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

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It\"s vital to have the ability to distinguish atom of one aspect from atom of an additional element. Elements are pure building materials that consist of all other matter, so each one is given a unique name. The name of elements are also represented by unique one- or two-letter symbols, such together \$$\\ceH\$$ for hydrogen, \$$\\ceC\$$ for carbon, or \$$\\ceHe\$$ because that helium. However, that would more powerful if these names might be offered to recognize the numbers of protons and neutrons in the atoms. That\"s where atomic number and also mass numberare useful.

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

## Atomic Number

Scientists distinguish between different elements by counting 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 v two proton is always a helium atom. If researchers count 4 protons in one atom, they recognize it\"s a beryllium atom. An atom with three protons is a lithium atom, one atom with five protons is a boron atom, an atom with 6 protons is a carbon atom . . . The list goes on.

Since an atom that one aspect can be distinguished from an atom of one more element by the number of protons in that is nucleus, scientists are always interested in this number, and how this number differs between different elements. The variety of protons in an atom is called its atomic number (\$$Z\$$). This number is very important because it is distinctive for atoms of a offered element. All atoms that an aspect have the same number of protons, and also every element has a different variety of protons in that atoms. Because that example, every helium atoms have two protons, and also no other aspects have atoms v two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table \$$\\PageIndex1\$$: atom of the an initial Six elements
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 likewise tells girlfriend how countless electrons are in a neutral atom of that element. Because that example, hydrogen has actually an atomic variety of 1. This means that one atom the hydrogen has actually one proton, and, if it\"s neutral, one electron together well. Gold, ~ above the various other hand, has actually an atomic variety 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 one atom is the total number of protons and neutrons in that nucleus. The mass of the atom is a unit referred to as the atom mass unit \$$\\left( \\textamu \\right)\$$. One atomic mass unit is the massive of a proton, or about \$$1.67 \\times 10^-27\$$ kilograms, i m sorry is an extremely tiny mass. A neutron has just a small bit an ext mass than a proton, however its fixed is regularly assumed to it is in one atomic mass unit together well. Since electrons have virtually no mass, just about all the massive of one atom is in that protons and also neutrons. Therefore, the total number of protons and also neutrons in an atom identify its mass in atom mass units (Table \$$\\PageIndex1\$$).

Consider helium again. Many helium atoms have actually two neutrons in addition to 2 protons. Therefore the massive of most helium atom is 4 atomic mass units (\$$2 \\: \\textamu\$$ for the protons + \$$2 \\: \\textamu\$$ because that the neutrons). However, some helium atoms have an ext or much less than two neutrons. Atoms with the same number of protons but different numbers of neutron are called isotopes. Since the variety of neutrons deserve to vary because that a offered element, the mass numbers of various atoms of an element may also vary. For example, part helium atoms have three neutrons rather of two (these are referred to as isotopes and also are disputed in information later on).

Why carry out you think that the \"mass number\" contains protons and also neutrons, yet not electrons? You understand that most of the fixed of one atom is focused in its nucleus. The fixed of one atom counts on the number of protons and neutrons. You have already learned that the mass of one electron is very, very tiny compared come the massive of either a proton or a neutron (like the fixed of a penny compared to the fixed of a bowling ball). Count the variety of protons and also neutrons speak scientists about the complete mass of an atom.

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\\<\\textmass number \\: A = \\left( \\textnumber the protons \\right) + \\left( \\textnumber the neutrons \\right)\\>

An atom\"s massive number is really easy to calculate, provided that you know the number of protons and neutrons in one atom.

Example 4.5.1

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

Solution

\$$\\left( \\textnumber that protons \\right) = 2\$$ (Remember that an atom the helium constantly has 2 protons.)

\$$\\left( \\textnumber that neutrons \\right) = 2\$$

\$$\\textmass number = \\left( \\textnumber the protons \\right) + \\left( \\textnumber of neutrons \\right)\$$