2.1: ATOMIC NUMBER, PROTON NUMBER AND NUCLEON NUMBER; IDENTITY OF AN ELEMENT
1n 1913, Moseley observed that when different elements were bombarded with cathode rays, the X-rays of some characteristic frequencies were produced. It was found that the square root of the frequency of the X rays was directly proportional to the atomic number of an element Z.
He concluded that this number, i.e. the atomic number Z was a fundamental property of an element. It is also called proton number. The number of protons and neutrons in the nucleus of an atom is collectively called its nucleon number (A), also called mass number.
Atomic number is related to the mass number by the following equation:
A = Z + N
For example, an atom of an element X having atomic number Z and mass number A is described as , e.g., . Number of neutrons in an atom can be calculated as:
N = A – Z
Let us consider ,
Atomic number/ proton number (Z) = 13
Mass number/ nucleon number (A) = 27
N = 27-13 = 14
Similarly, the number of electrons, protons, and neutrons can be justified for an ion as in the following example:
13Al27 atom loses three electrons to form Al3+, then;
No. of protons = 13
No. of neutrons =14
No of electrons = 13 – 3 = 10
Similarly, gains an electron to form Cl– ion;
No. of protons = 17
No. of neutrons =18
No of electrons = 17 + 1 = 18
In case, electron gain happens by the neutral atoms, say 8O16 to 8O2–, 15P31 to 15P3– and 16S32 to 16S2–, the number of neutrons, protons and electrons are as in Table 2.1.
Table 2.1: The Number of protons, electrons and neutrons in different ions
| Species | Neutrons | Protons | Electrons |
| O2- | 8 | 8 | 10 |
| S2- | 16 | 16 | 18 |
| P3- | 16 | 15 | 18 |
Thus, the atomic number and proton number represent the same concept.
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2.2: EFFECT OF ELECTRIC FIELD ON FUNDAMENTAL PARTICLES
The behaviour of particles in an electric field depends upon their mass and charge. If we allow the beams of electrons, protons and neutrons to pass one by one at the same speed through an electric field, they show their behaviour as follows.
1) Neutrons being neutral are not deflected but travel in a straight path perpendicular to the direction of electric field.
2) Protons being positively charged are deflected towards the negative plate.
3) Electrons being negatively charged are deflected towards the positive plate, to greater extent since they are 1836 times lighter than protons.
Figure
The amount of deviation from its original direction of movement is measured in two ways.
Equations
This is possible if we imagine that after deflection, the particle moves in a circular path. Hence, the factors affecting the radius of deflection are reciprocal to that for the angle of deflection.