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

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It"s vital to have the ability to distinguish atom of one facet from atoms of an additional element. Aspects are pure building materials that comprise all other matter, so each one is given a unique name. The names of aspects are likewise represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, it would much more powerful if this names might be supplied to recognize the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): the is complicated to find characteristics that differ in between each element, and to differentiate one facet from another. Every element, however, does have a unique variety of protons. Sulfur has actually 16 protons, silicon has 14 protons, and gold has 79 protons. Photos used through permission (public domain because that sulfur and silicon, yellow is license is granted by CC-BY-SA-NC-ND; Allifwynnfoundation.orgist-hp).

Atomic Number

Scientists distinguish between different aspects by count the variety of protons in the cell core (Table (PageIndex1)). If an atom has only one proton, we know that it"s a hydrogen atom. One atom through two proton is always a helium atom. If scientists count four protons in an 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, an atom with six protons is a carbon atom . . . The perform goes on.

Since an atom that one facet can be identified from an atom of an additional element by the variety of protons in that is 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 very important because it is unique for atom of a given element. All atoms that an element have the same number of protons, and also every aspect has a different number of protons in the atoms. Because that example, every helium atoms have actually two protons, and no other aspects have atoms through two protons.

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

Mass Number

The mass number ((A)) of one atom is the total number of protons and neutrons in that is nucleus. The massive of the atom is a unit called the atom mass unit (left( extamu ight)). One atom mass unit is the fixed of a proton, or about (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 fixed is frequently assumed to it is in one atom mass unit together well. Because electrons have actually virtually no mass, just around all the massive of an atom is in its protons and also neutrons. Therefore, the total variety of protons and also neutrons in one atom identify its mass in atomic mass devices (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in addition to two protons. As such the massive of many helium atoms is 4 atom mass systems ((2 : extamu) for the proton + (2 : extamu) because that the neutrons). However, some helium atoms have an ext or much less than 2 neutrons. Atoms through the same variety of protons yet different number of neutron are called isotopes. Since the number of neutrons have the right to vary because that a provided element, the mass numbers of different atoms the an element may additionally vary. For example, part helium atoms have actually three neutrons instead of 2 (these are dubbed isotopes and are discussed in detail later on).

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

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

An atom"s fixed number is very easy come calculate, noted that you understand the variety of protons and neutrons in one atom.

Example 4.5.1

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


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

(left( extnumber that neutrons ight) = 2)

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