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Learning Objectives

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

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It"s vital to be able to distinguish atoms of one element from atom of another element. Elements are pure building materials that consist of all other matter, so each one is given a distinct name. The surname of facets are likewise represented by distinct one- or two-letter symbols, such together $$\ceH$$ for hydrogen, $$\ceC$$ for carbon, or $$\ceHe$$ for helium. However, that would an ext powerful if these names might be supplied to determine the number of protons and neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure $$\PageIndex1$$: it is complicated to find attributes that differ between each element, and also to identify 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 actually 79 protons. Images used with permission (public domain because that sulfur and silicon, gold is license is granted by CC-BY-SA-NC-ND; Alyellowcomic.comist-hp).

## Atomic Number

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

Since one atom of one element can be distinguished from one atom of one more element by the variety of protons in its nucleus, scientists are constantly 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 very important since it is distinctive for atoms of a given element. Every atoms that an facet have the same number of protons, and every facet has a different number of protons in that is atoms. Because that example, all helium atoms have actually two protons, and no other aspects have atoms through two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table $$\PageIndex1$$: atoms of the an initial 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 have to have one electron for every proton, one element"s atomic number likewise tells girlfriend how numerous electrons room in a neutral atom of the element. For example, hydrogen has actually an atomic number of 1. This method that one atom that hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, top top the various other hand, has an atomic variety of 79, which means 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 also neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit $$\left( \textamu \right)$$. One atomic mass unit is the mass of a proton, or around $$1.67 \times 10^-27$$ kilograms, i beg your pardon is an extremely little mass. A neutron has just a small bit much more mass than a proton, but its mass is often assumed to it is in one atom mass unit together well. Due to the fact that electrons have virtually no mass, just about all the mass of one atom is in the protons and neutrons. Therefore, the total number of protons and also neutrons in an atom determines its mass in atomic mass devices (Table $$\PageIndex1$$).

Consider helium again. Many helium atoms have two neutron in addition to 2 protons. Because of this the fixed of most helium atom is 4 atom mass devices ($$2 \: \textamu$$ because that the protons + $$2 \: \textamu$$ because that the neutrons). However, some helium atom have more or less than 2 neutrons. Atoms with the same variety of protons but different numbers of neutrons are dubbed isotopes. Because the number of neutrons deserve to vary for a given element, the mass numbers of different atoms of an aspect may additionally vary. Because that example, some helium atoms have actually three neutrons instead of 2 (these are dubbed isotopes and also are disputed in detail later on).

Why carry out you think that the "mass number" has protons and neutrons, yet not electrons? You know that most of the mass of an atom is concentrated in the nucleus. The massive of one atom depends on the number of protons and neutrons. Friend have currently learned that the massive of an electron is very, very tiny compared to the fixed of one of two people a proton or a spirit (like the mass of a penny compared to the fixed of a bowling ball). Counting the variety of protons and also neutrons tells scientists about the total mass of an atom.

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

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

Example 4.5.1

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

Solution

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

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

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