define atomic number. Specify mass number. Identify 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 facet from atom of an additional element. Aspects are pure building material that consist of all various other matter, so each one is given a distinctive name. The names of aspects are also represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) for carbon, or (ceHe) because that helium. However, the would more powerful if this names can be offered to identify the numbers of protons and also neutrons in the atoms. That"s where atomic number and also mass numberare useful.

Figure (PageIndex1): that is an overwhelming to find characteristics that differ in between each element, and to identify one element from another. Each element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has 14 protons, and gold has 79 protons. Pictures used through permission (public domain for sulfur and silicon, yellow is licensed by CC-BY-SA-NC-ND; Alcivicpride-kusatsu.netist-hp).

Atomic Number

Scientists distinguish between different aspects by counting the number of protons in the cell nucleus (Table (PageIndex1)). If one atom has only one proton, we know that it"s a hydrogen atom. One atom with two proton is always a helium atom. If researchers count four protons in one atom, they know it"s a beryllium atom. An atom with three protons is a lithium atom, one atom with 5 protons is a boron atom, one atom with six protons is a carbon atom . . . The list goes on.

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

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms 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, since neutral atoms need to have one electron for every proton, one element"s atom number additionally tells you how numerous electrons room in a neutral atom of the element. Because that example, hydrogen has an atomic variety of 1. This means that one atom of hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, on the other hand, has actually an atomic variety of 79, which method that an atom that gold has actually 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number ((A)) of one atom is the total number of protons and also neutrons in that is nucleus. The mass of the atom is a unit called the atomic mass unit (left( extamu ight)). One atom mass unit is the massive of a proton, or around (1.67 imes 10^-27) kilograms, i m sorry is one extremely small mass. A neutron has actually just a small bit much more mass than a proton, however its mass is regularly assumed to be one atom mass unit together well. Since electrons have virtually no mass, just around all the mass of an atom is in the protons and neutrons. Therefore, the total variety of protons and also neutrons in one atom determines its massive in atom mass devices (Table (PageIndex1)).

Consider helium again. Most helium atoms have two neutron in enhancement to 2 protons. Because of this the mass of most helium atoms is 4 atomic mass systems ((2 : extamu) for the protons + (2 : extamu) because that the neutrons). However, some helium atom have an ext or much less than two neutrons. Atoms with the same variety of protons however different number of neutrons are dubbed isotopes. Due to the fact that the variety of neutrons deserve to vary for a provided element, the mass number of different atoms that an facet may likewise vary. Because that example, part helium atoms have actually three neutrons rather of two (these are called isotopes and also are discussed in detail later on).

Why perform you think the the "mass number" contains protons and neutrons, however not electrons? You recognize that many of the mass of an atom is concentrated in its nucleus. The massive of an atom relies on the number of protons and also neutrons. Friend have already learned the the massive of an electron is very, very tiny compared to the mass of one of two people a proton or a spirit (like the fixed of a penny contrasted to the mass of a bowling ball). Count the number of protons and also neutrons speak scientists about the total mass of an atom.

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

An atom"s fixed number is very easy come calculate, provided that you recognize the variety 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 of protons ight) = 2) (Remember the an atom of helium constantly has 2 protons.)

(left( extnumber of neutrons ight) = 2)

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