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

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It"s vital to be able to distinguish atom of one aspect from atoms of one more element. Facets are pure building material that comprise all other matter, so every one is offered a distinct name. The surname of aspects are additionally represented by distinct one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) for helium. However, that would more powerful if these names might be used to recognize the number of protons and neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

Figure (PageIndex1): it is challenging to find features that differ in between each element, and also to identify one facet from another. Each element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Photos used through permission (public domain because that sulfur and silicon, gold is licensed by CC-BY-SA-NC-ND; Alchathamtownfc.netist-hp).

Atomic Number

Scientists distinguish in between different facets by counting the variety of protons in the nucleus (Table (PageIndex1)). If one atom has only one proton, we understand that it"s a hydrogen atom. An atom with two proton is constantly 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, an atom with 5 protons is a boron atom, one atom with 6 protons is a carbon atom . . . The list goes on.

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

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the an initial 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 for every proton, an element"s atomic number additionally tells you how plenty of electrons room in a neutral atom of that element. For example, hydrogen has an atomic variety of 1. This means that an atom of hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, on the various other hand, has actually an atomic number 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 one atom is the total variety of protons and also neutrons in that nucleus. The fixed of the atom is a unit dubbed the atomic mass unit (left( extamu ight)). One atom mass unit is the massive of a proton, or about (1.67 imes 10^-27) kilograms, which is one extremely little mass. A neutron has just a small bit more mass than a proton, however its massive is frequently assumed to it is in one atom mass unit together well. Since electrons have virtually no mass, just about all the fixed of an atom is in its protons and neutrons. Therefore, the total variety of protons and also neutrons in an atom identify its mass in atomic mass devices (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in addition to 2 protons. As such the fixed of many helium atoms is 4 atom mass systems ((2 : extamu) for the protons + (2 : extamu) for the neutrons). However, some helium atom have more or much less than 2 neutrons. Atoms v the same number of protons yet different numbers of neutron are called isotopes. Since the variety of neutrons can vary because that a given element, the mass numbers of different atoms of an aspect may also vary. Because that example, part helium atoms have actually three neutrons instead of two (these are referred to as isotopes and are discussed in detail later on).

Why perform you think that the "mass number" includes protons and neutrons, however not electrons? You know that many of the fixed of an atom is focused in that is nucleus. The mass of one atom depends on the variety of protons and also neutrons. Girlfriend have already learned that the mass of an electron is very, very small compared to the mass of one of two people a proton or a spirit (like the mass of a penny compared to the mass of a bowling ball). Count the number of protons and also neutrons tells scientists around the full mass of one atom.

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

An atom"s massive number is an extremely easy to calculate, provided that you understand the number of protons and neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber that neutrons ight) = 2)

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