Chemistry - Atomic number and atomic mass number

Atomic number and atomic mass number :

The number of protons in the nucleus is chemical identity of an element. This number is called as atomic number (Z) of the element The positive charge on the nucleus is due to the protons present in it (+Ze). Atom being electrically neutral, it contains the same number of extranuclear electrons in it as its atomic number. Therefore the total electronic charge on an atom is -Ze.

Atomic number (Z) = Number of protons = Number of electrons

As can be seen from Table, mass of an electron is negligibly small compared to that of the nucleons.

Properties of subatomic particles

As a result the mass of an atom can be considered to be concentrated in its nucleus. The approximate mass of one proton or one neutron is 1u. Therefore approximate atomic mass in daltons is numerically equal to the number of nucleons in the atom. The numberof neutrons in the nucleus is designated by the symbol N; and the total number of protons and neutrons, that is nucleons, in an atom is called its atomic mass number (A).

Mass number (A) = Number of protons (Z) + Number of Neutrons (N)Therefore A = Z + N N = A - Z

The composition of any atom is represented by element symbol (X) with the atomic mass number (A) as superscript on left and atomic

number (Z) as subscript its :

The atom or nucleus having a unique composition as specified by AZX is called a nuclide.

Isotopes, isobars and isotones :

Similarities in composition of nuclides results in three types of relationships.

i. Isotopes : Some elements exist as single natural nuclide. For example 9 19F .

However, many elements exist naturally as mixture of two or more types of atoms or nuclides. These individual nuclides are called isotopes of that element. 

Isotopes of Carbon

All the isotopes of an element have the same number of protons but different number of neutrons in their nuclei. As the proton number is the atomic number, all the isotopes of an element have the same position in the modern periodic table which exhibit similar chemical properties. All the natural isotopes of an element coexist and a definite natural abundance. Table shows various features of the three common isotopes of carbon.

ii. Isobars : The atoms of different elements having the same mass number but different atomic numbers are called isobars. Isobars are different elements.

Isobars

They have different chemical properties and occupy different positions in modern periodic table. Table shows an illustration of isobars.

iii. Isotones : The atoms of different elements having same number of neutrons in their nuclei are called isotones. Table shows examples of isotones.

Isotones

Drawbacks of Rutherford atomic model :

i. Let us now go back to the point of time when 

Rutherford put forth his nuclear model of atom. It is akin to a miniature of the solar system, the nucleus playing the role of the massive sun and the electrons are lighter planets. Electrons in this model could not be stationary as the electrostatic force of attraction exerted would pull them towards itself, and this would form a miniature version of Thomson’s atomic model. However, the electrons revolving about the nucleus, as described by Rutherford, also pose a problem. Electrons in the Rutherford model are negatively charged particles in orbital motion. Such orbital motion is an acceleraled motion accompanied by a continuous change in the velocity of electron as noticed from the continuously changing direction. According to the electromagnetic theory of Maxwell, accelerated charged particles would emit electromagnetic radiation. An electron in an orbit would emit radiation, equivalent energy possessed by the radiation associated with the electronic motion. The orbit would, therefore, shrink continuously. Thus, an electron orbiting about the nucleus would follow a spiral path to the nucleus. It can be seen that the Rutherford atomic model has an intrinsic instability of atom. However, real atoms are stable.

ii. The second serious drawback of the Rutherford model is that it does not describe the distribution of electrons around the nucleus and their energies. The drawbacks of the Rutherford model were overcome in the Bohr atomic model.