one facet can be distinguished from one atom of one more element by the number of protons in the nucleus, researchers are always interested in this number, and also how this number differs between different elements. The variety of protons in one atom is called its atomic number (\(Z\)). This number is an extremely important because it is distinct for atoms of a offered element. Every atoms the an facet have the same variety of protons, and also every facet has a different variety of protons in its atoms. For example, every helium atoms have actually two protons, and also 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 likewise tells you how countless electrons space in a neutral atom of the element. For example, hydrogen has actually an atomic number of 1. This means that an atom the hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, ~ above the various other hand, has actually an atomic variety of 79, which way that an atom that gold has 79 protons, and, if it"s neutral, 79 electrons together well.
The mass number (\(A\)) of one atom is the total variety of protons and also neutrons in that is nucleus. The fixed of the atom is a unit referred to as the atomic mass unit \(\left( \textamu \right)\). One atom mass unit is the massive of a proton, or around \(1.67 \times 10^-27\) kilograms, i beg your pardon is an extremely small mass. A neutron has actually just a tiny bit more mass than a proton, however its fixed is often assumed to be one atomic mass unit together well. Since electrons have virtually no mass, just about all the mass of an atom is in the protons and neutrons. Therefore, the total variety of protons and neutrons in one atom identify its mass in atomic mass units (Table \(\PageIndex1\)).
Consider helium again. Many helium atoms have two neutrons in enhancement to two protons. Because of this the mass of most helium atoms is 4 atom mass systems (\(2 \: \textamu\) because that the proton + \(2 \: \textamu\) because that the neutrons). However, some helium atoms have an ext or much less than two neutrons. Atoms through the same variety of protons yet different numbers of neutron are dubbed isotopes. Because the variety of neutrons deserve to vary because that a given element, the mass numbers of different atoms the an aspect may likewise vary. For example, part helium atoms have actually three neutrons instead of 2 (these are called isotopes and are questioned in information later on).
Why do you think the the "mass number" has protons and neutrons, however not electrons? You know that most of the massive of an atom is concentrated in that is nucleus. The fixed of an atom relies on the variety of protons and neutrons. You have already learned that the fixed of an electron is very, very little compared come the fixed of either a proton or a spirit (like the massive of a penny compared to the massive of a bowling ball). Counting the number of protons and also neutrons tells scientists around the complete mass of one atom.
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\<\textmass number \: A = \left( \textnumber that protons \right) + \left( \textnumber of neutrons \right)\>
An atom"s mass number is very easy to calculate, detailed that you recognize the variety of protons and neutrons in one atom.
What is the mass variety of an atom of helium that includes 2 neutrons?
\(\left( \textnumber the protons \right) = 2\) (Remember that an atom that helium always has 2 protons.)
\(\left( \textnumber the neutrons \right) = 2\)
\(\textmass number = \left( \textnumber that protons \right) + \left( \textnumber the neutrons \right)\)