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Learning Objectives

define atomic number. Specify mass number. Determine the variety of protons, neutrons, and also electrons in one atom.

You are watching: What one factor identifies an atom as a specific element?

It"s necessary to have the ability to distinguish atom of one aspect from atom of another element. Aspects are pure substances that make up all other matter, so every one is given a unique name. The names of elements are additionally represented by distinct one- or two-letter symbols, such as $$\ceH$$ for hydrogen, $$\ceC$$ because that carbon, or $$\ceHe$$ for helium. However, the would an ext powerful if these names could be offered to determine the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

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

## Atomic Number

Scientists distinguish between different elements by counting the number of protons in the cell nucleus (Table $$\PageIndex1$$). If one atom has actually only one proton, we know that it"s a hydrogen atom. An atom through two protons is always a helium atom. If researchers count 4 protons in one atom, they recognize 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 element can be distinguished from an atom of an additional element by the number of protons in that is nucleus, researchers are always interested in this number, and also how this number differs between different elements. The number of protons in an atom is called 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 that an aspect have the same number of protons, and also every facet has a different variety of protons in its atoms. Because that example, every helium atoms have two protons, and also no other aspects have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table $$\PageIndex1$$: atom 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 need to have one electron because that every proton, one element"s atom number likewise tells girlfriend how many electrons room in a neutral atom of that element. Because that example, hydrogen has actually an atomic number of 1. This way that one atom the hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, top top the various other hand, has actually an atomic number of 79, which means that an atom the gold has 79 protons, and, if it"s neutral, 79 electrons as well.

## Mass Number

The mass number ($$A$$) of one atom is the total variety of protons and also 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, which is an extremely small mass. A neutron has actually just a tiny bit an ext mass 보다 a proton, but its mass is frequently assumed to it is in one atom mass unit together well. Since electrons have virtually no mass, just around all the fixed of one atom is in its protons and also neutrons. Therefore, the total variety of protons and also neutrons in an atom identify its fixed in atomic mass systems (Table $$\PageIndex1$$).

Consider helium again. Most helium atoms have actually two neutrons in addition to 2 protons. Because of this the fixed of many helium atom is 4 atomic mass units ($$2 \: \textamu$$ for the proton + $$2 \: \textamu$$ for the neutrons). However, some helium atom have more or much less than two neutrons. Atoms through the same number of protons but different number of neutron are dubbed isotopes. Since the variety of neutrons deserve to vary for a provided element, the mass number of different atoms that an facet may additionally vary. Because that example, part helium atoms have actually three neutrons rather of 2 (these are called isotopes and are disputed in detail later on).

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

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

An atom"s fixed number is an extremely easy to calculate, provided that you understand the number of protons and neutrons in one atom.

Example 4.5.1

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

Solution

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

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

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