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

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It"s essential to be able to distinguish atoms of one element from atom of another element. Elements are pure building material that make up all various other matter, so every one is given a distinct name. The surname 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, the would much more powerful if these names might be provided to determine the number of protons and neutrons in the atoms. That"s wherein atomic number and mass numberare useful.

Figure (PageIndex1): the is challenging to find features that differ between each element, and also to identify one element from another. Each element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has 79 protons. Pictures used with permission (public domain because that sulfur and silicon, yellow is license is granted by CC-BY-SA-NC-ND; Alaltoalsimce.orgist-hp).

Atomic Number

Scientists distinguish between different elements by counting the number of protons in the cell core (Table (PageIndex1)). If an atom has actually only one proton, we understand that it"s a hydrogen atom. One atom through two protons is constantly a helium atom. If scientists count 4 protons in an atom, they recognize it"s a beryllium atom. One atom with 3 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 an atom that one aspect can be differentiated from one atom of another element by the number of protons in the nucleus, scientists are constantly interested in this number, and how this number differs between different elements. The number of protons in one atom is referred to as its atomic number ((Z)). This number is an extremely important because it is unique for atom of a offered element. All atoms the an facet have the same number of protons, and every aspect has a different number of protons in that is atoms. For example, every helium atoms have two protons, and no other facets have atoms through two protons.

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

Mass Number

The mass number ((A)) of one atom is the total variety of protons and neutrons in its nucleus. The massive of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atomic mass unit is the massive of a proton, or around (1.67 imes 10^-27) kilograms, i beg your pardon is one extremely little mass. A neutron has just a small bit much more mass than a proton, but its massive is frequently assumed to it is in one atomic mass unit as well. Due to the fact that electrons have actually virtually no mass, just about all the mass of an atom is in the protons and neutrons. Therefore, the total number of protons and neutrons in one atom determines its massive in atom mass units (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutrons in enhancement to 2 protons. Thus the fixed of many helium atoms is 4 atomic mass systems ((2 : extamu) for the protons + (2 : extamu) for the neutrons). However, part helium atoms have more or much less than two neutrons. Atoms through the same number of protons however different numbers of neutrons are referred to as isotopes. Due to the fact that the variety of neutrons can vary for a provided element, the mass number of different atoms the an aspect may additionally vary. For example, part helium atoms have three neutrons instead of two (these are dubbed isotopes and also are discussed in detail later on).

Why execute you think the the "mass number" has protons and also neutrons, but not electrons? You know that many of the fixed of one atom is concentrated in the nucleus. The fixed of one atom depends on the variety of protons and neutrons. You have currently learned that the massive of one electron is very, very small compared to the fixed of either a proton or a spirit (like the fixed of a penny contrasted to the mass of a bowling ball). Counting the variety of protons and neutrons speak scientists about the total mass of an atom.

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

An atom"s massive number is really easy to calculate, listed that you know the variety of protons and neutrons in an atom.

Example 4.5.1

What is the mass variety of an atom that helium that has 2 neutrons?


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

(left( extnumber the neutrons ight) = 2)

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