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

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It"s necessary to have the ability to distinguish atom of one aspect from atoms of another element. Facets are pure building material that consist of all other matter, so each one is offered a distinct name. The names of elements are additionally represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, that would an ext powerful if this names might be offered to determine the number of protons and also neutrons in the atoms. That"s wherein atomic number and also mass numberare useful.

Figure (PageIndex1): it is an overwhelming to find attributes that differ in between each element, and to differentiate one element from another. Each element, however, does have actually a unique number of protons. Sulfur has 16 protons, silicon has 14 protons, and gold has 79 protons. Pictures used v permission (public domain for sulfur and silicon, gold is licensed by CC-BY-SA-NC-ND; Alaramuseum.orgist-hp).

## Atomic Number

Scientists distinguish in between different facets by count the number of protons in the cell core (Table (PageIndex1)). If an atom has actually only one proton, we know that it"s a hydrogen atom. An atom v two protons is always a helium atom. If scientists count 4 protons in one atom, they know it"s a beryllium atom. One atom with three protons is a lithium atom, an atom with five protons is a boron atom, one atom with 6 protons is a carbon atom . . . The perform goes on.

Since an atom that one aspect can be distinguished from an atom of one more element by the variety of protons in that 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 really important since it is distinctive for atom of a given element. All atoms the an facet have the same variety of protons, and every aspect has a different variety of protons in its atoms. Because that example, every helium atoms have two protons, and no other aspects have atoms through two protons.

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

## Mass Number

The mass number ((A)) of an atom is the total number of protons and also neutrons in the nucleus. The mass of the atom is a unit dubbed the atomic mass unit (left( extamu ight)). One atomic mass unit is the massive of a proton, or around (1.67 imes 10^-27) kilograms, i m sorry is one extremely little mass. A neutron has just a small bit more mass 보다 a proton, yet its fixed is frequently assumed to it is in one atomic mass unit as well. Because electrons have actually virtually no mass, just around all the mass of an atom is in that protons and also neutrons. Therefore, the total variety of protons and also neutrons in one atom identify its fixed in atom mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutrons in enhancement to 2 protons. As such the massive of most helium atom is 4 atom mass systems ((2 : extamu) for the protons + (2 : extamu) because that the neutrons). However, some helium atoms have much more or less than 2 neutrons. Atoms with the same number of protons yet different numbers of neutrons are called isotopes. Due to the fact that the variety of neutrons can vary for a offered element, the mass numbers of different atoms of an element may likewise vary. For example, part helium atoms have three neutrons instead of 2 (these are called isotopes and are questioned in detail later on).

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

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

An atom"s mass number is really easy to calculate, listed that you understand the number of protons and also neutrons in one atom.

Example 4.5.1

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

Solution

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

(left( extnumber the neutrons ight) = 2)

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