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2.3: Protons, Neutrons, and Electrons in Atoms

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    49024
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    Skills to Develop

    • Describe the locations, charges, and masses of the three main subatomic particles.
    • Define atomic number.
    • Describe the size of the nucleus in relation to the size of the atom.
    • Define mass number.
    • Explain what isotopes are and how isotopes affect an element's atomic mass.
    • Determine the number of protons, neutrons, and electrons in an atom.

    Dalton's Atomic Theory explained a lot about matter, chemicals, and chemical reactions. Nevertheless, it wasn't entirely accurate, because contrary to what Dalton believed, atoms can, in fact, be broken apart into smaller subunits or subatomic particles. We have been talking about the electron in great detail, but there are two other particles of interest to use: protons and neutrons. In this section, we'll look at the atom a little more closely.

    Protons, Electrons, and Neutrons

    We already learned that J. J. Thomson discovered a negatively charged particle, called the electron. Rutherford proposed that these electrons orbit a positive nucleus. In subsequent experiments, he found that there is a smaller positively charged particle in the nucleus which is called a proton. There is a third subatomic particle, known as a neutron. Ernest Rutherford proposed the existence of a neutral particle, with the approximate mass of a proton. Years later, James Chadwick proved that the nucleus of the atom contains this neutral particle that had been proposed by Ernest Rutherford. Chadwick observed that when berylliCK12 Screenshot 2-3-1.pngum is bombarded with alpha particles, it emits an unknown radiation that has approximately the same mass as a proton, but no electrical charge. Chadwick was able to prove that the beryllium emissions contained a neutral particle - Rutherford's neutron.

    As you might have already guessed from its name, the neutron is neutral. In other words, it has no charge whatsoever, and is therefore neither attracted to nor repelled from other objects. Neutrons are in every atom (with one exception), and they're bound together with other neutrons and protons in the atomic nucleus.

    Before we move on, we must discuss how the different types of subatomic particles interact with each other. When it comes to neutrons, the answer is obvious. Since neutrons are neither attracted to nor repelled from objects, they don't really interact with protons or electrons (beyond being bound into the nucleus with the protons).

    Even though electrons, protons, and neutrons are all types of subatomic particles, they are not all the same size. When you compare the masses of electrons, protons, and neutrons, what you find is that electrons have an extremely small mass, compared to either protons or neutrons. On the other hand, the masses of protons and neutrons are fairly similar, although tCK12 Screenshot 2-3-2.pngechnically, the mass of a neutron is slightly larger than the mass of a proton. Because protons and neutrons are so much more massive than electrons, almost all of the mass of any atom comes from the nucleus, which contains all of the neutrons and protons.

    The table shown gives the properties and locations of electrons, protons, and neutrons. The second column shows the masses of the three subatomic particles in "atomic mass units." An atomic mass unit (\(\text{amu}\)) is defined as one-twelfth the mass of a carbon-12 atom. Atomic mass units (\(\text{amu}\)) are useful, because, as you can see, the mass of a proton and the mass of a neutron are almost exactly \(1.0\) in this unit system.

    In addition to mass, another important property of subatomic particles is their charge. You already know that neutrons are neutral, and thus have no charge at all. Therefore, we say that neutrons have a charge of zero. What about electrons and protons? You know that electrons are negatively charged and protons are positively charged, but what's amazing is that the positive charge on a proton is exactly equal in magnitude (magnitude means "absolute value" or "size when you ignore positive and negative signs") to the negative charge on an electron. The third column in the table shows the charges of the three subatomic particles. Notice that the charge on the proton and the charge on the electron have the same magnitude.

    Negative and positive charges of equal magnitude cancel each other out. This means that the negative charge on an electron perfectly balances the positive charge on the proton. In other words, a neutral atom must have exactly one electron for every proton. If a neutral atom has 1 proton, it must have 1 electron. If a neutral atom has 2 protons, it must have 2 electrons. If a neutral atom has 10 protons, it must have 10 electrons. You get the idea. In order to be neutral, an atom must have the same number of electrons and protons.

    Atomic Number and Mass NumberCK12 Screenshot 2-3-3.png

    Scientists can distinguish between different elements by counting the number of protons. If an atom has only one proton, we know it's a hydrogen atom. An atom with two protons is always a helium atom. If scientists count four protons in an atom, they know it's a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . the list goes on.

    Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs between different elements. Therefore, scientists give this number a special name. An element's atomic number is equal to the number of protons in the nuclei of any of its atoms. The periodic table gives the atomic number of each element. The atomic number is a whole number usually written above the chemical symbol of each element. The atomic number for hydrogen is 1, because every hydrogen atom has 1 proton. The atomic number for helium is 2 because every helium atom has 2 protons. What is the atomic number of carbon?

    Of course, since neutral atoms have to have one electron for every proton, an element's atomic number also tells you how many electrons are in a neutral atom of that element. Fore example, hydrogen has an atomic number of 1. This means that an atom of hydrogen has one proton, and, if it's neutral, one electron as well. Gold, on the other hand, has an atomic number of 79, which means that an atom of gold has 79 protons, and, if it's neutral, 79 electrons as well.

    The mass number of an atom is the total number of protons and neutrons in its nucleus. Why do you think that the "mass number" includes protons and neutrons, but not electrons? You know that most of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the number of protons and neutrons. You have already learned that the mass of an electron is very, very small compared to the mass of either a proton or a neutron (like the mass of a penny compared to the mass of a bowling ball). Counting the number of protons and neutrons tells scientists about the total mass of an atom.

    \[\text{mass number} \: A = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \right)\]

    An atom's mass number is very easy to calculate provided you know the number of protons and neutrons in an atom.

    Example 2.3.1

    What is the mass number of an atom of helium that contains 2 neutrons?

    Solution

    \(\left( \text{number of protons} \right) = 2\) (Remember that an atom of helium always has 2 protons.)

    \(\left( \text{number of neutrons} \right) = 2\)

    \(\text{mass number} = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \right)\)

    \(\text{mass number} = 2 + 2 = 4\)

    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:

    CK12 Screenshot 2-3-4.png

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

    \[^{59}_{28}\ce{Ni} \: \: \: \: \: \: \: \: \: \: \: ^{238}_{92}\ce{U}\]

    In the nickel nucleus represented above, the atomic number 28 indicates 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 do all uranium nuclei and this particular uranium nucleus has 146 neutrons.

    The other way of representing these nuclei would be Nickel-59 and Uranium-238, where 59 and 238 are the mass numbers of the two atoms, respectively. Note that the mass numbers (not the number of neutrons) is given to the side of the name.

    Isotopes

    Unlike the number of protons, which 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, containing the same number of protons, but different numbers of neutrons, are known as isotopes. Since the isotopes of any given element all contain the same number of protons, they have the same atomic number (for example, the atomic number of helium is always 2). However, since the isotopes of a given element contain different numbers of neutrons, different isotopes have different mass numbers. The following two examples should help to clarify this point.

    Example 2.3.1

    1. What is the atomic number and the mass number of an isotope of lithium containing 3 neutrons. A lithium atom contains 3 protons in its nucleus.
    2. What is the atomic number and the mass number of an isotope of lithium containing 4 neutrons?

    Solution

    a) \(\text{atomic number} = \left( \text{number of protons} \right) = 3\)

    \(\left( \text{number of neutrons} \right) = 3\)

    \(\text{mass number} = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \right)\)

    \(\text{mass number} = 3 + 3 = 6\)

    b) \(\text{atomic number} = \left( \text{number of protons} \right) = 3\)

    \(\left( \text{number of neutrons} \right) = 4\)

    \(\text{mass number} = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \right)\)

    \(\text{mass number} = 3 + 4 = 7\)

    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.

    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 abundances"). 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.

    We can use what we know about atomic number and mass number to find the number of protons, neutrons, and electrons in any given atom or isotope. Consider the following examples.

    Example 2.3.2

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

    Solution

    Finding the number of protons is simple. The atomic number, the number of protons, is listed in the bottom right corner. \(\text{number of protons} = 19\).

    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 2.3.3

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

    Solution

    Finding the number of protons is simple. The atomic number, the number of protons, is found on the periodic table. All zinc atoms have \(\text{number of protons} = 30\).

    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. \(\text{number of neutrons} = 65 - 30 = 35\).

    Summary

    • Electrons are a type of subatomic particle with a negative charge.
    • Protons are a type of subatomic particle with a positive charge. Protons are bound together in an atom's nucleus as a result of the strong nuclear force.
    • Neutrons are a type of subatomic particle with no charge (they're neutral). Like protons, neutrons are bound into the atom's nucleus as a result of the strong nuclear force.
    • Protons and neutrons have approximately the same mass, but they are bot much more massive than electrons (approximately 2,000 times as massive as an electron).
    • The positive charge on a proton is equal in magnitude to the negative charge on an electron. As a result, a neutral atom must have an equal number of protons and electrons.
    • Each element has a unique number of protons. An element's atomic number is equal to the number of protons in the nuclei of any of its atoms.
    • The mass number of an atom is the sum of the protons and neutrons in the atom.
    • Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons in their atomic nuclei.

    Vocabulary

    • Neutron: a subatomic particle with no charge
    • Atomic mass unit (\(\text{amu}\)): a unit of mass equal to one-twelfth the mass of a carbon-12 atom
    • Atomic number: the number of protons in the nucleus of an atom
    • Mass number: the total number of protons and neutrons in the nucleus of an atom
    • Isotopes: atoms of the same element that have the same number of protons but different numbers of neutrons

    Further Reading/Supplemental Links

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    2.3: Protons, Neutrons, and Electrons in Atoms is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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