The Atomic Number

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In 1913, English physicist Henry Moseley observed the properties of x-rays from about a dozen consecutive elements in the periodic table. In doing so, he discovered that the wavelength of the x-rays became shorter as the atomic weight increased.

This meant that both the frequency and energy were increasing because of the inverse relationship between wavelength and frequency. By taking the square root of the frequency, Moseley found that the increase was constant from one element to the next. 

Moseley suggested that this regular increase from element to element must be caused by something in the atom. He then showed that this something was the positive charge (proton) in the nucleus. Moseley referred to this positive charge collectively as the atomic number.

The atomic number is the number of protons in the atomic nucleus.

The new concept of the atomic number brought even more power to the periodic table. Instead of the elements being arranged by relative atomic masses, elements could now be arranged by atomic numbers without one element out of order.

Now, for the first time scientist could tell how many elements remained to be discovered because of the numerical gaps. It was obvious that it is the number of protons that determine which element is which and not the electrons or other particles. Moseley had truly made a great discovery. 

The Modern Periodic Law states that the properties of the elements are periodic functions of the atomic number.

The discovery of the nuclear charge (atomic number) also resulted in being able to determine the number of electrons in an atom. It was previously known, that a neutral atom must have just enough electrons to neutralize the positive nuclear charge and if an atom lost or gained electrons it would become a charged atom (ion).

Therefore, in a stable atom the number of electrons must equal the number of protons.