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

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

Figure (PageIndex1): the is challenging to find characteristics that differ between each element, and to differentiate one aspect from another. Each element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has 14 protons, and also gold has actually 79 protons. Images used through permission (public domain for sulfur and silicon, yellow is licensed by CC-BY-SA-NC-ND; Albsci-ch.orgist-hp).

Atomic Number

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

Since an atom the one facet can be differentiated from an atom of an additional element through the variety of protons in that nucleus, scientists are always interested in this number, and also how this number differs between different elements. The number of protons in one atom is referred to as its atomic number ((Z)). This number is really important because it is distinctive for atoms of a offered element. Every atoms that an facet have the same number of protons, and also every aspect has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other aspects have atoms through two protons.

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

Mass Number

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

Consider helium again. Many helium atoms have two neutrons in addition to two protons. Because of this the massive of many helium atoms is 4 atomic mass systems ((2 : extamu) for the proton + (2 : extamu) because that the neutrons). However, part helium atom have much more or much less than 2 neutrons. Atoms v the same variety of protons but different number of neutron are dubbed isotopes. Due to the fact that the variety of neutrons have the right to vary for a given element, the mass number of various atoms of an element may likewise vary. Because that example, some helium atoms have three neutrons instead of 2 (these are called isotopes and are disputed in detail later on).

Why execute you think that the "mass number" includes protons and also neutrons, yet not electrons? You know that most of the mass of one atom is concentrated in its nucleus. The massive of one atom counts on the variety of protons and neutrons. Friend have currently learned that the massive of an electron is very, very small compared come the massive of either a proton or a neutron (like the fixed of a penny contrasted to the massive of a bowling ball). Counting the number of protons and neutrons tells scientists about the full mass of an atom.

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

An atom"s mass number is very easy come calculate, provided that you understand the number of protons and neutrons in one atom.

Example 4.5.1

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


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

(left( extnumber the neutrons ight) = 2)

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