Although all atoms that an element have the same variety of protons, the atoms may differ in the number of neutrons they have (Table 1-2). These differing atoms of the same element are called isotopes. 4 isotopes of helium (He) are shown in figure 1-1. Every atoms the chlorine (Cl) have actually 17 protons, but there space chlorine isotopes having actually 15 come 23 neutrons. Just two chlorine isotope exist in far-reaching amounts in nature, those through 18 neutron (75.53% of every chlorine atoms discovered in nature), and those with 20 neutrons (24.47%). To compose the symbol for an isotope, place the atomic number together a subscript and the mass number (protons plus neutrons) together a superscript come the left the the atom symbol. The signs for the two naturally occurring isotopes that chlorine then would be Cl and also

Cl. Strictly speaking, the subscript is unnecessary, because all atom of chlorine have actually 17 protons. For this reason the isotope symbols space usually composed without the subscript: 35Cl and 37Cl. In pointing out these isotopes, we use the. State chlorine-35 and also chlorine-37. Because that a cell nucleus to be stable, the number of neutrons should (for the first couple of elements) same or contempt exceed the number of protons. The more protons, the higher the proportion of neutrons to proton to ensure stability. Nuclei that have too countless of either sort of fundamental particle room unstable, and failure radioactively in methods that are debated in chapter 23.

You are watching: Cl-35 protons neutrons electrons


Figure 1-1 4 isotopes of helium (He). Every atoms of helium have actually two proton (hence 2 electrons), yet the variety of neutrons deserve to vary. Many helium atom in nature have actually two neutron (helium-4), and also fewer than one helium atom per million in nature has actually just one spirit (helium-3). The various other helium isotopes, helium-5, helium-6, and also helium-8 (not shown) are unstable and are seen just briefly in atom reactions (see thing 23). The dimension of the cell core is grossly exaggerated here. If the nucleus were of the size shown, the atom would be fifty percent a kilometer across.

Example 1.2.1

How countless protons, neutrons, and also electrons are there in an atom the the many stable isotope of uranium, uranium-238? create the symbol for this isotope. Refer to Figure. 1-1.


The atomic variety of uranium (see the inside ago cover) is 92, and also the mass variety of the isotope is given as 238. For this reason it has 92 protons, 92 electrons,and 238 - 92 = 146 neutrons. Its prize is

U (or 238U).

The complete mass of an atom is called its atomic weight, and also this is nearly but not specifically the amount of the masses that its constituent protons, neutrons and also electrons. * once protons, neutrons, and also electrons incorporate to type an atom, several of their massive is converted to energy and also is provided off. (This is the resource of power in nuclear blend reactions.) due to the fact that the atom cannot be damaged down into its fundamental particles uneven the power for the missing mass is gave from exterior it, this energy is referred to as the binding energy that the nucleus.

Note: Atomic weight vs. Atomic Mass

The state atomic weight and molecular weight room universally used by working scientists, and also will be supplied in this book, even though these are technically masses rather than weights.

Table 1-2. Composition of typical Atoms and Ions

Electrons proton Neutrons



Atomic Weight


Total Charge

(electron units)

Hydrogen atom, 1H or H 1 1 0 1 1.008 0
Deuterium atom, 2H or D 1 1 1 1 2.014 0
Tritium atom, 3H or T 1 1 2 1 3.016 0
Hydrogen ion, H+ 0 1 0 1 1.007 +1
Helium atom, 4He 2 2 2 2 4.003 0
Helium nucleus or alpha particle, He2+ or α 0 2 2 2 4.002 +2
Lithium atom, 7Li 3 3 4 3 7.016 0
Carbon atom, 12Ca 6 6 6 6 12.000 0
Oxygen atom, 16O 8 8 8 8 15.995 0
Chlorine atom, 35Cl 17 17 18 17 34.969 0
Chlorine atom, 37Cl 17 17 20 17 36.966 0
Naturally developing mixture that chlorine 17 17 18 or 20 17 35.453 0
Uranium atom, 234U 92 92 142 92 234.04 0
Uranium atom, 235U 92 92 143 92 235.04 0
Uranium atom, 238U 92 92 146 92 238.05 0
Naturally arising mixture that uranium 92 92 varied 92 238.03 0

Example 1.2.2

Calculate the mass that is shed when one atom of carbon-12 is developed from protons, electrons, and neutrons.


Since the atomic number of every carbon atom is 6, carbon-12 has actually 6 protons and also therefore 6 electrons. To discover the variety of neutrons, us subtract the variety of protons native the fixed number: 12 - 6 = 6 neutrons. We have the right to use the data in Table 1-1 to calculation the full mass of this particles:

Protons: 6 X 1.00728 amu = 6.04368 amu
Neutrons: 6 X 1.00867 amu = 6.05202 amu
Electrons: 6 X 0.00055 amu = 0.00330 amu
Total bit mass: 12.09900 amu

But through the definition of the scale of atom mass units, the mass of one carbon-12 atom is precisely 12 amu. Thus 0.0990 amu of mass has disappeared in the procedure of building the atom indigenous its particles.

Example 1.2.3

Calculate the meant atomic weight of the isotope the chlorine that has 20 neutrons. To compare this v the actual atomic load of this isotope as offered in Table 1-2.


The chlorine isotope has 17 protons and also 20 neutrons:

Protons: 17 X 1.00728 amu = 17.1238 amu
Neutrons: 20 X 1.00867 amu = 20.1734 amu
Electrons: 17 X 0.00055 amu = 0.0094 amu
Total bit mass: 37.3066 amu
Actual observed atom weight: 36.966 amu
Mass Loss: 0.341 amu

Each isotope of an facet is defined by an atomic number (total number of protons), a massive number (total variety of protons and also neutrons), and an atomic weight (mass the atom in atom mass units). Since mass accident upon development of an atom space small, the mass number is commonly the exact same as the atomic load rounded come the nearest integer. (For example, the atomic load of chlorine-37 is 36.966, i beg your pardon is rounded come 37.) If over there are number of isotopes that an facet in nature, climate of course the experimentally observed atomic load (the natural atomic weight) will be the weighted average of the isotope weights. The median is weighted according to the percent abundance of the isotopes. Chlorine occurs in nature together 75.53% chlorine-35 (34.97 amu) and also 24.47% chlorine-37 (36.97 amu), so the weighted mean of the isotope weights is

\<(0.7553 \times 34.97 \;amu) + (0.2447 \times 36.97\; amu) = 35.46\; amu\>

The atom weights given inside the back cover of this book are all weighted averages of the isotopes emerging in nature, and also these room the numbers we shall use henceforth-unless we room specifically mentioning one isotope. All isotopes of an facet behave the same means snucongo.orgically because that the most part. Their actions will different in regard to mass-sensitive properties such together diffusion rates, i m sorry we"ll look at at later in this book.

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Example 1.2.4

Magnesium (Mg) has actually three significant natural isotopes: 78.70% of all magnesium atoms have an atomic load of 23.985 amu, 10.13% have an atomic load of 24.986 amu, and also 11.17% have an atomic weight of 25.983 amu. How plenty of protons and neutrons are current in every of these 3 isotopes? just how do we compose the icons for each isotope? Finally, what is the weighted average of the atomic weights?


There room 12 protons in all magnesium isotopes. The isotope who atomic load is 23.985 amu has a mass variety of 24 (protons and also neutrons), for this reason 24 - 12 protons offers 12 neutrons. The symbol because that this isotope is 24Mg. Similarly, the isotope who atomic weight is 24.986 amu has actually a mass number of 25, 13 neutrons, and also 25Mg together a symbol. The 3rd isotope (25.983 amu) has a mass variety of 26, 14 neutrons, and also 26Mg together a symbol. We calculate the mean atomic weight as follows:

(0.7870 X 23.985) + (0.1013 X 24.986) + (0.1117 X 25.983) = 24.31 amu

Example 1.2.5

Boron has two naturally emerging isotopes, lOB and 11B. We know that 80.22% the its atoms space 11B, atomic load 11.009 amu. From the organic atomic weight given on the inside earlier cover, calculate the atomic weight of the lOB isotope.


If 80.22% of all boron atoms room 11B, climate 100.00 - 80.22, or 19.78%, space the unknown isotope. We can use W to represent the unknown atomic weight in our calculation:

(0.8022 X 11.009) + (0.1978 X W) = 10.81 amu (natural atomic weight) W =
= 10.01 amu