Atomic Models

We have studied about particles of the atom but still, the arrangement of particles of an atom in space were still unknown. Attempts were made to find out the spatial arrangement of atomic particles viz. neutron, proton and electron. Different atomic models were proposed which were telling about the arrangement of particles inside an atom.

The very first attempt was done by Thomson which is called as Thomson Model of Atom.

Let us take a look at Thomson Model of Atom:

Thomson’s Model:

J.J. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping the electrons in a matrix.

This model is called the plum – pudding model after a type of Victorian dessert in which bits of plums were surrounded by matrix of pudding.

However, this model could not satisfactorily explain the results of scattering experiment carried out by Rutherford who worked with Thomson.

Rutherford’s Scattering Experiment:

Rutherford bombarded a very thin sheet of gold (thickness of the order of 0.0004 cm) with a narrow beam of α – particles (He+2 ion having atomic mass 4 units and charge +2 units) and determined the subsequent path of these α – particles with the help of a fluorescent screen of zinc sulphide.

He made the following observation with this experiment:

(1) Most of the α – particles pass through the gold sheet without any deflection.

(2) Some of the α – particles gets deflected at certain angles (less than 90°).

(3) A very few α – particles (1 in every 20,000) gets deflected backwards i.e. reversed back at angle of near about 180°.

Conclusion:

Since most of the α – particles passed straight through the gold foil, It showsthat, most of the part of the atom is empty.
Only a few α – particles are deflected at large angles,shows the presence of a heavy positively charge body, since for such a large deflection, the α– particles must come closer to or collided with a massive positively charged body.
The fact that 1 in 20,000 α – particle have deflected at 180° backwards, it shows that volume occupied by the heavy positively charged body in the atom is very small in comparison to the total volume of the atom.

Rutherford’s Atomic Model:

On the basis of the results obtained from the experiment, Rutherford proposed an atomic model. According to him,

(i) All the positive charge of the atom resides in a very small volume inside the atom i.e. all the protons and neutrons of the atom are present in a very small region at the centre of the atom, which is called nucleus. Nearly the total mass of the atom is present in the nucleus.

(ii) The size of the atom is around 10⁵ times that of size of nucleus i.e. size of nucleus is very small in comparison to the size of atom. The diameter of the nucleus is about 10^-13 cm while the atom has a diameter of the order of 10^-8 cm.

(iii) Since all the neutrons and protons are contained by the nucleus, so most of the space inside the atom is empty.

(iv) The electrons revolve around the nucleus in well-defined orbits and the required centripetal force is balanced by the colombic force between positively charged nucleus and electrons.

Limitations of Rutherford’s Model of Atom:

Position of Electron – Rutherford said that electron revolves around the nucleus however, he was unable to determine the exact position of electron inside an atom.
Stability of Atom – According to Maxwell, when a charged particle accelerates, it radiates energy. Since electron is a charged particle and According to Rutherford, it is revolving around the nucleus means it is accelerating (even if moving with constant speed since direction is constantly changing), so it should continuously emit radiation and lose energy. As a result of this a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus.

Interesting Fact - It was calculated that the electron should fall into the nucleus in less than 10-8 sec.

Terms related to Atom

Atomic Number – Atomic number is the number of proton present in the nucleus. It is denoted by Z. It is also equal to the number of electrons present in a neutral atom.
Mass Number – Total number of proton and neutron present in the nucleus of the atom is called mass number of an atom. Mass number is denoted by A.
Nucleons – Proton and neutron are collectively called as nucleons.
Isotopes – Such atoms which have same atomic number but different mass number are called isotopes of each other. Isotopes have same number of proton but different number of neutrons. For e.g. ₁H¹, ₁H²,₁H³ are the isotopes of Hydrogen.

Note – All isotopes of a given element shows same chemical behaviour. This is since chemical properties of an atom depends upon number of electron and number of electron depends upon number of proton. Neutron do not have any significant effect on chemical properties of an atom.

Isobars – Such atoms which have same mass number but different atomic number are called isobars. For e.g. ₁₅P³²,₁₆P³² are isobars.
Isotones – Atoms having same number of neutrons but different number of protons or different mass number are termed as isotones. For e.g. ₆C¹⁴,₇N¹⁵,₈O¹⁶ are isotones.
Isoelectronic – Atoms having same number of electrons are called isoelectronic. For e.g. N₂, CO, CN^– are isoelectronic species.
Nuclear Isomer – Nuclear isomers (isomeric nuclei) are the atoms with the same atomic number and same mass number but with different radioactive properties.

Example of nuclear isomers is
Uranium -X (half-life 1.4 min) and
Uranium -Z (half-life 6.7 hours)
The reason for nuclear isomerism is the different energy states of the two isomeric nuclei.
Other examples are

₃₀Zn⁶⁹ (T_1/2 = 13.8 hour)
₃₀Zn⁶⁹ (T_1/2 = 57 min)
₃₅Br⁸⁰ (T_1/2 = 4.4 hour)
₃₅Br⁸⁰ (T_1/2 = 18 min)

Isosters: Such molecules which have same number of electrons as well as same number of atom are called isosters. For e.g. N₂ and CO both have 2 atom and same number of electron so they are isosters. Other examples are CO₂ and N₂O etc.
Atomic mass unit – it is equal to the 1/12th mass of carbon-12 atom.