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

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It"s essential to be able to distinguish atom of one element from atoms of one more element. Facets are pure building materials that make up all other matter, so each one is offered a distinctive name. The name of elements are additionally represented by unique one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) because that helium. However, it would more powerful if this names might be offered to identify the numbers of protons and also neutrons in the atoms. That"s whereby atomic number and mass numberare useful.

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

Atomic Number

Scientists distinguish in between different aspects by counting the variety of protons in the cell core (Table (PageIndex1)). If one atom has actually only one proton, we know that it"s a hydrogen atom. An atom with two protons is constantly a helium atom. If researchers count 4 protons in one atom, they recognize it"s a beryllium atom. An atom with 3 protons is a lithium atom, one 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 aspect can be distinguished from an atom of another element by the number of protons in that is nucleus, scientists are constantly interested in this number, and also how this number differs between different elements. The variety of protons in an atom is dubbed its atomic number ((Z)). This number is very important since it is distinct for atom of a offered element. Every atoms the an aspect have the same number of protons, and also every facet has a different number of protons in the atoms. Because that example, all helium atoms have actually two protons, and also no other elements have atoms through two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms 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, due to the fact that neutral atoms have to have one electron for every proton, an element"s atomic number additionally tells friend how plenty of electrons are in a neutral atom of the element. For example, hydrogen has an atomic number of 1. This method that an atom that hydrogen has actually one proton, and, if it"s neutral, one electron together well. Gold, ~ above the various other hand, has an atomic variety of 79, which method that one atom of 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 the nucleus. The massive of the atom is a unit dubbed the atom mass unit (left( extamu ight)). One atom mass unit is the fixed of a proton, or around (1.67 imes 10^-27) kilograms, which is an extremely little mass. A neutron has just a small bit an ext mass 보다 a proton, but its fixed is often assumed to be one atom mass unit together well. Because electrons have virtually no mass, just about all the massive of an atom is in that is protons and also neutrons. Therefore, the total number of protons and neutrons in one atom determines its massive in atomic mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in enhancement to 2 protons. Because of this the fixed of most helium atom is 4 atom mass units ((2 : extamu) because that the protons + (2 : extamu) because that the neutrons). However, some helium atoms have more or less than 2 neutrons. Atoms through the same variety of protons but different numbers of neutron are called isotopes. Because the variety of neutrons can vary because that a given element, the mass numbers of various atoms the an element may likewise vary. Because that example, part helium atoms have three neutrons rather of 2 (these are dubbed isotopes and also are discussed in information later on).

Why perform you think the the "mass number" contains protons and also neutrons, but not electrons? You recognize that many of the massive of one atom is concentrated in that nucleus. The mass of one atom depends on the variety of protons and neutrons. Friend have already learned that the massive of an electron is very, very little compared come the massive of either a proton or a ghost (like the fixed of a penny contrasted to the massive of a bowling ball). Counting the number of protons and neutrons speak scientists around the total mass of an atom.

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

An atom"s mass number is very easy come 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 of helium that contains 2 neutrons?


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

(left( extnumber of neutrons ight) = 2)

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