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

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It"s necessary to have the ability to distinguish atom of one element from atom of an additional element. Facets are pure building materials that comprise all various other matter, so each one is given a distinctive name. The names of facets are additionally represented by unique one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) because that helium. However, that would much more powerful if this names can be supplied to recognize the numbers of protons and neutrons in the atoms. That"s where atomic number and also mass numberare useful.

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

Atomic Number

Scientists distinguish between different elements 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. One atom through two proton is constantly a helium atom. If researchers count four protons in an atom, they understand it"s a beryllium atom. An atom with 3 protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . The list goes on.

Since one atom the one element can be differentiated from one atom of an additional element by the number of protons in the nucleus, researchers are always interested in this number, and how this number differs in between different elements. The number of protons in one atom is dubbed its atomic number ((Z)). This number is really important since it is unique for atoms of a offered element. Every atoms that an facet have the same variety of protons, and every element has a different variety of protons in the atoms. For example, all helium atoms have actually two protons, and no other facets have atoms v two protons.

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

Mass Number

The mass number ((A)) of one atom is the total variety of protons and also neutrons in its nucleus. The fixed of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atomic mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, i m sorry is an extremely little mass. A neutron has actually just a tiny bit more mass than a proton, however its massive is regularly assumed to be one atom mass unit as well. Since electrons have virtually no mass, just around all the massive of one atom is in that protons and also neutrons. Therefore, the total number of protons and neutrons in an atom identify its mass in atomic mass devices (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutron in enhancement to two protons. Thus the fixed of many helium atoms is 4 atom mass units ((2 : extamu) for the proton + (2 : extamu) because that the neutrons). However, some helium atoms have an ext or much less than two neutrons. Atoms through the same number of protons yet different number of neutrons are dubbed isotopes. Because the number of neutrons can vary for a given element, the mass numbers of various atoms of an facet may also vary. Because that example, part helium atoms have three neutrons rather of two (these are called isotopes and also are questioned in information later on).

Why carry out you think the the "mass number" has protons and also neutrons, however not electrons? You recognize that most of the mass of an atom is concentrated in its nucleus. The fixed of an atom depends on the number of protons and also neutrons. Friend have already learned that the fixed of an electron is very, very small compared come the fixed of either a proton or a spirit (like the mass of a penny compared to the fixed of a bowling ball). Counting the variety of protons and neutrons speak scientists about the complete mass of an atom.

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

An atom"s fixed number is very easy to calculate, detailed that you recognize the number of protons and also neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber of neutrons ight) = 2)

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