define atomic number. Define mass number. Identify the number of protons, neutrons, and electrons in one atom.

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It"s crucial to be able to distinguish atoms of one facet from atoms of another element. Aspects are pure building materials that comprise all other matter, so each one is provided a unique name. The name of aspects are also represented by distinctive one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) for helium. However, that would more powerful if this names can be offered to recognize the numbers of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): it is difficult to find features that differ between each element, and to identify one facet from another. Every element, however, does have a unique variety of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Images used v permission (public domain because that sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Alinvernessgangshow.netist-hp).

Atomic Number

Scientists distinguish in between different elements by counting the variety of protons in the cell core (Table (PageIndex1)). If one atom has only one proton, we know that it"s a hydrogen atom. An atom with two protons is always a helium atom. If researchers count 4 protons in one atom, they understand it"s a beryllium atom. An atom with 3 protons is a lithium atom, an atom with 5 protons is a boron atom, an atom with 6 protons is a carbon atom . . . The perform goes on.

Since an atom that one aspect can be identified from an atom of an additional element by the number of protons in the nucleus, scientists are always interested in this number, and how this number differs between different elements. The variety of protons in one atom is referred to as its atomic number ((Z)). This number is really important because it is distinct for atoms of a provided element. Every atoms that an element have the same number of protons, and every aspect has a different number of protons in the atoms. For example, all helium atoms have actually two protons, and no other elements have atoms with 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 atom number likewise tells friend how numerous electrons room in a neutral atom of the element. For example, hydrogen has an atomic variety of 1. This method that one atom that hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, on the various other hand, has an atomic number of 79, which way that an 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 variety of protons and neutrons in its nucleus. The mass 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 m sorry is an extremely small mass. A neutron has actually just a small bit more mass than a proton, however its massive is often assumed to be one atom mass unit together well. Due to the fact that electrons have virtually no mass, just around all the fixed of an atom is in its protons and also neutrons. Therefore, the total number of protons and neutrons in one atom identify its fixed in atom mass devices (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutron in enhancement to 2 protons. Because of this the fixed of most helium atom is 4 atom mass units ((2 : extamu) because that the protons + (2 : extamu) because that the neutrons). However, part helium atoms have more or much less than 2 neutrons. Atoms v the same variety of protons however different number of neutron are called isotopes. Since the number of neutrons can vary for a offered element, the mass number of different atoms of an aspect may also vary. Because that example, some helium atoms have actually three neutrons rather of 2 (these are referred to as isotopes and are disputed in information later on).

Why execute you think the the "mass number" has protons and neutrons, yet not electrons? You know that many of the massive of one atom is focused in its nucleus. The massive of one atom relies on the variety of protons and also neutrons. Friend have already learned the the massive of one electron is very, very small compared come the mass of either a proton or a ghost (like the mass of a penny contrasted to the massive of a bowling ball). Counting the variety of protons and also neutrons tells scientists around the total mass of one atom.

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

An atom"s mass number is really easy to calculate, provided that you understand the variety of protons and neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber the neutrons ight) = 2)

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