specify atomic number. Define mass number. Determine the variety of protons, neutrons, and electrons in an atom.

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It"s necessary to be able to distinguish atoms of one element from atoms of another element. Elements are pure building materials that consist of all various other matter, so each one is offered a unique name. The name of aspects are also represented by unique one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, that would more powerful if these names can be provided to determine the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): the is difficult to find attributes that differ in between each element, and to identify one facet from another. Each element, however, does have actually a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Photos used with permission (public domain because that sulfur and also silicon, yellow is licensed by CC-BY-SA-NC-ND; Alsmashville247.netist-hp).

Atomic Number

Scientists distinguish in between different elements by counting the variety of protons in the cell nucleus (Table (PageIndex1)). If an atom has actually only one proton, we recognize that it"s a hydrogen atom. One atom v two proton is constantly a helium atom. If researchers count four protons in one atom, they know 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 six protons is a carbon atom . . . The perform goes on.

Since an atom the one element can be differentiated from an atom of an additional element by the variety of protons in its nucleus, researchers are constantly interested in this number, and how this number differs in between different elements. The variety of protons in one atom is referred to as its atomic number ((Z)). This number is very important due to the fact that it is unique for atom of a offered element. All atoms that an aspect have the same number of protons, and also every aspect has a different number of protons in its atoms. For example, every helium atoms have actually two protons, and no other aspects have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms of the first 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, since neutral atoms have to have one electron because that every proton, an element"s atomic number also tells you how countless electrons are in a neutral atom of that element. For example, hydrogen has an atomic number of 1. This means that one atom of hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, ~ above the various other hand, has an atomic variety of 79, which means that an atom of gold has actually 79 protons, and, if it"s neutral, 79 electrons together well.

Mass Number

The mass number ((A)) of an atom is the total number of protons and also neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit (left( extamu ight)). One atomic 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 actually just a tiny bit an ext mass than a proton, yet its fixed is frequently assumed to be one atom mass unit as well. Due to the fact that electrons have actually virtually no mass, just around all the massive of one atom is in that protons and neutrons. Therefore, the total number of protons and also neutrons in an atom identify its massive in atom mass units (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in enhancement to 2 protons. Therefore the mass of most helium atom is 4 atom mass devices ((2 : extamu) because that the protons + (2 : extamu) for the neutrons). However, part helium atom have much more or much less than two neutrons. Atoms v the same number of protons but different numbers of neutron are dubbed isotopes. Due to the fact that the number of neutrons can vary for a given element, the mass number of various atoms the an aspect may additionally vary. Because that example, part helium atoms have three neutrons instead of two (these are referred to as isotopes and also are disputed in information later on).

Why execute you think that the "mass number" has protons and also neutrons, however not electrons? You understand that many of the mass of one atom is focused in that is nucleus. The massive of an atom depends on the number of protons and also neutrons. Girlfriend have currently learned the the mass of an electron is very, very small compared to the mass of either a proton or a ghost (like the massive of a penny compared to the massive of a bowling ball). Counting the number of protons and also neutrons tells scientists about 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 massive number is very easy to calculate, noted that you understand the variety of protons and also neutrons in one atom.

Example 4.5.1

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


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

(left( extnumber the neutrons ight) = 2)

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