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

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

Figure $$\PageIndex1$$: the is challenging to find qualities that differ between each element, and also to distinguish one aspect from another. Every element, however, does have a unique number of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has 79 protons. Pictures used v permission (public domain because that sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Algimpppa.orgist-hp).

## Atomic Number

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

Since one atom that one facet can be differentiated from one atom of an additional element by the number of protons in the nucleus, scientists are always interested in this number, and how this number differs in between different elements. The number of protons in one atom is referred to as its atomic number ($$Z$$). This number is an extremely important since it is unique for atoms of a offered element. Every atoms of an facet have the same number of protons, and every aspect has a different variety of protons in the atoms. Because that example, all helium atoms have actually two protons, and no other facets have atoms with 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 for every proton, an element"s atom number additionally tells friend how plenty of electrons space in a neutral atom of that element. For example, hydrogen has actually an atomic number of 1. This means that an atom that hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, on the other hand, has an atomic number of 79, which way that one atom of 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 is nucleus. The fixed of the atom is a unit called the atom mass unit $$\left( \textamu \right)$$. One atom mass unit is the fixed of a proton, or about $$1.67 \times 10^-27$$ kilograms, i m sorry is an extremely tiny mass. A neutron has actually just a tiny bit an ext mass than a proton, but its fixed is regularly assumed to it is in one atom mass unit as well. Because electrons have actually virtually no mass, just about all the mass of an atom is in that is protons and also neutrons. Therefore, the total number of protons and neutrons in one atom identify its massive in atom mass systems (Table $$\PageIndex1$$).

Consider helium again. Many helium atoms have two neutrons in enhancement to two protons. As such the massive of most helium atoms is 4 atom mass devices ($$2 \: \textamu$$ for the proton + $$2 \: \textamu$$ for the neutrons). However, some helium atoms have an ext or much less than two neutrons. Atoms with the same variety of protons but different number of neutron are called isotopes. Due to the fact that the variety of neutrons can vary because that a provided element, the mass numbers of different atoms the an element may additionally vary. Because that example, some helium atoms have actually three neutrons instead of 2 (these are dubbed isotopes and are questioned in information later on).

Why execute you think that the "mass number" consists of protons and neutrons, but not electrons? You know that most of the massive of an atom is concentrated in the nucleus. The mass of one atom counts on the variety of protons and also neutrons. Girlfriend have already learned that the mass of an electron is very, very little compared to the mass of one of two people a proton or a spirit (like the mass of a penny compared to the massive of a bowling ball). Counting the number of protons and also neutrons speak scientists about the full mass of an atom.

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

An atom"s mass number is an extremely easy come calculate, noted that you understand the number of protons and also neutrons in an atom.

Example 4.5.1

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

Solution

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

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

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