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

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It"s vital to have the ability to distinguish atom of one aspect from atoms of one more element. Aspects are pure building material that comprise all other matter, so each one is provided a unique name. The names of elements are additionally represented by distinct one- or two-letter symbols, such as (ceH) because that hydrogen, (ceC) for carbon, or (ceHe) for helium. However, that would more powerful if these names could be supplied to identify the number of protons and neutrons in the atoms. That"s wherein atomic number and also mass numberare useful.

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

Atomic Number

Scientists distinguish in between different elements by count the variety of protons in the nucleus (Table (PageIndex1)). If one atom has only one proton, we know that it"s a hydrogen atom. An 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 the one aspect can be differentiated from an atom of one more element by the variety of protons in that nucleus, researchers are constantly interested in this number, and also 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 really important since it is distinct for atom of a offered element. All atoms the an aspect have the same number of protons, and also every element has a different number of protons in the atoms. Because that example, every helium atoms have two protons, and also no other facets have atoms with 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 have to have one electron because that every proton, one element"s atomic number also tells friend how countless electrons space in a neutral atom of that element. For example, hydrogen has an atomic number of 1. This means that an atom of hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, top top the various other hand, has actually an atomic number of 79, which way that an atom the 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 variety of protons and also neutrons in that nucleus. The fixed of the atom is a unit referred to as the atomic mass unit (left( extamu ight)). One atomic mass unit is the massive of a proton, or about (1.67 imes 10^-27) kilograms, i m sorry is one extremely tiny mass. A neutron has actually just a tiny bit more mass than a proton, yet its fixed is regularly assumed to it is in one atom mass unit together well. Since electrons have actually virtually no mass, just around all the massive of one atom is in that is protons and also neutrons. Therefore, the total number of protons and neutrons in one atom determines its mass in atom mass devices (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutron in enhancement to 2 protons. Thus the fixed of many helium atom is 4 atomic mass devices ((2 : extamu) for the protons + (2 : extamu) because that the neutrons). However, part helium atom have much more or less than 2 neutrons. Atoms v the same number of protons yet different numbers of neutrons are called isotopes. Because the variety of neutrons can vary because that a offered element, the mass number of different atoms of an facet may additionally vary. Because that example, some helium atoms have actually three neutrons rather of 2 (these are dubbed isotopes and also are debated in information later on).

Why carry out you think the the "mass number" includes protons and neutrons, yet not electrons? You recognize that many of the fixed of an atom is focused in its nucleus. The mass of one atom counts on the number of protons and neutrons. You have already learned that the mass of one electron is very, very little compared to the mass of either a proton or a neutron (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 one atom.

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

An atom"s mass number is very easy to calculate, provided that you know 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 that protons ight) = 2) (Remember that an atom the helium constantly has 2 protons.)

(left( extnumber the neutrons ight) = 2)

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