S Atomic Number

The atomic number (represented by the letter Z) of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as. The atomic number represents the number of protons present in the nucleus of an atom. The atomic number of an element is located on the upper-left corner of that element in the periodic table. Krypton's atomic number is 36, which means that there are 36 protons in the nucleus of one atom of krypton. Krypton also has 36 electrons.

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.


S Block Atomic Number

  • 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.

Atomic 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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Moseley's Discovery - The Modern Concept of Atomic Number

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Today, we know that the atomic number gives the number of protons (positive charges) in the nucleus. This was the discovery made by Henry Gwyn-Jefferies Moseley. He found that certain lines in the X-ray spectrum of each element moved the same amount each time you increased the atomic number by one.

Rutherford (in 1914) described Moseley's discovery thus:

'Recently Moseley has supplied very valuable evidence that this rule [atomic numbers changing by one from element to element] also holds for a number of the lighter elements. By examination of the wave-length of the characteristic X rays emitted by twelve elements varying in atomic weight between calcium (40) and zinc (65.4), he has shown that the variation of wave-length can be simply explained by supposing that the charge on the nucleus increases from element to element by exactly one unit. This holds true for cobalt and nickel, although it has long been known that they occupy an anomalous relative position in the periodic classification of the elements according to atomic weights.'

I. Atomic Structure: 1903 - 1911

Exactly where the positive protons (and the negative electrons) were in the atom took time to be worked out. Keep in mind that the electron (the first sub-atomic particle discovered) was not discovered until 1897.

(1) J.J. Thomson in 1903, had electrons as negative particles with mass, while the positive charge was spread out through the space of the atom.
(2) In 1911 Rutherford announced his atomic model: (a) a nucleus - a dense concentration of positive charge with (b) electrons orbiting the nucleus in an unspecified manner.
(3) In 1913, Bohr took up the question of where the negative electrons are (in the atom) and Moseley studied where the positive charges were.

By the way, Moseley was part of Rutherford's research group -- having arrived in Manchester just weeks before Rutherford published his great nucleus paper -- when he started his atomic number work. Rutherford was not all that excited by Moseley wanting to study X-rays, but the energy and enthusiasm of the younger man soon wore Rutherford down.

[You might notice that neutrons have not been mentioned. It would not be until 1920 that Rutherford proposed the existence of a neutral particle -- the neutron. Another of Rutherford's students -- James Chadwick -- won the 1935 Nobel Prize for discovering the neutron in 1932.]

Within a few months of Rutherford's nucleus paper being published, the true, physical meaning of 'atomic number' was suggested by A. van den Broek. In 1913, he wrote:

'In a previous letter to NATURE (July 20, 1911, p. 78) the hypothesis was proposed that the atomic weight being equal to about twice the intra-atomic charge, 'to each possible intra-atomic charge corresponds a possible element,' or that (Physik. Zeitschr, xiv., 1912, p. 39), 'if all elements be arranged in order of increasing atomic weights, the number of each element in that series must be equal to its intra-atomic charge.' '

II. Moseley's X-Ray Spectra Work

Moseley's problem was to find a linear relationship between the atomic number and a measureable property of the nucleus. The atomic number increased by steps of one (18, 19, 20, 21, and so on). Moseley needed some function of a nuclear property that increased in the same pattern, that is, by one for each element in turn. He found it in the K line of the X-ray spectra of each element. It turns out that the square root of the frequency moves by a constant value (let's call it 'one unit') for each one unit move by the atomic number.

Oxygen's Atomic Number

Why did he choose to study this area for what he needed? We can find the answer in the work of Charles Barkla. He had demonstrated that the elements emitted characteristic X-rays, called K and L rays. These X-rays were independent of the physical or chemical state the element was in. Someone, perhaps Barkla or Bohr or Moseley, realized that this meant the X-rays were characteristic of the nucleus.

So Moseley set about to determine the wavelengths of the K radiation using recently discovered techniques by the father-and-son team of W.L Bragg and W.H. Bragg. It seems to me as I write this that Moseley was pretty confident going into this experiment that all he needed to do was find the proper linear relationship. Getting the equipment working so as to give reliable data was probably the most time-consuming task of the entire research he carried out.

However, the research was carried out and Moseley determined the relationship mentioned above. It was linear, with the frequency square root value moving up the same amount for each one unit jump in the atomic number. Here, using Moseley's data is graph which shows linear behavior:

About this data, Moseley himself said:

S-2 Atomic Number

'We have here a proof that there is in the atom a fundamental quantity, which increases by regular steps as we pass from one element to the next. This quantity can only be the change on the central positive nucleus, of the existence of which we already have definite proof.'