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

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

Figure (PageIndex1): that is difficult to find qualities that differ in between each element, and also to differentiate one element from another. Every element, however, does have actually a unique number of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and gold has 79 protons. Pictures used with permission (public domain because that sulfur and silicon, yellow is licensed by CC-BY-SA-NC-ND; Albarisalcity.orgist-hp).

Atomic Number

Scientists distinguish in between different elements by counting the variety of protons in the cell nucleus (Table (PageIndex1)). If one atom has only one proton, we understand that it"s a hydrogen atom. One atom with two protons is always a helium atom. If scientists count 4 protons in an atom, they recognize it"s a beryllium atom. One atom with three protons is a lithium atom, one atom with five protons is a boron atom, one atom with 6 protons is a carbon atom . . . The perform goes on.

Since an atom the one aspect can be differentiated from an atom of an additional element by the number of protons in that is nucleus, researchers 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 due to the fact that it is distinct for atom of a offered element. Every atoms the an facet have the same number of protons, and also every element has a different number of protons in that atoms. Because that 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, since neutral atoms have to have one electron for every proton, an element"s atom number additionally tells friend how countless electrons are in a neutral atom of the element. Because that example, hydrogen has an atomic variety of 1. This way that an atom that hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, ~ above the other hand, has an atomic variety of 79, which way that one atom the 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 neutrons in the nucleus. The massive of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atom mass unit is the massive of a proton, or about (1.67 imes 10^-27) kilograms, i m sorry is one extremely little mass. A neutron has just a tiny bit more mass than a proton, yet its massive is often assumed to be one atomic mass unit together well. Since electrons have actually virtually no mass, just about all the massive of an atom is in that protons and also neutrons. Therefore, the total number of protons and also neutrons in one atom determines its fixed in atomic mass systems (Table (PageIndex1)).

Consider helium again. Most helium atoms have two neutron in addition to two protons. As such the massive of most helium atoms is 4 atom mass devices ((2 : extamu) for the proton + (2 : extamu) for the neutrons). However, some helium atoms have more or much less than two neutrons. Atoms through the same number of protons however different number of neutrons are called isotopes. Because the variety of neutrons deserve to vary because that a offered element, the mass numbers of different atoms the an element may also vary. For example, some helium atoms have three neutrons rather of 2 (these are dubbed isotopes and are disputed in information later on).

Why perform you think the the "mass number" has protons and also neutrons, but not electrons? You recognize that most of the mass of an atom is focused in its nucleus. The mass of an atom depends on the number of protons and also neutrons. Girlfriend have already learned that the massive of an electron is very, very small compared come the massive of either a proton or a neutron (like the mass of a penny compared to the fixed of a bowling ball). Count the variety of protons and neutrons speak scientists about the complete mass of one atom.

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

An atom"s mass number is really easy come calculate, detailed that you recognize the number of protons and also 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 the helium constantly has 2 protons.)

(left( extnumber of neutrons ight) = 2)

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