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

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It"s essential to have the ability to distinguish atom of one element from atom of one more element. Aspects are pure building material that comprise all other matter, so every one is provided a distinct name. The names of facets are likewise represented by distinctive one- or two-letter symbols, such together (ceH) for hydrogen, (ceC) because that carbon, or (ceHe) because that helium. However, the would much more powerful if these names can be offered to identify the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): the is an overwhelming to find qualities that differ in between each element, and also to distinguish one aspect from another. Each element, however, does have a unique variety of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and also gold has 79 protons. Pictures used with permission (public domain because that sulfur and also silicon, yellow is license is granted by CC-BY-SA-NC-ND; Alpiersonforcongress.comist-hp).

Atomic Number

Scientists distinguish between different facets by count the variety of protons in the cell core (Table (PageIndex1)). If an atom has only one proton, we know that it"s a hydrogen atom. One atom through two protons is constantly a helium atom. If researchers count 4 protons in an atom, they understand it"s a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, one atom with 6 protons is a carbon atom . . . The list goes on.

Since an atom the one facet can be distinguished from an atom of one more element by the number of protons in that nucleus, researchers are always interested in this number, and also how this number differs between different elements. The number of protons in an atom is dubbed its atomic number ((Z)). This number is very important since it is distinctive for atom of a provided element. Every atoms that an element have the same number of protons, and also every element has a different number of protons in the atoms. For example, every helium atoms have actually two protons, and also no other aspects have atoms v two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom 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, because neutral atoms have to have one electron for every proton, an element"s atomic number additionally tells friend how plenty of electrons room in a neutral atom of that element. For example, hydrogen has actually an atomic number of 1. This means that one atom the hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, top top the various other hand, has actually an atomic number 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 one atom is the total number of protons and neutrons in that is nucleus. The massive of the atom is a unit referred to as the atomic mass unit (left( extamu ight)). One atom mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, i beg your pardon is an extremely tiny mass. A neutron has actually just a tiny bit much more mass than a proton, however its massive is regularly assumed to it is in one atom mass unit as well. Since electrons have actually virtually no mass, just around all the fixed of one atom is in the protons and also neutrons. Therefore, the total variety of protons and neutrons in an atom determines its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutron in addition to 2 protons. Because of this the mass of many helium atom is 4 atomic mass units ((2 : extamu) because that the proton + (2 : extamu) because that the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons yet different numbers of neutrons are called isotopes. Because the number of neutrons deserve to vary because that a offered element, the mass number of different atoms that an aspect may additionally vary. Because that example, part helium atoms have three neutrons rather of 2 (these are called isotopes and are disputed in information later on).

Why carry out you think the the "mass number" consists of protons and neutrons, however not electrons? You know that most of the massive of one atom is focused in that nucleus. The massive of an atom relies on the number of protons and neutrons. Friend have already learned that the mass of one electron is very, very tiny compared to the fixed of either a proton or a neutron (like the fixed of a penny contrasted to the fixed of a bowling ball). Count the number of protons and neutrons speak scientists around the full mass of one atom.

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

An atom"s mass number is an extremely easy to calculate, provided that you recognize 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 includes 2 neutrons?


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

(left( extnumber of neutrons ight) = 2)

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