Although every atoms the an element have the same number of protons, individual atoms may have various numbers that neutrons. These differing atom are dubbed isotopes. All atoms that chlorine (Cl) have actually 17 protons, but there room chlorine isotopes through 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 also those v 20 neutrons (24.47%). To write the symbol for an isotope, location the atomic number as a subscript and the fixed number (protons plus neutrons) as a superscript come the left of the atom symbol. The symbols for the 2 naturally occurring isotopes that chlorine are written together follows: ( ^35_17 m Cl) and also ( ^37_17 m Cl). The subscript is somewhat unnecessary, due to the fact that all atoms of chlorine have 17 protons; isotope symbols are usually created without the subscript, as in 35Cl and 37Cl. In pointing out these isotopes, the terms chlorine-35 and chlorine-37 are used to differentiate in between them. In general, for an atom to it is in stable, it should have an ext neutrons 보다 protons. Nuclei with too numerous of either sort of basic particle room unstable, and failure radioactively.

You are watching: Express your answer as an isotope.

Example (PageIndex1):

How plenty of protons, neutrons, and electrons space there in an atom the uranium-238? compose the symbol for this isotope.


The atomic number of uranium (see periodic table) is 92, and the mass variety of the isotope is provided as 238. Therefore, it has 92 protons, 92 electrons, and also 238 — 92 : 146 neutrons. Its price is (ce^238_92U ) (or 238U).

The complete mass of an atom is called its atomic weight, and is the approximate amount of the masses the its ingredient protons, neutrons, and also electrons. Once protons, neutrons, and also electrons combine to kind an atom, few of their massive is convert to energy and also is provided off. (This is the source of power in nuclear fusion reactions. Since the atom can not be damaged down right into its fundamental particles uneven the energy for the missing mass is supplied from exterior it, this energy is called the binding energy the the nucleus.

Example (PageIndex2):

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


Because the atomic variety of every carbon atom is 6, carbon-12 has actually 6 protons and also therefore 6 electrons. To uncover the number of neutrons, us subtract the variety of protons native the mass number: 12 – 6 = 6 neutrons. The data in Table 1-1 can be supplied to calculate the complete mass of these particles:

Protons: 6 x 1.00728 amu = 6.04368 uNeutrons: 6 x 1.00867 amu = 6.05202 uElectrons: 6 x 0.00055 amu = 0.00330 uTotal bit mass = 12.09900 u

However, by the definition of the range of atomic mass units, the massive of one carbon-12 atom is specifically 12 amu. Therefore, 0.0990 u of mass has actually disappeared in the procedure of building the atom indigenous its particles.

Each isotope that an facet is identified by an atom number (the number of protons), a mass number (the total number of protons and also neutrons), and also an atomic load (mass that atom in atomic mass units). Because mass losses upon formation of an atom room small, the fixed number is normally the same as the atomic load rounded come the nearest integer (for example, the atomic weight of chlorine-37 is 36.966, i m sorry is rounded to 37). If over there are several isotopes of an aspect in nature, then the experimentally it was observed atomic weight (the natural atomic weight) is the weighted average of the isotope weights. The mean is weighted according to the percent variety of the isotopes. Chlorine occurs in nature as 75.53% chlorine-35 (34.97 u) and also 24.47% chlorine-37 (36.97 u), therefore the weighted median of the isotope weights is (07553 x 34.97 u) + (0.2447 x 36.97 u) = 35.46 u. The atom weights discovered in routine tables room all load averages that the isotopes emerging in nature, and these space the numbers used because that the remainder the this article, except when stating one isotope specifically. In general, all isotope of an element behave the same method tastecraftedmcd.comically. Their behaviors differ with regard come mass-sensitive properties such as diffusion rates.

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Example (PageIndex3):

Magnesium (Mg) has actually three far-ranging natural isotopes: 78.70% of all magnesium atoms have an atomic load of 23.985 u, 10.13% have an atomic load of 24.986 u, and also 11.17% have actually an atomic weight of 25.983 u. How plenty of protons and neutrons are current in every of these 3 isotopes? just how are the signs for each isotope written? Finally, what is the weighted median of the atomic weights?


There are 12 protons in all magnesium isotopes. The isotope whose atomic weight is 23.985 u has actually a mass number of 24 (protons and neutrons), for this reason 24 - 12 protons gives 12 neutrons. The symbol for this isotope is 24Mg. Similarly, the isotope whose atomic weight is 24.986 amu has actually a mass variety of 25, 13 neutrons, and also 25Mg together a symbol. The third isotope (25.983 amu) has actually a mass variety of 26, 14 neutrons, and also 26Mg as a symbol. The typical atomic load is calculated as follows:

(0.7870 x 23.985) + (0.1013 x 24.986) + (0.1117 x 25.983) = 24.31 u

Example (PageIndex4): Boron

Boron has two naturally emerging isotopes, 10B and 11B. In nature, 80.22% that its atoms room 11B, through atomic load 11.009 u. Indigenous the herbal atomic weight, calculation the atomic weight of the 10B isotope.


If 80.22% of all boron atoms space 11B, then 100.00 — 80.22, or 19.78%, are the unknown isotope. In the routine table the atomic weight of boron is discovered to it is in 10.81 u. We deserve to use W to represent the unknown atomic weight in our calculation:

< (0.8022 imes 11.009) + (0.1978 imes W) = 10.81 m u quad m (natural~atomic~weight) >

< W=dfrac10.81-8.8310.1978=10.01 m u >

Contributors and also Attributions

Dickerson, Richard E. And also Gray, take care of B. And Haight, Gilbert p (1979) tastecraftedmcd.comical principles.