Atomic number and mass number

Quantitative Properties of the Atom:

1- Atomic Number:

The basic property of an element is the atomic number. Any element has the same number of protons in the nucleus that is why atomic number is used as the basic property of any element.

The atomic number is given the symbol Z where;

Z= number of protons

The number of protons in the nucleus is exactly equal to the number of electrons outside the nucleus in a neutral atom.

For example, the neutral hydrogen atom has one proton and one electron.

H atom: 1 proton, 1 electron Z = 1

2- Mass Numbers; Isotopes:

The number of protons and neutrons in the nucleus is called the mass number. Mass number is given the symbol A. For the same element mass number can vary.

A = number of protons + number of neutrons

There are three different mass numbers for element hydrogen are Z=1, 2, and 3. The three all have one proton and the atomic number is equal to one. A light hydrogen element (the most common type) has no neutrons in the nucleus (A = 1). The second element hydrogen (deuterium) has one neutron (A = 2). The third element hydrogen (tritium) has two neutrons (A = 3).

Atoms that contain the same number of protons but a different number of neutrons are called isotopes. The three kinds of hydrogen atoms just described are isotopes of that element. They have masses that are very nearly in the ratio 1:2:3.

The two famous isotopes are Uranium-235 and Uranium-238.

Isotope	Z	A	Number of protons	Number of Neutrons
Uranium -235	92	235	92	143
Uranium -238	92	238	92	146

In the periodic table, the atomic number appears at the lower left of the symbol of the element. The mass number appears at the upper left of the element symbol.

The nuclear symbols for the isotopes of hydrogen element are: