The Atomic Number Of An Element Is The Number Of

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Learning Objective

  • Determine the relationship between the mass number of an atom, its atomic number, its atomic mass, and its number of subatomic particles

Key Points

  • Neutral atoms of each element contain an equal number of protons and electrons.
  • The number of protons determines an element’s atomic number and is used to distinguish one element from another.
  • The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.
  • Together, the number of protons and the number of neutrons determine an element’s mass number.
  • Since an element’s isotopes have slightly different mass numbers, the atomic mass is calculated by obtaining the mean of the mass numbers for its isotopes.


  • atomic massThe average mass of an atom, taking into account all its naturally occurring isotopes.
  • mass numberThe sum of the number of protons and the number of neutrons in an atom.
  • atomic numberThe number of protons in an atom.
AtomicThe atomic number of an element is the number of what

Atomic number definition is - an experimentally determined number characteristic of a chemical element that represents the number of protons in the nucleus which in a neutral atom equals the number of electrons outside the nucleus and that determines the place of the element in the periodic table. Atomic number of element is fixed. But atomic mass doesn't. For example, atomic number of carbon atom is 6 but it has atomic mass of carbon may have 12, or 14(it is known as isotopic nature of elements) That is why atomic number of an element is considered to be a more appropriate parameter than its atomic mass.

Atomic Number

Atomic number, the number of a chemical element in the periodic system, whereby the elements are arranged in order of increasing number of protons in the nucleus. Accordingly, the number of protons, which is always equal to the number of electrons in the neutral atom, is also the atomic number. The atomic number of an element is equal to the total number of protons in the nucleus of the atoms of that element. The atomic number can provide insight into the electronic configuration of the element. For example, carbon has an electron configuration of He 2s 2 2p 2, since its atomic number is 6. What is the atomic number and mass number?

Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons. The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons. The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms). For instance, iron, Fe, can exist in its neutral state, or in the +2 and +3 ionic states.

Mass Number

An element’s mass number (A) is the sum of the number of protons and the number of neutrons. The small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.

Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine (Cl) is 35.45 amu because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 amu (17 protons and 18 neutrons) and some with an atomic mass of 37 amu (17 protons and 20 neutrons).

Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7 amu) contains three protons (found from Z), three electrons (as the number of protons is equal to the number of electrons in an atom), and four neutrons (7 – 3 = 4).

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The atomic number is the number of protons — positively charged particles — in the nucleus an atom of a chemical element. Elements are distinguished from one another by the numbers of these particles they have, and so each element has its own unique atomic number. The chemical properties of an element are determined by its number of electrons, but in a neutral atom, this is the same as the number of protons. Atoms can, however, gain or lose electrons to form negatively or positively charged ions, so the atomic number is defined as the number of protons, as this is always the same for a given element.

Element Name Atomic Number And Mass Number

Atomic Number, Mass Number and Atomic Weight

Element Atomic Number Mass Number

It is possible to confuse these values, but they are quite distinct from one another. Atoms consist of a nucleus containing positively charged protons and electrically neutral neutrons, with electrons orbiting some distance away. Protons and neutrons are relatively heavy, and similar in weight, but electrons are very much lighter and contribute very little to the weight of an atom. The mass number of an atom is the number of protons plus the number of neutrons and is nearly equal to the weight of the atom.

The number of neutrons in an element can vary. Forms of an element with different numbers of neutrons are known as isotopes. For example, the most common form of hydrogen has one proton and no neutrons, but two other isotopes of hydrogen exist, deuterium and tritium, with one and two neutrons, respectively. Naturally occurring elements are often mixtures of different isotopes. Carbon is another example, consisting of isotopes with mass numbers 12, 13 and 14. These all have six protons, but have six, seven and eight neutrons, respectively.

Although 19th century chemists had established good approximations of the atomic weights of the known elements, the precise calculations are not always straightforward, due to the occurrence of different isotopes in varying proportions. Often, the atomic weight is determined as an average, based on the relative abundance of isotopes. Since some isotopes are unstable, changing over time into other elements, atomic weights can vary, and may be represented as a range, rather than a single value. Isotopes are usually represented with the atomic number at the bottom left of the chemical symbol, and the mass number, or approximate atomic weight, at the top right. For example carbon 13 would be shown as 6C13.

The Periodic Table

The Atomic Number Of An Element Is The Number Of Quizlet

In the 1860s, the Russian chemist Dimitri Mendeleev worked on a table of the elements known at that time, initially listing them in order of atomic weight and arranging them in rows that grouped elements with similar chemical properties together. It had been noticed previously by other chemists that the properties of the elements, when ordered by weight, tended to repeat at more or less regular intervals. For example, lithium, sodium and potassium are all reactive metals that combine with non-metals in similar ways, while helium, neon and argon are all completely unreactive gases. For this reason, Mendeleev’s list became known as the periodic table.

Mendeleev’s first draft worked well, but there were a few inconsistencies. For example, listed in order of weight, iodine came before tellurium. The problem was that this grouped iodine with oxygen, sulfur and selenium, and tellurium with fluorine, chlorine and bromine. According to their chemical properties, the reverse should have been the case, so before publishing his table in 1869, Mendeleev simply swapped these elements round. It was not until the early 20th century, however, that the reason for these inconsistencies was revealed.

In 1913, the physicist H.G.J. Moseley established a relationship between the wavelengths of X-rays produced by different elements and their sequence in the periodic table. As the structure of the atom was revealed by other experiments around this time, it became clear that this relationship was dependent on the number of protons in an element’s nucleus, in other words, its atomic number. The periodic table could then be ordered by this number, putting the observed chemical properties of the elements on a sound theoretical basis. The occasional inconsistencies in the original table were due to the fact that variations in the number of neutrons could sometimes result in an element having a higher atomic weight than another element with a higher atomic number.

The modern periodic table shows the elements in boxes arranged into rows and columns, with atomic number ascending along each row. Each column groups together elements with similar chemical properties. The columns are determined by the number and arrangement of electrons in the atoms, which in turn is determined by the number of protons. Each box normally contains the chemical symbol for the element, with the atomic number above.