Isotopes Are Atoms Of The Same Element With Different Numbers Of

Posted : admin On 1/29/2022

Learning Objectives

Excellent question! As you know, an atom has three basic subatomic particles: electrons, neutrons and protons. These three basic particles are like the “fingerprints” of an element. These are what differentiate, let’s just say, a Hydrogen atom fro. Isotopes are atoms of the same element that contain an identical number of protons, but a different number of neutrons. Despite having different numbers of neutrons, isotopes of the same element have very similar physical properties. Some isotopes are unstable and will undergo radioactive decay to become other elements. Isotopes are a form of an element that has the same number of protons but a different number of neutrons. In general, atoms want to have the same number of neutrons and protons but the number of neutrons can change. There are 3 different isotopes of hydrogen which are shown in the figure below. A new element, Tyserium (Ty), has recently been discovered and consists of two isotopes. One isotope has a mass of 331 g/mol and is 35.0% abundant. The other isotope is 337 g/mole and is 65.0% abundant. What is the mass of Ty as it appears on the periodic table?

  • Explain what isotopes are and how an isotope affects an element's atomic mass.
  • Determine the number of protons, electrons, and neutrons of an element with a given mass number.

All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same physical and chemical properties because they have the same numbers of protons and electrons.

An Example: Hydrogen Isotopes

Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in Figure (PageIndex{1}). Most hydrogen atoms have just one proton, one electron, and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium.

For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13.

Isotopes Are Atoms Of The Same Element With Different Numbers Of

Example (PageIndex{1}): Lithium Isotopes

  1. What is the atomic number and the mass number of an isotope of lithium containing 3 neutrons?
  2. What is the atomic number and the mass number of an isotope of lithium containing 4 neutrons?

Solution

A lithium atom contains 3 protons in its nucleus irrespective of the number of neutrons or electrons.

a.

[ begin{align}text{atomic number} = left( text{number of protons} right) &= 3 nonumber left( text{number of neutrons} right) &= 3 nonumberend{align} nonumber ]

[ begin{align} text{mass number} & = left( text{number of protons} right) + left( text{number of neutrons} right) nonumber text{mass number} & = 3 + 3 nonumber &= 6 nonumber end{align}nonumber]

b.

Isotopes Of Atoms

[ begin{align}text{atomic number} = left( text{number of protons} right) &= 3 nonumber left( text{number of neutrons} right) & = 4nonumberend{align}nonumber]

[ begin{align}text{mass number} & = left( text{number of protons} right) + left( text{number of neutrons} right)nonumber text{mass number} & = 3 + 4nonumber &= 7 nonumber end{align}nonumber]

Which

Notice that because the lithium atom always has 3 protons, the atomic number for lithium is always 3. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in the isotope with 4 neutrons. In nature, only certain isotopes exist. For instance, lithium exists as an isotope with 3 neutrons, and as an isotope with 4 neutrons, but it doesn't exist as an isotope with 2 neutrons or as an isotope with 5 neutrons.

Stability of Isotopes

Atoms need a certain ratio of neutrons to protons to have a stable nucleus. Having too many or too few neutrons relative to protons results in an unstable, or radioactive, nucleus that will sooner or later break down to a more stable form. This process is called radioactive decay. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. When they decay, they release particles that may be harmful. This is why radioactive isotopes are dangerous and why working with them requires special suits for protection. The isotope of carbon known as carbon-14 is an example of a radioisotope. In contrast, the carbon isotopes called carbon-12 and carbon-13 are stable.

This whole discussion of isotopes brings us back to Dalton's Atomic Theory. According to Dalton, atoms of a given element are identical. But if atoms of a given element can have different numbers of neutrons, then they can have different masses as well! How did Dalton miss this? It turns out that elements found in nature exist as constant uniform mixtures of their naturally occurring isotopes. In other words, a piece of lithium always contains both types of naturally occurring lithium (the type with 3 neutrons and the type with 4 neutrons). Moreover, it always contains the two in the same relative amounts (or 'relative abundance'). In a chunk of lithium, (93%) will always be lithium with 4 neutrons, while the remaining (7%) will always be lithium with 3 neutrons.

Dalton always experimented with large chunks of an element—chunks that contained all of the naturally occurring isotopes of that element. As a result, when he performed his measurements, he was actually observing the averaged properties of all the different isotopes in the sample. For most of our purposes in chemistry, we will do the same thing and deal with the average mass of the atoms. Luckily, aside from having different masses, most other properties of different isotopes are similar.

There are two main ways in which scientists frequently show the mass number of an atom they are interested in. It is important to note that the mass number is not given on the periodic table. These two ways include writing a nuclear symbol or by giving the name of the element with the mass number written.

To write a nuclear symbol, the mass number is placed at the upper left (superscript) of the chemical symbol and the atomic number is placed at the lower left (subscript) of the symbol. The complete nuclear symbol for helium-4 is drawn below:

The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium nucleus with 146 neutrons.

[ce{^{59}_{28}Ni}]

[ ce{ ^{238}_{92}U}]

In the nickel nucleus represented above, the atomic number 28 indicates that the nucleus contains 28 protons, and therefore, it must contain 31 neutrons in order to have a mass number of 59. The uranium nucleus has 92 protons, as all uranium nuclei do; and this particular uranium nucleus has 146 neutrons.

Another way of representing isotopes is by adding a hyphen and the mass number to the chemical name or symbol. Thus the two nuclei would be Nickel-59 or Ni-59 and Uranium-238 or U-238, where 59 and 238 are the mass numbers of the two atoms, respectively. Note that the mass numbers (not the number of neutrons) are given to the side of the name.

Example (PageIndex{2}): Potassium-40

How many protons, electrons, and neutrons are in an atom of (^{40}_{19}ce{K})?

Solution

[text{atomic number} = left( text{number of protons} right) = 19]

For all atoms with no charge, the number of electrons is equal to the number of protons.

[text{number of electrons} = 19]

The mass number, 40, is the sum of the protons and the neutrons.

To find the number of neutrons, subtract the number of protons from the mass number.

[text{number of neutrons} = 40 - 19 = 21.]

Example (PageIndex{3}): Zinc-65

How many protons, electrons, and neutrons are in an atom of zinc-65?

Solution

[text{number of protons} = 30]

For all atoms with no charge, the number of electrons is equal to the number of protons.

[text{number of electrons} = 30]

The mass number, 65, is the sum of the protons and the neutrons.

To find the number of neutrons, subtract the number of protons from the mass number.

[text{number of neutrons} = 65 - 30 = 35]

Exercise (PageIndex{3})

How many protons, electrons, and neutrons are in each atom?

  1. (^{60}_{27}ce{Co})
  2. Na-24
  3. (^{45}_{20}ce{Ca})
  4. Sr-90
Answer a:
27 protons, 27 electrons, 33 neutrons
Answer b:
11 protons, 11 electrons, 13 neutrons
Answer c:
20 protons, 20 electrons, 25 neutrons
Answer d:
38 protons, 38 electrons, 52 neutrons

Summary

  • The number of protons is always the same in atoms of the same element.
  • The number of neutrons can be different, even in atoms of the same element.
  • Atoms of the same element that contain the same number of protons, but different numbers of neutrons, are known as isotopes.
  • Isotopes of any given element all contain the same number of protons, so they have the same atomic number (for example, the atomic number of helium is always 2).
  • Isotopes of a given element contain different numbers of neutrons, therefore, different isotopes have different mass numbers.

Contributions & Attributions

This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:

  • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

  • Marisa Alviar-Agnew (Sacramento City College)

  • Henry Agnew (UC Davis)

GENERAL CHEMISTRY TOPICS

Isotopes Are Atoms Of The Same Element With Different Numbers Of Neutrons

Isotopes

An isotope is made up of atoms of the same element that have the same atomic mass. Different isotopes of an element arise from atoms with differing numbers of neutrons. Uses of isotopes. Average atomic masses from natural abundances: The weighted-average calculation.

Atomic number, mass number and isotopes

Musical accompaniment for these lecture notes.

Isotopes Are Atoms Of The Same Element With Different Numbers Of Protons

The atomic number of an element (symbolized as Z) is the number of protons in the nuclei of its atoms. The mass number (A) is the total number of nucleons (neutrons and protons). An isotope is made up of atoms of the same element (which by definition have a characteristic and fixed atomic number) that also have the same mass number. Different isotopes of an element arise from atoms with differing numbers of neutrons. Because of this, chemists need a way to represent specific isotopes of an element. Isotopes of an element have the same atomic number, but different mass numbers. The atomic number, when represented along with the symbol of an element, is shown as a leading subscript. The mass number is shown as a leading superscript. Since the element symbol implies an atomic number, the latter is often dropped, and an isotope as commonly represented textually with just the mass number and the element symbol (for example 14C or 18O).

Two Atoms Are Isotopes If

In the periodic table, the elements, represented as their symbols, are arranged in a particular pattern that reflects (as we will see) a regularity, or periodicity in their properties. Typically in the table, the element symbol is contained within its own small box, along with other information including the atomic number and the average atomic mass. The average atomic mass of an element represents the averages of its naturally occurring isotopic masses weighted according to their natural abundance. The formula for calculation of average atomic mass and illustration of its use is presented below.

How do the isotopic forms of an element differ from one another, physically and chemically? Isotopes are defined by their subatomic particle composition, which we will think of as a physical property. The chemistry of an element is determined by, in a general sense, the number of valence electrons its atoms possess. The number of valence electrons associated with a neutral atom is in turn determined by the number protons in the nucleus. Thus, two atomic nuclei could have the same number of protons, but different numbers of neutrons. Yet since the atoms they are part of would still have the same number of valence electrons, these two atoms would be chemically indistinguishable.*

Uses of isotopes

There are a wide variety of applications of isotopes in nuclear chemistry, medicine, biochemistry, anthropology, paleontology, and geology. Many such uses are based on the phenomenon of radioactivity, shown by some of the isotopes of many of the elements. Such radioactive isotopes are unstable, undergoing spontaneous nuclear decay processes at a rate determined by the half-life of the isotope. One example is the use of 14C - the isotope of carbon with six protons and eight neutrons, which has a half-life of 5730 years - as a basis for dating of materials derived from living organisms that are many thousands of years old. This technique, called radiocarbon dating, is used widely in geosciences and anthropology.

Average atomic masses from natural abundances: The weighted-average calculation

The atomic masses given in the periodic table represent weighted averages based on the natural abundances of the isotopes of a given element. The formula for a weighted average is

Isotopes Are Atoms Of The Same Element With Different Numbers Of Which Results In Different Numbers

Here the xi's are the masses of the individual isotopes, and the wi's are the fractional abundances corresponding to the isotopes. Note that the weights must sum to 1 (equivalently the percent abundances must sum to 100%).

For example, chlorine exists in two isotopic forms, 35Cl and 37Cl. The mass of the 35Cl isotope is 34.97 amu and that of 37Cl is 36.97 amu. The abundances are 75.77% and 24.23%, repectively. Therefore in this case, the weighted average becomes

wa = (0.7577)(34.97 amu) + (0.2423)(36.97 amu) = 35.45 amu

The result of this calculation is the atomic mass of chlorine that appears in the periodic table.

* Actually, since isotopes of an element differ in atomic mass, they can be subtly distinguished by differences in reaction rates, or in physical processes - such as rate of diffusion - affected by mass.