Although every atoms that an element have the same number of protons, the atoms might differ in the variety of neutrons they have (Table 1-2). These differing atom of the same facet are dubbed isotopes. 4 isotopes the helium (He) are displayed in figure 1-1. All atoms the chlorine (Cl) have actually 17 protons, but there room chlorine isotopes having actually 15 come 23 neutrons. Only two chlorine isotopes exist in far-reaching amounts in nature, those with 18 neutrons (75.53% of all chlorine atoms uncovered in nature), and also those v 20 neutrons (24.47%). To create the symbol because that an isotope, location the atom number as a subscript and also the fixed number (protons to add neutrons) together a superscript come the left of the atomic symbol. The symbols for the two naturally occurring isotopes that chlorine then would certainly be Cl and

Cl. Strictly speaking, the subscript is unnecessary, since all atom of chlorine have actually 17 protons. Thus the isotope symbols space usually written without the subscript: 35Cl and 37Cl. In stating these isotopes, we use the. State chlorine-35 and chlorine-37. For a cell nucleus to be stable, the variety of neutrons have to (for the first couple of elements) same or slightly exceed the variety of protons. The more protons, the higher the proportion of neutrons to protons to for sure stability. Nuclei that have too plenty of of either type of an essential particle space unstable, and breakdown radioactively in methods that are debated in chapter 23.

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Figure 1-1 4 isotopes that helium (He). Every atoms of helium have two proton (hence two electrons), but the variety of neutrons deserve to vary. Many helium atoms in nature have two neutrons (helium-4), and also fewer 보다 one helium atom every million in nature has just one ghost (helium-3). The various other helium isotopes, helium-5, helium-6, and also helium-8 (not shown) space unstable and also are seen only briefly in atom reactions (see thing 23). The dimension of the nucleus is grossly exaggerated here. If the nucleus to be of the dimension shown, the atom would be fifty percent a kilometre across.

Example 1.2.1

How numerous protons, neutrons, and also electrons are there in an atom of the most stable isotope the uranium, uranium-238? compose the symbol because that this isotope. Refer to Figure. 1-1.


The atomic number of uranium (see the inside back cover) is 92, and the mass number of the isotope is offered as 238. Thus it has 92 protons, 92 electrons,and 238 - 92 = 146 neutrons. Its prize is

U (or 238U).

The total mass of one atom is referred to as its atom weight, and this is virtually but not specifically the amount of the masses the its constituent protons, neutrons and electrons. * when protons, neutrons, and also electrons incorporate to type an atom, several of their fixed is convert to energy and also is given off. (This is the source of power in nuclear blend reactions.) since the atom can not be damaged down right into its fundamental particles uneven the power for the lacking mass is offered from external it, this energy is called the binding energy the the nucleus.

Note: Atomic load vs. Atomic Mass

The state atomic weight and also molecular weight room universally provided by functioning scientists, and also will be used in this book, also though these space technically masses rather than weights.

Table 1-2. Composition of typical Atoms and also 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 cell core 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 arising 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 developing mixture that uranium 92 92 varied 92 238.03 0

Example 1.2.2

Calculate the mass the is lost when one atom that carbon-12 is created from protons, electrons, and also neutrons.


Since the atomic number of every carbon atom is 6, carbon-12 has actually 6 protons and therefore 6 electrons. To uncover the variety of neutrons, we subtract the variety of protons from the massive number: 12 - 6 = 6 neutrons. We have the right to use the data in Table 1-1 to calculate the full mass of these 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 fragment mass: 12.09900 amu

But by the meaning of the range of atom mass units, the fixed of one carbon-12 atom is exactly 12 amu. Hence 0.0990 amu the mass has actually disappeared in the procedure of structure the atom indigenous its particles.

Example 1.2.3

Calculate the supposed atomic load of the isotope the chlorine that has actually 20 neutrons. To compare this v the really atomic weight of this isotope as provided in Table 1-2.


The chlorine isotope has actually 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 identified by an atom number (total number of protons), a massive number (total variety of protons and also neutrons), and an atomic weight (mass of atom in atom mass units). Since mass losses upon development of an atom room small, the mass number is commonly the same as the atomic load rounded to the nearest integer. (For example, the atomic load of chlorine-37 is 36.966, which is rounded come 37.) If over there are number of isotopes the an element in nature, then of course the experimentally it was observed atomic weight (the natural atomic weight) will certainly be the weighted typical of the isotope weights. The mean is weighted follow to the percent variety of the isotopes. Chlorine occurs in nature as 75.53% chlorine-35 (34.97 amu) and also 24.47% chlorine-37 (36.97 amu), for this reason the weighted mean of the isotope weights is

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

The atomic weights provided inside the earlier cover of this book are all weighted averages the the isotopes arising in nature, and also these space the numbers we shall use henceforth-unless we are specifically discussing one isotope. All isotopes of an aspect behave the same way hunterriverpei.comically for the most part. Their behavior will different in regard to mass-sensitive properties such together diffusion rates, i beg your pardon we"ll look at later in this book.

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Example 1.2.4

Magnesium (Mg) has three far-ranging natural isotopes: 78.70% of all magnesium atoms have actually an atomic load of 23.985 amu, 10.13% have an atomic load of 24.986 amu, and 11.17% have actually an atomic weight of 25.983 amu. How plenty of protons and neutrons are current in every of these three isotopes? exactly how do we create the symbols for each isotope? Finally, what is the weighted mean of the atomic weights?


There room 12 proton in every magnesium isotopes. The isotope whose atomic weight is 23.985 amu has a mass number of 24 (protons and also neutrons), for this reason 24 - 12 protons gives 12 neutrons. The symbol because that this isotope is 24Mg. Similarly, the isotope whose atomic load is 24.986 amu has a mass variety of 25, 13 neutrons, and 25Mg together a symbol. The third isotope (25.983 amu) has actually a mass number of 26, 14 neutrons, and 26Mg together a symbol. Us calculate the mean atomic weight together 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 occurring isotopes, lOB and 11B. We know that 80.22% the its atoms are 11B, atomic weight 11.009 amu. Native the organic atomic weight provided on the inside ago cover, calculate the atomic weight of the lOB isotope.


If 80.22% of all boron atoms space 11B, climate 100.00 - 80.22, or 19.78%, are the unknown isotope. We can use W to stand for the unknown atomic load in ours calculation:

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