• In the early 1900’s many scientist had turned their attention to investigate the structure of atom . Many models were proposed and a handful have been adopted as ways to describe the atom. Neither of them are perfect but they have bought us a long way towards understanding these building blocks. These are –

• The Rutherford Model• The Bohr Model• The Dalton’s Model• The Thomson’s Model• The Quantum Mechanical Model

The Rutherford ModelThe Rutherford Model In 1909 Ernest Rutherford In 1909 Ernest Rutherford

conduct what is now a famous conduct what is now a famous experiment where he bombarded experiment where he bombarded gold foil with alpha particles gold foil with alpha particles (Helium nuclei) . A source which (Helium nuclei) . A source which undergoes alpha decay is placed undergoes alpha decay is placed in a lead box With a small hole in in a lead box With a small hole in it .Ant of the alpha particles it .Ant of the alpha particles which hit the inside of the box which hit the inside of the box are simply stopped by the are simply stopped by the box .Only those which pass box .Only those which pass through the opening are allowed through the opening are allowed to escape, and they follow a to escape, and they follow a straight line to the gold foil .straight line to the gold foil .

Observations –Observations – Most of the alpha particles Most of the alpha particles

pass straight through the pass straight through the gold foil.gold foil.

Some of the alpha particles Some of the alpha particles get deflected by very small get deflected by very small amount.amount.

A very few get deflected A very few get deflected greatly.greatly.

Even fewer get bounced of Even fewer get bounced of the foil and back to the left.the foil and back to the left.

Conclusions – Conclusions – The atom is 99.99% empty The atom is 99.99% empty

space.space. The nucleus contains a The nucleus contains a

positive charge and most of positive charge and most of the mass of atom .the mass of atom .

The nucleus is The nucleus is approximately 100,000 approximately 100,000 times smaller then the times smaller then the atom . atom .

The deflection of alpha particles through large angles and even The deflection of alpha particles through large angles and even bouncing off the gold foil is best described by the animation bouncing off the gold foil is best described by the animation above. Keep in mind that the gold nuclei have a charge of +79 above. Keep in mind that the gold nuclei have a charge of +79 and the alpha particle has a charge of +2. these two positive and the alpha particle has a charge of +2. these two positive charges repel each other. The closer they get, the greater the charges repel each other. The closer they get, the greater the force. The greater the force the greater the amount of force. The greater the force the greater the amount of deflection. deflection.

The Bohr Model

While The Rutherford Model focused on describing the nucleus , Niels Bohr turned his attention to describing the electron. Prior to the Bohr Model, the accepted model was one which depicted the electron as an orbiting planet.

The flaw with the planet like model is that an electron particle moving in the circular path would be accelerating . An accelerating electron creates a changing magnetic . The changing magnetic field would carry energy away from the electron , eventually slowing it down and allowing it to be captured by nucleus

Bohr built upon spectroscopic observations of atoms. Spectroscopists noticed that an atom can only absorb certain energies (colors) of light (the absorption spectrum) and once excited can only release certain energies (the emission spectrum) and these energies happen to be the same. Bhor uses these observations to argue the energy of a bound electron is “quantized”. Quantized is a fancy word mean only certain quantities of energy are allow. This explanation addresses the true origin of light. Science only certain energy levels are allow it is actually possible to diagram the atom in the terms of its energy level.

In the animation below You will see a hydrogen atom .In the animation you will notice that if the energy of the photon of light is just right , it will cause the electron to jump to a higher level.

When the electrons jumps back down, a photon is created for each jump down. A photon without the right amount of energy (the pink one) passes through the atom with no effect. Photons with too much energy will cause the electron to be ejected with ionizes the atom. A ionized electron is said to be the n = infinity energy level . Keep in mind that these rings are not actually orbits, but are levels that represent the electron wave .The number n corresponds to the number of complete waves of electron.

The Dalton’s ModelThe Dalton’s Model

Daltons theory was based on the premise that the atoms of the different elements could be distinguished by Daltons theory was based on the premise that the atoms of the different elements could be distinguished by difference in their weight . He stated his theory in a lecture to the royal institution in 1803 . The theory difference in their weight . He stated his theory in a lecture to the royal institution in 1803 . The theory proposed a number of basic ideas :proposed a number of basic ideas :

All matter is composed of atoms.All matter is composed of atoms. Atoms cannot be made or can destroyed.Atoms cannot be made or can destroyed.

All atoms of the same element are identical.All atoms of the same element are identical. Different elements have different types of atom.Different elements have different types of atom.

Chemical reactions are occur When atoms are rearranged.Chemical reactions are occur When atoms are rearranged.Compounds are formed from atoms of the constituent elements.Compounds are formed from atoms of the constituent elements.

Using his theory, Dalton rationalized the various laws Using his theory, Dalton rationalized the various laws of chemical combination Which were in existence at of chemical combination Which were in existence at that time .However, he made a mistake in assuming that time .However, he made a mistake in assuming that the simplest compound of the two elements must that the simplest compound of the two elements must binary, formed from atoms of each element in a 1:1 binary, formed from atoms of each element in a 1:1 ratio, and his system of atomic weights was not very ratio, and his system of atomic weights was not very accurate , he gave oxygen an atomic weight of seven accurate , he gave oxygen an atomic weight of seven instead of eight. Despite these errors , Dalton's theory instead of eight. Despite these errors , Dalton's theory proved a logical explanation of concepts. proved a logical explanation of concepts.

The Thomson's ModelThe Thomson's Model An atom is electrically neutral .It An atom is electrically neutral .It

contains positive charges (due contains positive charges (due to protons) and negative to protons) and negative charges (due to electrons). charges (due to electrons). Hence J.J. Thomson assumed Hence J.J. Thomson assumed that an atom is a uniform sphere that an atom is a uniform sphere of positive charges with of positive charges with electrons embedded into it. electrons embedded into it. Thus, an atom may be Thus, an atom may be visualized as a pudding or cake visualized as a pudding or cake of positive charges with raisins of positive charges with raisins (electrons) embedded into it . (electrons) embedded into it . Hence this is also known as Hence this is also known as “Raisin-Pudding Model”“Raisin-Pudding Model”

This model explains the This model explains the electrical neutrality of atom ,but electrical neutrality of atom ,but it failed to stand for a long time it failed to stand for a long time as it was not consistent with the as it was not consistent with the experimental results obtained experimental results obtained by Rutherford.by Rutherford.

The Quantum Mechanical ModelThe Quantum Mechanical Model

• As we learn that the Bhor model establishes the concept of definite electron energy levels within atoms . But Bhor’s model was rather simplistic and as scientist made more discoveries about more complex atoms , Bhor’s model was modified and eventually was replaced by more sophisticated models .

• The Quantum Mechanical Model of the atom presents a more accurate model of the atom. It us a more sophisticated model based on complex mathematical calculations and interpretations .

• The quantum model introduces the concept of -• Sublevels : According to the calculations in

quantum mechanics each energy level of an atom consists of one or more sublevels The principle energy level ,n, determines the number of sublevels in the principle energy level.

• n = The number of sublevels in the nth principal energy level

• Atomic orbits: Each sublevel has one or more atomic orbit with a specific 3-diminsional shape.