Isotopes and Mass numbers

Unlike the number of protons, i m sorry is constantly the very same for every atoms that the exact same element, the number of neutrons deserve to be different. Atom of the same aspect with various numbers of neutron are known asisotopes. Due to the fact that the isotope of any type of given element all save the same number of protons, they have the same atomic number. However, since the isotopes of a given element contain various numbers the neutrons, different isotopes have various mass numbers.

Review the adhering to two examples to assist clarify this point.

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Example:What is the atom number (Z) and also the massive number (A) of one isotope the lithium containing 3 neutrons? A lithium atom contains 3 proton in that is nucleus.

Given-atomic number (Z)= # of protons = 3# of neutron = 3Solve-mass number (A)= # of protons + # of neutrons= 3 + 3 = 6

Example:What is the atom number (Z) and the mass number (A) of one isotope of lithium containing 4neutrons? A lithium atom consists of 3protons in that nucleus.

Given-atomic numberZ= # of protons = 3# of neutrons = 4Solve-mass number (A)= # of proton + # of neutrons= 3 + 4 = 7

Notice that since the lithium atom always has 3protons, the atom number because that lithium is alwaysZ= 3. The massive number, however, isA= 6for the isotope through 3neutrons, andA= 7for the isotope v 4neutrons. In nature, only particular isotopes exist. Because that instance, lithium exists together an isotope through 3neutrons and also as an isotope with 4neutrons, however it doesn’t exists as an isotope through 2neutrons or as an isotope v 5neutrons.

This whole discussion of isotopes brings us back to our knowledge of atom theory. Based ~ above isotopes, atoms of a given element cannot all be identical. Atom of a given element can have different numbers the neutrons, and also therefore, different mass numbers. It transforms out that facets found in nature exist as uniform mixtures through a consistent ratio of your naturally emerging isotopes. In other words, a piece of lithium constantly contains both types of naturally arising lithium (the type with 3neutrons and also the type with 4neutrons). Moreover, it always contains the two in the exact same relative amounts (or “relative abundances”). In a chunk of lithium, 93% will always be lithium v 4neutrons, while the staying 7% will constantly be lithium through 3neutrons.

Isotope Notation

Because the nucleus for an facet can differ because that the same element, we need to specify or be able to calculate the atom number (Z) and the mass number (A) for the atom. The atom have the right to be shown with anuclear symbol which takes the form

where X is the chemical symbol for the element, Ais themass number, and also Zis the atom number. Because that example, a nitrogen cell core containing 7 protons and also 8 neutrons would certainly be. Due to the fact that all nitrogenatomsmust have actually 7 protons in their nucleus, periodically the 7 is omitted and also the symbol is written simply asThis exact same practice can be used when writing the name. Omit the number of protons (or atomic number) and also identify the fixed number after the name and also hyphen, because that example, nitrogen-15.

For one more example, let"s consider hydrogen"s 3 isotopes. All hydrogen atoms need to have precisely 1 proton in the nucleus. However, the variety of neutrons in the nucleus can vary, leading to various mass numbers (see the photo below).

One isotope the hydrogen has 1 proton and also 0 neutron in the nuclues (in the photo on the left above). The nuclear symbol is written over and has the name hydrogen-1. An additional isotope of hydrogenhas 1 proton and also 1 spirit in the nuclues (in the picture in the facility above). The atom symbol is written over and has actually the surname hydrogen-2.A third isotope that hydrogenhas 1 proton and 2 neutrons in the nuclues (in the image on the best above). The nuclear symbol is written above and has the surname hydrogen-3. (Unlike plenty of other isotopes, isotopes of hydrogen are additionally referred come with committed names. The hydrogen-1 may likewise be described as protium, hydrogen-2 together deuterium, and hydrogen-3 as tritium.)

Atomic Mass and also Isotope Abundance

Knowing around the various isotopes is vital when it comes to calculating atomic mass. Theatomic mass(sometimes referred to as atom weight) that an aspect is the weighted typical mass the the atom in a naturally arising sample of the element.Atomic massive is frequently reported in atomic mass units.Most periodic tables offer the atom mass of every element. The atom mass is frequently a decimal number usually written listed below the chemical symbol the each element in the table. You can also calculate the atomic mass of one element, provided you recognize the relative abundances the element’s naturally emerging isotopes and also the masses that those various isotopes. The examples below show exactly how this calculate is done.


Boron has actually two naturally developing isotopes. In a sample of boron, 20% that the atoms are boron-10, which is an isotope of boron with 5 neutrons and a mass variety of 10 amu. The various other 80% of the atoms space boron-11, i m sorry is an isotope of boron with 6 neutrons and a mass number of 11 amu. What is the atom mass the boron?


To execute this problem, we will calculate 20% the the massive of boron-10, which is how much the boron-10 isotope contributes come the “average boron atom.” us will likewise calculate 80% of the massive of boron-11, i m sorry is exactly how much the boron-11 isotope contributes to the “average boron atom.”

Step One:Convert the percentages offered in the question into their decimal forms by splitting each percent by 100%:

Decimal type of 20% = 0.20Decimal type of 80% = 0.80

Step Two:Multiply the mass of each isotope through its relative abundance (percentage) in decimal form:

20% the the mass of boron-10 = 0.20 x 10 amu = 2.00 amu80% that the massive of boron-11 = 0.80 x 11 amu = 8.80 amu

Step Three:Find the total mass that the “average atom” by adding together the contribute from the different isotopes:

Total fixed of typical atom = 2.00 amu + 8.80 amu = 10.80 amu

The mass of an average boron atom, and also thus boron’s atomic mass, is 10.80 amu.


Neon has three naturally developing isotopes. In a sample the neon, 90.48% the the atoms room neon-20, which is one isotope the neon v 10 neutrons and also a mass variety of 19.99 amu. An additional 0.27% the the atoms areneon-21, i m sorry is an isotope the neon v 11 neutrons and also a mass number of 20.99 amu. The final 9.25% that the atom areneon-22, which is one isotope the neon v 12 neutrons and also a mass number of 21.99 amu. What is the atomic mass that neon?


To do this problem, we will certainly calculate 90.48% that the mass of neon-20, i beg your pardon is exactly how muchneon-20 contributes to the “average neon atom.” we will likewise calculate 0.27% of the massive ofneon-21 and 9.25% the the fixed ofneon-22, i m sorry are just how much theneon-21 and theneon-22 isotopes add to the “average neon atom” respectively.

Step One:Convert the percentages offered in the question into their decimal develops by separating each percentage by 100%:

Decimal form of 90.48% = 0.9048Decimal type of 0.27% = 0.0027Decimal form of 9.25% = 0.0925

Step Two:Multiply the mass of each isotope by its loved one abundance (percentage) in decimal form:

90.48% that the mass ofneon-20 =0.9048 x 20 amu = 18.10 amu0.27% the the massive of neon-21 = 0.0027 x 21 amu = 0.057 amu9.25% of the massive ofneon-22 = 0.0925 x 22 amu = 2.04 amu

Step Three:Find the complete mass that the “average atom” by adding together the contributions from the various isotopes:

Total fixed of typical atom = 18.10 amu + 0.057 amu + 2.04 amu

The massive of an typical neon atom, and also thus neon’s atomic mass, is 20.20 amu.

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Notice the atomic mass native the routine table because that boron (symbol B) is 10.81 and also the atomic mass of neon (symbol Ne) is 20.18, both which are really close come what we calculated in our examples. Take it time to notice that not all routine tables have the atom number over the element’s symbol and the atomic mass below it. If girlfriend are ever confused, remember the the atomic number is constantly a entirety number and also should constantly be the smaller sized of the two, if the atomic mass for many numbers is in decimal type and should constantly be the bigger of the two. (The atomic mass must encompass both the variety of protons and the average number of neutrons.)

Bohr Model

As discussed previously,the Bohr version is helpful for visualizing the placement of the protons, neutrons, and electrons in the atom of an element. Through the sports of neutrons in isotopes, the Bohr design of the atom must readjust - yet only slightly. For example, think about an atom of sulfur v a mass variety of 32, sulfur-32 (see photo below),

and one atom of sulfur with a mass variety of 33, sulfur-33. The similarities between the isotopes would certainly be the atom identity, therefore, the atomic number (16) and also # of proton (16) remain the same. Because it is an atom the sulfur, the proton # (16) still equates to the electron # (16). The two major differences in the Bohr version diagrams would certainly be the mass number and also # that neutrons. The calculate of massive number = # of proton + # that neturons, therefore,mass number (33) = # of proton (16) + # that neturons (?). Neturon #, climate = 17 (see photo below).