What is the Mass Number of the Isotope Lithium 7

4.viii: Isotopes – When the Number of Neutrons Varies

Learning Objectives
• Explicate what isotopes are and how an isotope affects an element’southward diminutive mass.
• Determine the number of protons, electrons, and neutrons of an chemical element with a given mass number.

All atoms of the same element have the same number of protons, only some may have different numbers of neutrons. For example, all carbon atoms have half-dozen protons, and most accept six neutrons besides. Only some carbon atoms have seven or viii neutrons instead of the usual half dozen. Atoms of the same element that differ in their numbers of neutrons are chosen
isotopes. Many isotopes occur naturally. Usually one or ii isotopes of an element are the most stable and mutual. Different isotopes of an element generally have the same concrete and chemic properties considering 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 but 1 proton, one electron, and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms accept i neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms take two neutrons. These atoms are the isotope named tritium.

For virtually elements other than hydrogen, isotopes are named for their mass number. For instance, 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 diminutive mass of 13 (6 protons + 7 neutrons = thirteen). These atoms are the isotope chosen carbon-13.

Case $$\PageIndex{1}$$: 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{marshal}\text{diminutive number} = \left( \text{number of protons} \correct) &= 3 \nonumber \\ \left( \text{number of neutrons} \right) &= iii \nonumber\end{align} \nonumber

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\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{marshal}\nonumber

b.

\begin{align}\text{atomic number} = \left( \text{number of protons} \right) &= 3 \nonumber\\ \left( \text{number of neutrons} \correct) & = 4\nonumber\stop{marshal}\nonumber

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

Notice that because the lithium atom always has iii protons, the atomic number for lithium is e’er three. 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 be. For instance, lithium exists as an isotope with 3 neutrons, and every bit an isotope with iv neutrons, only it doesn’t exist every bit an isotope with 2 neutrons or equally an isotope with 5 neutrons.

Stability of Isotopes

Atoms demand a certain ratio of neutrons to protons to have a stable nucleus. Having also many or likewise few neutrons relative to protons results in an unstable, or radioactive, nucleus that volition sooner or later break downwards to a more stable form. This process is chosen radioactive decay. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. When they decay, they release particles that may exist harmful. This is why radioactive isotopes are unsafe and why working with them requires special suits for protection. The isotope of carbon known as carbon-fourteen is an example of a radioisotope. In contrast, the carbon isotopes called carbon-12 and carbon-xiii are stable.

This whole discussion of isotopes brings usa back to Dalton’southward Diminutive Theory. According to Dalton, atoms of a given element are identical. But if atoms of a given chemical element tin can accept different numbers of neutrons, then they can have dissimilar masses besides! How did Dalton miss this? Information technology turns out that elements plant in nature exist as abiding 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, information technology always contains the two in the same relative amounts (or “relative affluence”). In a clamper of lithium, $$93\%$$ will always be lithium with 4 neutrons, while the remaining $$7\%$$ will always be lithium with 3 neutrons.

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Dalton always experimented with big chunks of an element—chunks that contained all of the naturally occurring isotopes of that chemical 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 about of our purposes in chemistry, we will practice the same matter and deal with the boilerplate mass of the atoms. Luckily, aside from having different masses, most other backdrop of different isotopes are like.

In that location are ii primary ways in which scientists oftentimes evidence 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 means 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 chemic symbol and the atomic number is placed at the lower left (subscript) of the symbol. The complete nuclear symbol for helium-4 is fatigued beneath:

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

$\ce{^{59}_{28}Ni} \nonumber$

$\ce{ ^{238}_{92}U} \nonumber$

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 gild to have a mass number of 59. The uranium nucleus has 92 protons, every bit all uranium nuclei do; and this particular uranium nucleus has 146 neutrons.

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

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Example $$\PageIndex{2}$$: Potassium-twoscore

How many protons, electrons, and neutrons are in an cantlet of $$^{twoscore}_{nineteen}\ce{Grand}$$?

Solution

$\text{atomic number} = \left( \text{number of protons} \correct) = 19 \nonumber$

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

$\text{number of electrons} = 19 \nonumber$

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.

$\text{number of neutrons} = 40 – 19 = 21. \nonumber$

Example $$\PageIndex{3}$$: Zinc-65

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

Solution

$\text{number of protons} = 30 \nonumber$

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

$\text{number of electrons} = xxx \nonumber$

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.

$\text{number of neutrons} = 65 – 30 = 35 \nonumber$

Practice $$\PageIndex{3}$$

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

1. $$^{60}_{27}\ce{Co}$$
2. Na-24
3. $$^{45}_{xx}\ce{Ca}$$
4. Sr-90
27 protons, 27 electrons, 33 neutrons
11 protons, 11 electrons, 13 neutrons
Respond c:
20 protons, xx electrons, 25 neutrons
38 protons, 38 electrons, 52 neutrons

Summary

• The number of protons is always the same in atoms of the aforementioned element.
• The number of neutrons can be different, even in atoms of the aforementioned element.
• Atoms of the same chemical element that incorporate the same number of protons, merely unlike numbers of neutrons, are known as
isotopes.
• Isotopes of any given element all contain the same number of protons, so they have the aforementioned atomic number (for case, the atomic number of helium is always ii).
• Isotopes of a given element contain dissimilar numbers of neutrons, therefore, different isotopes accept unlike mass numbers.

What is the Mass Number of the Isotope Lithium 7

Source: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Introductory_Chemistry_(Tro)/04%3A_Atoms_and_Elements/4.08%3A_Isotopes_-_When_the_Number_of_Neutrons_Varies