Atomic Number, Atomic Mass and Isotopes (Article) | Khan Academy (2023)

Basic properties of atoms, including atomic number and atomic mass. The atomic number is the number of protons in an atom, and isotopes have the same atomic number but differ in the number of neutrons.

Radioactivity is in the news quite often. You may have read about this in discussions of nuclear power, the Fukushima reactor tragedy, or the development of nuclear weapons. It's showing up in popular culture, too: Many superhero origin stories involve exposure to radiation, or, in the case of Spider-Man, being bitten by a radioactive spider. But what exactly does it mean when something is radioactive?

Radioactivity is actually a property of an atom. Radioactive atoms have unstable nuclei and eventually release subatomic particles to become more stable, releasing energy (radiation) in the process. There are often elements in both radioactive and non-radioactive forms, which differ in the number of neutrons they contain. These different versions of the elements are called isotopes, and small amounts of radioactive isotopes are often found in nature. For example, a small amount of carbon exists in the atmosphere as radioactive carbon-14, and the amount of carbon-14 found in fossils allows paleontologists to determine their age.

In this article, we'll take a closer look at subatomic particles that contain different atoms and what makes an isotope radioactive.

The atoms of each element contain a characteristic number of protons. In fact, the number of protons determines which atom we're looking at (for example, all atoms with six protons are carbon atoms); The number of protons in an atom is calledatomic number. Conversely, the number of neutrons in a given element can vary. They are called forms of the same atom that differ only in their number of neutrons.Isotope. The number of protons and the number of neutrons together determine that of an element.mass number: mass number = protons + neutrons. If you want to calculate how many neutrons an atom has, you can simply subtract the number of protons or the atomic number from the mass number.

(Video) Atomic number, mass number, and isotopes | Chemistry | Khan Academy

A property that is closely related to the mass number of an atom is itsAtomic mass. The atomic mass of an individual atom is simply its total mass and is usually expressed in atomic mass units, or amu. By definition, a carbon atom with six neutrons, carbon-12, has an atomic mass of 12 amu. Other atoms generally do not have round atomic masses for reasons that are beyond the scope of this article. In general, however, the atomic mass of an atom is very close to its mass number, but with some deviation to the decimal places.

Since the isotopes of an element have different atomic masses, scientists can also determine themrelative atomic mass- sometimes also calledatomic weight- for one article. The relative atomic mass is an average of the atomic masses of all the different isotopes in a sample, and each isotope's contribution to the average is determined by the proportion of the sample that it is composed of. The relative atomic masses given in periodic table entries, such as that for hydrogen below, are calculated for all naturally occurring isotopes of each element, weighted by the abundance of those isotopes on Earth. Extraterrestrial objects such as asteroids or meteors can have very different isotopic abundances.

Image showing the "anatomy" of an entry in the periodic table. The upper left is the atomic number or number of protons. In the middle is the symbol for the letter of the element (for example, H). Below is the relative atomic mass calculated for naturally occurring isotopes on Earth. At the bottom is the name of the element (for example, hydrogen).

(Video) Nuclide Symbols: Atomic Number, Mass Number, Ions, and Isotopes

Image credit: Modified from OpenStax CNX Biology

As mentioned above, isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Many elements, such as carbon, potassium, and uranium, have multiple naturally occurring isotopes. A neutral carbon-12 atom contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 (six protons plus six neutrons). Neutral carbon 14 contains six protons, eight neutrons, and six electrons; its mass number is 14 (six protons plus eight neutrons). These two alternate forms of carbon are isotopes.

Some isotopes are stable, but others can emit or eject subatomic particles to achieve a more stable, lower energy configuration. Such isotopes are calledRadioisótopo, and the process by which they release particles and energy is calleddecay. Radioactive decay can cause a change in the number of protons in the nucleus; When this happens, the identity of the atom changes (for example, carbon-14 breaks down into nitrogen-14).

Radioactive decay is a random but exponential process and that of an isotopehalf lifeIt is the period of time during which half of the material decomposes into another relatively stable product. The ratio of the parent isotope to its decay product and to stable isotopes changes in a predictable way; This predictability allows the relative abundance of the isotope to be used as a clock, measuring the time from the incorporation of the isotope (for example, in a fossil) to the present.

(Video) How To Calculate The Number of Protons, Neutrons, and Electrons - Chemistry

Diagram of the radioactive decay of carbon-14. The amount of carbon-14 decreases exponentially with time. The time at which half of the original carbon-14 has decayed (and the other half is still there) is called t 1/2. This time is also known as the half-life of the radioisotope and is 5730 years for carbon-14.

Photo Credit: Modified from CK-12 Biology

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For example, carbon is normally present in the atmosphere in the form of gases such as carbon dioxide, and it exists in three isotopic forms: carbon-12 and carbon-13, which are stable, and carbon-14, which is radioactive. These forms of carbon occur in relatively constant proportions in the atmosphere, with carbon-12 being the major form at about 99%, carbon-13 being the minor form at about 1%, and carbon-14 being present in trace amounts. .1^ 11Initial superscript, 1, final superscript. Because plants extract carbon dioxide from the air to make sugars, the relative amount of carbon-14 in their tissues is equal to the concentration of carbon-14 in the atmosphere. If the animals eat the plants, or other animals that eat the plants, the carbon-14 concentrations in their bodies will also match the atmospheric concentration. When an organism dies, it stops absorbing carbon-14, so the ratio of carbon-14 to carbon-12 in its remains, such as fossilized bones, decreases as the carbon-14 gradually breaks down into nitrogen-14.2^ 22checked.

After a half-life of about 5,730 years, half of the original carbon-14 will have been converted to nitrogen-14. This property can be used to date previously living objects, such as ancient bone or wood. By comparing the ratio of the concentrations of carbon 14 and carbon 12 in an object to the same ratio in the atmosphere, which corresponds to the initial concentration of the object, one can determine the fraction of the isotope that has not yet decayed. This fraction can be used to accurately estimate the age of the material if it is no more than 50,000 years old. Other elements have isotopes with different half-lives and can therefore be used to measure age on different time scales. For example, potassium-40 has a half-life of 1.25 billion years and uranium-235 has a half-life of about 700 million years and has been used to measure the age of lunar rocks.2^ 22checked.

[Attribution and references]

(Video) What are Isotopes?


1. Worked example: Identifying isotopes and ions | Chemistry | Khan Academy
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2. Isotopes | Atomic structure and properties | AP Chemistry | Khan Academy
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3. Atomic Number, Mass Number, and Net Electric Charge
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4. Calculating atomic weight | Chemistry | Khan Academy
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