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

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It"s vital to have the ability to distinguish atom of one element from atoms of an additional element. Facets are pure substances that comprise all other matter, so every one is given a distinctive name. The name of aspects are also represented by unique one- or two-letter symbols, such as (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, the would an ext powerful if these names might be provided to identify the number of protons and neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): that is difficult to find characteristics that differ in between each element, and to differentiate one facet from another. Every element, however, does have actually a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has actually 79 protons. Photos used through permission (public domain for sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Albarisalcity.orgist-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 with two protons is constantly a helium atom. If scientists count 4 protons in one atom, they know it"s a beryllium atom. An atom with 3 protons is a lithium atom, an atom with 5 protons is a boron atom, one atom with 6 protons is a carbon atom . . . The list goes on.

Since an atom of one element can be differentiated from an atom of another element by the variety of protons in the nucleus, researchers are always interested in this number, and also how this number differs in between different elements. The number of protons in an atom is called its atomic number ((Z)). This number is very important due to the fact that it is unique for atoms of a provided element. Every atoms of an element have the same number of protons, and also every aspect has a different number of protons in that atoms. For example, all helium atoms have actually two protons, and no other aspects have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the 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, because neutral atoms have to have one electron because that every proton, an element"s atomic number also tells girlfriend how numerous electrons are in a neutral atom of the element. For example, hydrogen has an atomic variety of 1. This method that one 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 that gold has 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number ((A)) of an atom is the total variety of protons and neutrons in that is 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 small bit an ext mass than a proton, yet its mass is regularly assumed to be one atomic mass unit as well. Due to the fact that electrons have actually virtually no mass, just about all the fixed of one atom is in its protons and neutrons. Therefore, the total variety of protons and neutrons in one atom determines its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutrons in enhancement to 2 protons. Therefore the fixed of many helium atom is 4 atom mass systems ((2 : extamu) for the proton + (2 : extamu) because that the neutrons). However, part helium atom have much more or less than 2 neutrons. Atoms v the same number of protons yet different number of neutrons are called isotopes. Because the number of neutrons can vary for a provided element, the mass numbers of various atoms the an element may also vary. For example, part helium atoms have three neutrons instead of two (these are called isotopes and also are questioned in information later on).

Why carry out you think that the "mass number" consists of protons and also neutrons, however not electrons? You understand that many of the mass of an atom is concentrated in its nucleus. The massive of one atom depends on the variety of protons and neutrons. You have already learned the the fixed of one electron is very, very little compared to the mass of one of two people a proton or a neutron (like the mass of a penny contrasted to the massive of a bowling ball). Counting the variety of protons and also neutrons tells scientists about the complete mass of one atom.

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

An atom"s fixed number is an extremely easy to calculate, provided 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 contains 2 neutrons?


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

(left( extnumber of neutrons ight) = 2)

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