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

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It"s essential to have the ability to distinguish atoms of one element from atom of one more element. Facets are pure building material that comprise all other matter, so every one is provided a unique name. The surname of aspects are likewise represented by distinct one- or two-letter symbols, such as (ceH) because that hydrogen, (ceC) for carbon, or (ceHe) because that helium. However, that would an ext powerful if this names could be provided to recognize the number of protons and neutrons in the atoms. That"s wherein atomic number and also mass numberare useful.

Figure (PageIndex1): it is complicated to find features that differ in between each element, and also to distinguish one aspect from another. Every element, however, does have a unique number of protons. Sulfur has 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Pictures used v permission (public domain for sulfur and silicon, gold is license is granted by CC-BY-SA-NC-ND; Aljulianum.netist-hp).

Atomic Number

Scientists distinguish between different elements by counting the variety of protons in the cell core (Table (PageIndex1)). If one atom has only one proton, we know that it"s a hydrogen atom. An atom v two protons is constantly a helium atom. If scientists count four protons in an atom, they know it"s a beryllium atom. One atom with 3 protons is a lithium atom, one atom with 5 protons is a boron atom, an atom with six protons is a carbon atom . . . The list goes on.

Since one atom that one aspect can be distinguished from one atom of another element through the number of protons in the nucleus, scientists are always interested in this number, and how this number differs in between different elements. The variety of protons in one atom is dubbed its atomic number ((Z)). This number is really important because it is distinctive for atom of a given element. Every atoms that an aspect have the same variety of protons, and every aspect has a different number of protons in the atoms. Because that example, all helium atoms have two protons, and also no other aspects have atoms v two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the an initial Six facets
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, because neutral atoms need to have one electron for every proton, an element"s atom number likewise tells girlfriend how numerous electrons are in a neutral atom of the element. For example, hydrogen has an atomic variety of 1. This way that one atom the hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, on the various other hand, has actually an atomic variety of 79, which way that one atom the gold has 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number ((A)) of one atom is the total number of protons and also neutrons in that nucleus. The massive of the atom is a unit dubbed the atom mass unit (left( extamu ight)). One atom mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, which is one extremely little mass. A neutron has just a tiny bit more mass 보다 a proton, yet its massive is regularly assumed to it is in one atomic mass unit together well. Because electrons have virtually no mass, just about all the mass of an atom is in that protons and neutrons. Therefore, the total variety of protons and also neutrons in an atom identify its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutrons in addition to two protons. Because of this the massive of many helium atom is 4 atomic mass units ((2 : extamu) because that the proton + (2 : extamu) for the neutrons). However, some helium atoms have an ext or less than 2 neutrons. Atoms v the same variety of protons yet different numbers of neutrons are called isotopes. Since the variety of neutrons have the right to vary for a offered element, the mass numbers of different atoms that an element may also vary. For example, part helium atoms have three neutrons instead of 2 (these are called isotopes and also are discussed in information later on).

Why do you think the the "mass number" includes protons and also neutrons, yet not electrons? You know that most of the fixed of one atom is concentrated in that is nucleus. The mass of an atom depends on the number of protons and also neutrons. Girlfriend have currently learned the the mass of one electron is very, very small compared to the mass of one of two people a proton or a ghost (like the fixed of a penny contrasted to the mass of a bowling ball). Counting the number of protons and also neutrons tells scientists around the complete mass of an atom.

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< extmass number : A = left( extnumber that protons ight) + left( extnumber that neutrons ight)>

An atom"s mass number is an extremely easy to calculate, listed that you recognize the variety of protons and neutrons in one atom.

Example 4.5.1

What is the mass number of an atom of helium that consists of 2 neutrons?


(left( extnumber that protons ight) = 2) (Remember the an atom that helium constantly has 2 protons.)

(left( extnumber that neutrons ight) = 2)

( extmass number = left( extnumber that protons ight) + left( extnumber the neutrons ight))