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B. Atomic Number and Mass Number

Scientists have established the relationship between the number of subatomic particles in

an atom with the characteristics of the atom that are denoted by its atomic number and mass
number.

Atomic Number (Z)
If an element it struck with a high energy beam of

electrons, the element will emit X-rays, electromagnetic
waves with a high frequency. Each chemical element has a

characteristic of X-ray spectrum, that is, of a certain
frequency. In 1912, Henry Moseley (1887 – 1915)

conducted an experiment on the X-ray spectra of chemical

elements. He measured the frequency of the X-rays
produced when the cathode rays struck various elements that

acted as the anode. Moseley noticed a regularity between the

X-ray frequencies emitted by elements and the positive
Figure 11. Henry Moseley
charges in the nucleus. He used the term atomic number (Z)
to state the number of positive charges inside the nucleus. It turned out that the atomic number

(Z) was no other than the number of protons. (The term proton had not been introduced yet).

The experiment proved that the atomic number Z (the number of proton) was indeed a
characteristic of an element. Different elements have different atomic numbers. Take a look at

table .

As an atom is neutral, the number of positive charges (protons) in the atom has to be the
same as the number of negative charges (electrons). So, the atomic number (Z) of a neutral atom

also states the number of electrons in the atom.

The atomic number (Z) states the number of protons (p). For a neutral

atom, the atomic number (Z) also states the number of electrons.

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