define atomic number. Specify mass number. Recognize the variety of protons, neutrons, and electrons in an atom.

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It"s important to have the ability to distinguish atoms of one aspect from atoms of an additional element. Facets are pure substances that comprise all other matter, so each one is given a distinct name. The names of aspects are also represented by distinctive one- or two-letter symbols, such together \(\ceH\) for hydrogen, \(\ceC\) because that carbon, or \(\ceHe\) for helium. However, the would more powerful if this names can be offered to identify the numbers of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure \(\PageIndex1\): the is daunting to find qualities that differ in between each element, and to identify one element from another. Every element, however, does have actually a unique number of protons. Sulfur has actually 16 protons, silicon has 14 protons, and gold has 79 protons. Images used v permission (public domain for sulfur and silicon, gold is license is granted by CC-BY-SA-NC-ND; Alyellowcomic.comist-hp).

Atomic Number

Scientists distinguish in between different aspects by counting the number of protons in the cell nucleus (Table \(\PageIndex1\)). If one atom has actually only one proton, we understand that it"s a hydrogen atom. An atom v two protons is constantly a helium atom. If scientists count 4 protons in an atom, they understand it"s a beryllium atom. One atom with three protons is a lithium atom, an atom with 5 protons is a boron atom, one atom with six protons is a carbon atom . . . The perform goes on.

Since an atom of one aspect can be identified from one atom of another element through the number of protons in its nucleus, scientists are constantly interested in this number, and how this number differs between different elements. The variety of protons in an atom is referred to as its atomic number (\(Z\)). This number is really important because it is unique for atom of a provided element. Every atoms of an facet have the same variety of protons, and every aspect has a different variety of protons in its atoms. Because that example, every helium atoms have actually two protons, and also no other aspects have atoms through two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table \(\PageIndex1\): atoms of the an initial Six aspects
Hydrogen 1 0 1 1 1
Helium 2 2 2 2 4
Lithium 3 4 3 3 7
Beryllium 4 5 4 4 9
Boron 5 6 5 5 11
Carbon 6 6 6 6 12

Of course, due to the fact that neutral atoms have to have one electron because that every proton, one element"s atom number additionally tells girlfriend how plenty of electrons are 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 actually one proton, and, if it"s neutral, one electron as well. Gold, top top the other hand, has an atomic variety of 79, which means that one atom of gold has 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number (\(A\)) of an atom is the total variety of protons and neutrons in that nucleus. The massive of the atom is a unit referred to as the atomic mass unit \(\left( \textamu \right)\). One atomic mass unit is the massive of a proton, or about \(1.67 \times 10^-27\) kilograms, i m sorry is one extremely little mass. A neutron has actually just a tiny bit much more mass 보다 a proton, but its massive is regularly assumed to be one atomic mass unit together well. Due to the fact that electrons have actually virtually no mass, just about all the fixed of an atom is in its protons and also neutrons. Therefore, the total number of protons and also neutrons in one atom determines its fixed in atom mass systems (Table \(\PageIndex1\)).

Consider helium again. Most helium atoms have actually two neutrons in enhancement to 2 protons. As such the massive of most helium atoms is 4 atomic mass devices (\(2 \: \textamu\) for the proton + \(2 \: \textamu\) for the neutrons). However, part helium atom have more or much less than two neutrons. Atoms through the same variety of protons but different numbers of neutron are called isotopes. Because the variety of neutrons deserve to vary because that a offered element, the mass number of different atoms the an aspect may likewise vary. Because that example, part helium atoms have actually three neutrons instead of two (these are referred to as isotopes and are disputed in detail later on).

Why do you think the the "mass number" consists of protons and neutrons, but not electrons? You understand that most of the mass of one atom is focused in that is nucleus. The fixed of an atom depends on the number of protons and neutrons. Friend have already learned that the fixed of one electron is very, very tiny compared come the mass of one of two people a proton or a neutron (like the massive of a penny contrasted to the fixed of a bowling ball). Counting the variety of protons and neutrons tells scientists about the total mass of an atom.

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\<\textmass number \: A = \left( \textnumber the protons \right) + \left( \textnumber the neutrons \right)\>

An atom"s mass number is an extremely easy to calculate, detailed that you know the number of protons and also neutrons in an atom.

Example 4.5.1

What is the mass variety of an atom of helium that contains 2 neutrons?


\(\left( \textnumber of protons \right) = 2\) (Remember the an atom of helium constantly has 2 protons.)

\(\left( \textnumber of neutrons \right) = 2\)

\(\textmass number = \left( \textnumber of protons \right) + \left( \textnumber the neutrons \right)\)