What is it called when two atoms have the same number of protons but different number of electrons?

Like everything we see in the world, isotopes are a type of atom, the smallest unit of matter that retains all the chemical properties of an element. Isotopes are forms of a chemical element with specific properties.

You can see the different chemical elements on the periodic table.

(Graphic: A. Vargas/IAEA)

Each element is distinguished by the number of protons, neutrons and electrons that it possesses. The atoms of each chemical element have a defining and same number of protons and electrons, but – crucially – not neutrons, whose numbers can vary.

Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.

(Graphic: A. Vargas/IAEA)

Nuclear techniques are used to measure the amounts and proportions of isotopes in matter, and trace their origin, history and sources based on this information. These measurements help experts to understand, for instance, terrestrial and aquatic systems, the volume of certain vitamins absorbed by the body, or the amount of fertiliser plants take up.

Learn more about the different types of isotopes.

An isotopic signature is the set of ratios between the amount of the various isotopes of an element in a sample.

Isotopic signatures are commonly known as fingerprints, because they are similar to human fingerprints and are used to track and trace. They are found in water, land, plants and animals. By tracing these fingerprints, scientists can evaluate:

(Graphic: A. Vargas/IAEA)

For example, the naturally occurring isotope carbon-14 present in water is used to understand the age of water and other organic materials.

Read the IAEA Bulletins to learn more about the importance and uses of isotopes and their signatures.

The first 80 elements on the periodic table have stable isotopes. The properties of stable isotopes allow them to be used to understand and manage water and land resources. They are also used in environmental studies, nutrition assessments and forensics.

Naturally occurring stable isotopes such as isotopes of hydrogen are used by measuring their amounts and ratio in samples of water to determine the water’s age and origin, understand its history and acknowledge its sources. This is known as isotope hydrology.

(Graphic: A. Vargas/IAEA)

Stable isotopes can be used to study land, humans, animals, insects and plants. For example, isotopes are used to map the migration path of butterflies and help protect the resources in their breeding environment.

They can also be used in agriculture. Using bio nitrogen fertilisers labelled with the nitrogen-15 stable isotope (15N), scientists can track and determine how effectively crops are taking up the fertiliser. This is important as plants need to absorb nitrogen to convert it into necessary proteins. Using 15N allows scientists to determine how much fertiliser is needed for crops to reach maximum yield.

Read about stable isotopes.

(Graphic: A. Vargas/IAEA)

There are more than 3000 known radioisotopes. They are the unstable form of an element. They emit different levels of radiation, which makes them useful in medicine, industry, agriculture, radiopharmaceutical sciences, industrial applications, environmental tracing and biological studies.

Radioisotopes are artificially and safely produced in research reactors and accelerators. One use of radioisotopes is to manage cancer and chronic diseases using radioisotope therapy, which treats cancerous cells in a safe and effective manner. Other uses range from creating better health care products by removing or neutralising chemicals, bacteria and toxins which pose a hazard.  

Find out the further uses of radioisotopes and their products.

• The IAEA helps countries build capacity to use isotopic techniques and to produce radioisotopes and radiopharmaceuticals.
• The IAEA assists national experts in enhancing the quality of their analytical measurement data through the provision of reference materials, validated procedures, proficiency tests and guidelines for environmental protection.
• The IAEA’s Technical Cooperation Programme often uses isotopic tools to address priorities in developing health and nutrition standards, improving food and agriculture production, tracing back the origin of food and the source of greenhouse gasses and pollutants and managing water supplies.
• Coordinated Research Projects led by the IAEA brings together institutes to establish networks and databases of isotopes and isotopic research.  
• The Global Network of Isotopes in Precipitation, operated by the IAEA and the World Meteorological Organization, offers scientific advice, logistics and technical support for research and the use of the technology.
• The IAEA also operates the Global Network of Isotopes in Rivers, which collects data on isotopes of rivers around the world and assists water resource management.
• The IAEA assists countries interested in setting up radioisotope production facilities, from consultation and selection of facilities and methods to supervising the establishment of production centres.

Now that we know how atoms are generally constructed, what do atoms of any particular element look like? How many protons, neutrons, and electrons are in a specific kind of atom? First, if an atom is electrically neutral overall, then the number of protons equals the number of electrons. Because these particles have the same but opposite charges, equal numbers cancel out, producing a neutral atom.

In the 1910s, experiments with x-rays led to this useful conclusion: the magnitude of the positive charge in the nucleus of every atom of a particular element is the same. In other words, all atoms of the same element have the same number of protons. Furthermore, different elements have a different number of protons in their nuclei, so the number of protons in the nucleus of an atom is characteristic of a particular element. This discovery was so important to our understanding of atoms that the number of protons in the nucleus of an atom is called the atomic number (Z).

For example, hydrogen has the atomic number 1; all hydrogen atoms have 1 proton in their nuclei. Helium has the atomic number 2; all helium atoms have 2 protons in their nuclei. There is no such thing as a hydrogen atom with 2 protons in its nucleus; a nucleus with 2 protons would be a helium atom. The atomic number defines an element. Table \(\PageIndex{1}\) lists some common elements and their atomic numbers. Based on its atomic number, you can determine the number of protons in the nucleus of an atom. The largest atoms have over 100 protons in their nuclei.

How many electrons are in an atom? Previously we said that for an electrically neutral atom, the number of electrons equals the number of protons, so the total opposite charges cancel. Thus, the atomic number of an element also gives the number of electrons in an atom of that element. (Later we will find that some elements may gain or lose electrons from their atoms, so those atoms will no longer be electrically neutral. Thus we will need a way to differentiate the number of electrons for those elements.)

How many neutrons are in atoms of a particular element? At first it was thought that the number of neutrons in a nucleus was also characteristic of an element. However, it was found that atoms of the same element can have different numbers of neutrons. Atoms of the same element (i.e., same atomic number, Z) that have different numbers of neutrons are called isotopes. For example, 99% of the carbon atoms on Earth have 6 neutrons and 6 protons in their nuclei; about 1% of the carbon atoms have 7 neutrons in their nuclei. Naturally occurring carbon on Earth, therefore, is actually a mixture of isotopes, albeit a mixture that is 99% carbon with 6 neutrons in each nucleus.

An important series of isotopes is found with hydrogen atoms. Most hydrogen atoms have a nucleus with only a single proton. About 1 in 10,000 hydrogen nuclei, however, also has a neutron; this particular isotope is called deuterium. An extremely rare hydrogen isotope, tritium, has 1 proton and 2 neutrons in its nucleus. Figure \(\PageIndex{1}\) compares the three isotopes of hydrogen.

Most elements exist as mixtures of isotopes. In fact, there are currently over 3,500 isotopes known for all the elements. When scientists discuss individual isotopes, they need an efficient way to specify the number of neutrons in any particular nucleus. The mass number (A) of an atom is the sum of the numbers of protons and neutrons in the nucleus. Given the mass number for a nucleus (and knowing the atomic number of that particular atom), you can determine the number of neutrons by subtracting the atomic number from the mass number.

A simple way of indicating the mass number of a particular isotope is to list it as a superscript on the left side of an element’s symbol. Atomic numbers are often listed as a subscript on the left side of an element’s symbol. Thus, we might see

\[\mathrm{^{mass\: number\xrightarrow{\hspace{45px}} 56}_{atomic\: number \xrightarrow{\hspace{35px}} 26}Fe} \label{Eq1} \]

which indicates a particular isotope of iron. The 26 is the atomic number (which is the same for all iron atoms), while the 56 is the mass number of the isotope. To determine the number of neutrons in this isotope, we subtract 26 from 56: 56 − 26 = 30, so there are 30 neutrons in this atom.

It is not absolutely necessary to indicate the atomic number as a subscript because each element has its own unique atomic number. Many isotopes are indicated with a superscript only, such as 13C or 235U. You may also see isotopes represented in print as, for example, carbon-13 or uranium-235.

The atom consists of discrete particles that govern its chemical and physical behavior. Each atom of an element contains the same number of protons, which is the atomic number (Z). Neutral atoms have the same number of electrons and protons. Atoms of an element that contain different numbers of neutrons are called isotopes. Each isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons.

Almost all of the mass of an atom is from the total protons and neutrons contained within a tiny (and therefore very dense) nucleus. The majority of the volume of an atom is the surrounding space in which the electrons reside. A representation of a carbon-12 atom is shown below in Figure \(\PageIndex{2}\).