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

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It"s necessary to have the ability to distinguish atoms of one element from atoms of one more element. Aspects are pure building material that comprise all various other matter, so every one is given a distinctive name. The name of elements are additionally represented by distinctive one- or two-letter symbols, such together $$\ceH$$ for hydrogen, $$\ceC$$ because that carbon, or $$\ceHe$$ for helium. However, the would much more powerful if this names might be supplied to determine the numbers of protons and also neutrons in the atoms. That"s wherein atomic number and mass numberare useful.

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

## Atomic Number

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

Since one atom of one element can be differentiated from an atom of an additional element by the number of protons in that is nucleus, researchers are constantly interested in this number, and also how this number differs between different elements. The number of protons in an atom is dubbed its atomic number ($$Z$$). This number is very important due to the fact that it is distinct for atoms of a given element. All atoms the an facet have the same variety of protons, and also every aspect has a different number of protons in that is atoms. Because that example, all helium atoms have actually two protons, and no other elements have atoms v 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 for every proton, one element"s atomic number likewise tells you how countless electrons are in a neutral atom of the element. Because that 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 together well. Gold, ~ above the various other hand, has actually an atomic variety of 79, which means that an atom the gold has actually 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 the nucleus. The fixed of the atom is a unit called the atomic mass unit $$\left( \textamu \right)$$. One atom mass unit is the mass of a proton, or about $$1.67 \times 10^-27$$ kilograms, i m sorry is an extremely little mass. A neutron has just a tiny bit more mass than a proton, yet its fixed is regularly assumed to be one atomic mass unit together well. Since electrons have virtually no mass, just around all the massive of one atom is in the protons and neutrons. Therefore, the total number of protons and neutrons in one atom identify its massive in atomic mass devices (Table $$\PageIndex1$$).

Consider helium again. Most helium atoms have actually two neutrons in enhancement to 2 protons. Because of this the fixed of most helium atom is 4 atom mass units ($$2 \: \textamu$$ because that the protons + $$2 \: \textamu$$ because that the neutrons). However, part helium atom have much more or much less than two neutrons. Atoms through the same variety of protons however different number of neutrons are dubbed isotopes. Due to the fact that the variety of neutrons deserve to vary for a offered element, the mass number of different atoms that an element may additionally vary. For example, some helium atoms have three neutrons instead of two (these are called isotopes and are disputed in detail later on).

Why carry out you think the the "mass number" contains protons and also neutrons, however not electrons? You recognize that most of the massive of one atom is concentrated in its nucleus. The massive of an atom counts on the number of protons and neutrons. Girlfriend have currently learned that the fixed of an electron is very, very small compared come the massive of either a proton or a spirit (like the fixed of a penny compared to the massive of a bowling ball). Counting the variety of protons and neutrons tells scientists around the complete mass of one atom.

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

An atom"s fixed number is an extremely easy come calculate, listed that you understand the variety 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

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

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

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