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

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It"s vital to be able to distinguish atom of one element from atom of another element. Elements are pure building materials that consist of all various other matter, so every one is given a distinctive name. The names of facets are also represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) for carbon, or (ceHe) for helium. However, it would much more powerful if these names can be used to identify the numbers of protons and also neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

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

Atomic Number

Scientists distinguish in between different facets by counting the number of protons in the cell core (Table (PageIndex1)). If one atom has only one proton, we understand that it"s a hydrogen atom. One atom through two proton is always a helium atom. If scientists count four protons in one atom, they recognize it"s a beryllium atom. An atom with 3 protons is a lithium atom, one atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . The perform goes on.

Since an atom that one aspect can be identified from one atom of one more element through the number of protons in the nucleus, researchers are constantly interested in this number, and how this number differs between different elements. The variety of protons in an atom is called its atomic number ((Z)). This number is an extremely important due to the fact that 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 is atoms. Because that 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 very first 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 for every proton, one element"s atom number additionally tells friend how plenty of electrons room in a neutral atom of that element. Because that example, hydrogen has actually an atomic number of 1. This means that one atom that hydrogen has actually one proton, and, if it"s neutral, one electron together well. Gold, top top the various other hand, has actually an atomic variety of 79, which way that one atom that 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 is nucleus. The mass of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atom mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, i m sorry is one extremely small mass. A neutron has just a small bit much more mass than a proton, however its fixed is regularly assumed to be one atomic mass unit together well. Because electrons have actually virtually no mass, just about all the massive of an atom is in its protons and neutrons. Therefore, the total variety of protons and neutrons in one atom identify its fixed in atomic mass devices (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutron in addition to two protons. Thus the fixed of most helium atom is 4 atomic mass devices ((2 : extamu) because that the protons + (2 : extamu) for the neutrons). However, some helium atom have much more or less than two neutrons. Atoms through the same variety of protons but different numbers of neutrons are called isotopes. Due to the fact that the number of neutrons can vary because that a offered element, the mass numbers of different atoms of an aspect may likewise vary. For example, some helium atoms have three neutrons instead of two (these are called isotopes and also are questioned in information later on).

Why do you think that the "mass number" has protons and also neutrons, yet not electrons? You know that most of the mass of one atom is focused in that is nucleus. The mass of one atom relies on the number of protons and also neutrons. You have currently learned the the mass of an electron is very, very little compared come the fixed of either a proton or a ghost (like the massive of a penny compared to the mass of a bowling ball). Count the number of protons and also neutrons speak scientists about the full mass of an atom.

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

An atom"s mass number is an extremely easy come calculate, noted that you understand the variety of protons and also neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber that neutrons ight) = 2)

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