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

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It"s crucial to have the ability to distinguish atoms of one facet from atom of an additional element. Aspects are pure building materials that consist of all other matter, so each one is offered a unique name. The names of aspects are likewise represented by distinctive one- or two-letter symbols, such as (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) because that helium. However, that would much more powerful if this names might be offered to identify the number of protons and neutrons in the atoms. That"s wherein atomic number and mass numberare useful.

Figure (PageIndex1): it is an overwhelming to find attributes that differ between each element, and also to differentiate one aspect from another. Every element, however, does have actually a unique variety of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Images used v permission (public domain for sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Alaufdercouch.netist-hp).

Atomic Number

Scientists distinguish in between different elements by count the number of protons in the cell core (Table (PageIndex1)). If an atom has actually only one proton, we recognize that it"s a hydrogen atom. One atom with two protons is constantly a helium atom. If scientists count 4 protons in one atom, they understand it"s a beryllium atom. An atom with three protons is a lithium atom, an atom with 5 protons is a boron atom, one atom with six protons is a carbon atom . . . The perform goes on.

Since one atom of one element can be differentiated from an atom of an additional element through the variety of protons in that nucleus, researchers are always interested in this number, and also how this number differs between different elements. The variety of protons in an atom is called its atomic number ((Z)). This number is very important since it is distinctive for atoms of a offered element. Every atoms the an aspect have the same variety of protons, and also every aspect has a different number of protons in the atoms. Because that example, all helium atoms have two protons, and also no other facets have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms of the very first Six elements
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, due to the fact that neutral atoms have to have one electron because that every proton, an element"s atomic number likewise tells girlfriend how countless electrons space in a neutral atom of the element. For example, hydrogen has actually an atomic variety of 1. This means that an atom that hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, on the various other hand, has an atomic variety of 79, which means 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 one atom is the total number of protons and also neutrons in the nucleus. The fixed of the atom is a unit called the atom mass unit (left( extamu ight)). One atom mass unit is the fixed of a proton, or about (1.67 imes 10^-27) kilograms, which is an extremely little mass. A neutron has just a tiny bit much more mass 보다 a proton, however its mass is often assumed to it is in one atomic mass unit together well. Because electrons have virtually no mass, just about all the fixed of one atom is in its protons and also neutrons. Therefore, the total variety of protons and also neutrons in an atom determines its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Most helium atoms have two neutron in enhancement to two protons. Therefore the mass of most helium atom is 4 atom mass systems ((2 : extamu) because that the proton + (2 : extamu) for the neutrons). However, part helium atoms have an ext or less than two neutrons. Atoms v the same variety of protons but different number of neutron are dubbed isotopes. Since the number of neutrons deserve to vary for a provided element, the mass numbers of different atoms the an element may likewise vary. Because that example, part helium atoms have actually three neutrons instead of two (these are called isotopes and also are discussed in detail later on).

Why do you think the the "mass number" contains protons and neutrons, yet not electrons? You recognize that most of the massive of an atom is concentrated in that nucleus. The massive of one atom relies on the variety of protons and also neutrons. You have currently learned the the mass of one electron is very, very little compared come the massive of one of two people a proton or a spirit (like the mass of a penny contrasted to the massive of a bowling ball). Counting the variety of protons and neutrons speak scientists around the full mass of an atom.

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

An atom"s fixed number is very easy to calculate, provided that you know the variety of protons and neutrons in one atom.

Example 4.5.1

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


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

(left( extnumber the neutrons ight) = 2)

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