The Number Of Neutrons In An Atom Is Equal To


Learning Objectives

Mass number: The mass number is equal to the: number of protons and neutrons: The atomic number is equal to the number of in an atom: protons: The number below the chemical symbol on the periodic table that has a few decimal places is known as the. Average atomic mass: If an element has a mass # of 23 and an atomic # of 11, how many. The number of neutrons in an atom is equal to the mass number the mass number +the atomic number the number of protons the mass number - the atomic number the atomic number. The number of neutrons is equal to the difference between the mass number of the atom (A) and the atomic number (Z) and is represented as n 0 = A-Z or numberofneutrons = Mass number-Atomic number. Mass number is the sum of protons and neutrons in an atom of an element and Atomic number is the number of protons present inside the nucleus of an. The easiest way to find the number of protons, neutrons, and electrons for an element is to look at the element’s atomic number on the periodic table. That number is equal to the number of protons. The number of protons is equal to the number of electrons, unless there’s an ion superscript listed after the element. In an atom the number of neutron is 58.7%more than that of protons. The number of electrons in a neutral atom is 92. Find out the number of proton,neutrons and mass number and represent the atom with atomic number and mass number.

  • 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.

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?


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


[ 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]

CHEM Chapter 4: Atoms Flashcards Quizlet


[ 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]

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{ ^{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})?


[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?



[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.

The Number Of Neutrons In An Atom Is Equal To ..

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


  • 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)

Below is a quick explanation of all the items on the fact sheets

Basic Information

Symbol- Each element is assigned a chemical symbol. This symbol usually originates from its name or its Latin name. For example, silicon has a chemical symbol 'Si'. Each element's symbol is composed of a capital letter followed by one or two lowercase letters.
Atomic Number- Each atom has an atomic number. This atomic number is equal to the number of protons in the nucleus of that particular atom. For example, the element cobalt (Co) has an atomic number of 27. This atomic number is also the number of protons in the atom. Therefore, Co has 27 protons.
Mass- The mass of an atom, expressed in atomic mass units (AMU), is roughly equal to the number of protons plus the number of neutrons. This is because both the protons and the neutrons in an atom have a relatively equal mass. The mass of an electron is so insignificant that it is not represented in the atomic mass. Since not all atoms have only one isotope1, the atomic mass is the average of all isotopes, once abundance is computed. For example, if you took a container of the element hydrogen (H), 99.984% of it would be H-1, 0.0156% of it would be H-2, and 0% of the hydrogen would be H-3. Since H-1 has one proton and no neutrons, its mass is 1. Because H-2 has one proton and one neutron, its mass is 2. Therefore, when you compute the percentages of the isotopes of H in any container, you find that the atomic mass of H is actually 1.0079. If the atomic mass of a particular element is shown in parentheses, such as (145) for Promethium (Pm), the atomic mass reflects that of the most stable isotope1, and is not the average atomic mass for all isotopes of the element. Atomic masses used on this periodic table are from the IUPAC 1995 recommendations.
Melting Point- The melting point of any element is the temperature at which the element changes from a solid to a liquid or from a liquid to a solid. Even though water is not an element, I will be using it in this example. Water freezes and ice melts at 0 °C (32 °F). Therefore, the melting point of water is 0 °C. The melting point is provided in degrees Celsius, Fahrenheit, and Kelvin. The melting point of a substance is also the freezing point.
Boiling Point- The boiling point of any element is the temperature at which it changes from a liquid to a gas or from a gas to a liquid. You probably know that water changes to steam and steam changes to water at a temperature of 100 °C (212 °F). The boiling point of water is 100 °C. Therefore, the boiling point is also the condensation point. The boiling point is provided in degrees Celsius, Fahrenheit, and Kelvin.
Number of Protons/Electrons- The number of protons/electrons in any atom is always equal to the atomic number of the atom. Each atom has a neutral charge, and since a proton has a positive charge and an electron has a negative charge, in order to achieve a neutral charge, the number of protons and electrons must equal. A particle that is not neutral (has either more or less electrons) is known as an ion.
Number of Neutrons- The number of neutrons in an atom is equal to the number of protons in an atom subtracted from the mass of the atom rounded to the nearest integer. This is true because both neutrons and protons have an atomic weight of approximately 1 AMU2 (see mass). Since atoms often have more than one isotope1, the number of neutrons listed on the element fact sheets is only valid for the most abundant isotope of any element.
For example, boron (B) has an atomic mass of 10.81 and an atomic number of 5. When you round 10.81 to the nearest integer, the result is 11. When you subtract the number of protons (equal to the atomic number) from the atomic mass, the result is 6. Therefore, the most common isotope of boron has 6 neutrons.
Classification- The classification of any element relates to its properties. Each periodic table may use different group names and classify each element a little differently. This periodic table uses 9 families:

Crystal Structure- The term 'Crystal Structure' refers to the way in which the atoms are arranged within an a substance (element). This property explains the way an element cleaves, or breaks apart physically. For example, an element with a cubic crystal structure, such as aluminum (Al), will break into cubes. Each side of the cube should have a straight edge.
Density- The density of an element refers to how closely its atoms are packed together. This is measured in grams per cubic centimeter. Take, for example, magnesium (Mg). Its density at 293 degrees Kelvin (20 degrees Celsius, 67 degrees Fahrenheit) is 1.738 g/cm3. This means that if you have a block of magnesium at room temperature (293 Kelvin), and you decide to cut a cube measuring 1 x 1 x 1 cm, the mass that you will cut will be 1.738 grams. The greater the density of an element is, the 'heavier' the element is.
Color- The color of an element refers to its physical reflection of light under normal conditions. For example, tin (Sn), will have a white color at room temperature. These properties may change if tin was heated to its melting point, where it would become a liquid, or if it was shown under a light with a color other than white.
Other Names- Some elements have more than one name or spelling. This may be caused by either local spelling or a naming dispute. For example, the element aluminum (Al) is spelled aluminum in the United States, but is spelled (and pronounced) aluminium in most otherEnglish-speaking countries, including Great Britain, Canada, and Australia.
A naming dispute has occurred between the American Chemical Society (ACS) and the International Union for Pure and Applied Chemistry (IUPAC) over elements 104-109. ACS has used the discoverer's suggested names, while IUPAC decided to leave the naming process up to a panel of 20 members. Until this naming dispute is resolved, this periodic table will use the systematic Latin names automatically assigned to newly discovered elements.
More information about the naming of heavy elements is available.

Atomic Structure

Number of Energy Levels- The number of energy levels refers to how many 'electron shells' or places where electrons can be an element has. An element with 4 shells, such as zinc (Zn), has 4 different areas where an electron is likely to be found.
Electron Arrangement- The electron arrangement of an atom refers to the number of electrons in each energy level. For example, carbon (C) has 6 electrons. Its atom arrangement shows that the six electrons are divided up into two shells, with 2 and 4 electrons, respectively.
Electron Configuration- The electron arrangement described above can be further described to include information about orbitals, shells, and more. This explanation is beyond the scope of this document, but if you are already aware of what these numbers mean, they are provided here for you.
Bohr Models- On this periodic table, Bohr models are now available for all 112 known elements. These models are designed to give some idea of how the electrons are spread over the energy levels. However, the Bohr model is now considered inaccurate among most scientists. This is because Bohr models show that electrons travel on specific paths or orbitals, a theory which has now been replaced by one that states that an electron has a greater probability of being in a certain area (or 'energy level') of the atom.

Half Lives

Half Lives- Half lives are defined as being the average time it takes for half of the atoms of a radioactive element to decay into their daughter elements. For example, carbon-14 (an isotope of carbon used for dating fossils) has a half life of 5730 years. This means that if you take a container of carbon-14, and leave it unchanged for 5730 years, about 50% of the carbon will remain as carbon-14, and the other 50% will decay to carbon-14's daughter element (nitrogen). If you wait for another 5730 years, about 25% of the container will be composed of the original carbon, and the other 75% will be atoms of nitrogen. Some elements, especially the heavier ones, have half lives of just a few milliseconds. For example, ununbium-277 (Uub) has a half-live of just 280 milliseconds. This means that in one second of ununbium's existence, 94% of it will radioactively decay into its daughter element.


Date of Discovery- The date of discovery of any element refers to the year in which is was first isolated and identified as an element. Some elements were discovered by early civilizations, and have an unknown discovery date.
Discoverer(s)- The discoverer of an element is defined as the first person to have identified the element. In more recent years, teams of scientists have been working on the identification of new elements, allowing more than one name to be put in this field.
Name Origin- The name origin of an element is the language/object/property/person that gives an element its name. Some elements have been assigned names of famous scientists, important mythological characters, or places. Other element's names come from foreign languages, such as Latin. The most recently discovered elements have a temporary, systematic name, assigned by IUPAC3.
Symbol Origin- When the chemical symbol of an element does not correspond to its name, its symbol origin is given on this periodic table. For example, the element lead has the chemical symbol 'Pb'. The symbol origin is from the Latin word 'plumbum', which means 'lead'.
Uses- Each element's most common uses, as an element or a compound containing the element, is written in this field.
Obtained from- The method of obtaining an element is also given under this section. Some elements are obtained from minerals, others are obtained from methods such as electrolysis of a mineral, while others are man-made.

True Or False. 1. The Number Of Neutrons In An Atom Is ..

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What Is A Neutron

1: An isotope is an atom of any element with the same number of protons and electrons as all the other atoms of this particular element, but with a different atomic mass (and number of neutrons).
2: AMU- Atomic Mass Unit(s)
3: IUPAC- International Union for Pure and Applied Chemistry

Let's Start At The Beginning. What Makes An Element An Element Is The Number Of Protons. The Periodic Table Lists Elements By Increasing Atomic N..

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