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Physical Chemistry

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22 Nov 2008 17:55:43 IST
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How should we visualise the structure of an atom ?
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How should we visualise the structure of an atom ?

Blazing goIITian

Joined: 3 Nov 2008
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22 Nov 2008 19:52:14 IST
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Solving Schrodinger's Equation for a particular atom gives many solutions, each one corresponding to a different possible orbital. The equations for these orbitals do not really describe how the electron "travels" around the nucleus, but, rather, the probability that the electron will be at in a certain position around the nucleus. By examining the probabilities given by a particular orbital, a "shape" of the orbital can be seen. This shape represents a pocket of space around the nucleus that the electron is most likely to be found.

Thus, the equation for an orbital does not tell you how an electron moves about the atom, but where it is most likely to be located. We actually have no idea what the path of travel might look like.

p-type orbital
(partial courtesy Zumdahl)

d-type orbitals
(partial courtesy Zumdahl) Orbital Visualization Examples
The dotted images correspond to the probability distribution for a particular solution (orbital) to Schrodinger's Equation. Based on the probability of where the electron might be, a boundary can be drawn, and a "shape" to the orbital can be described. While different atoms have different solutions to the Schrodinger Equation, many of the solutions look similar between atoms, and these orbitals have specific names. The ones above and below are called p-type, and the ones to the left are called d-type.

p-type orbital for Boron
2 overlapped p-type orbitals for carbon. The different orbitals are different colors (one is horizontal and one is vertical).

The simplest orbital, and first one to fill with electrons, is the s-type. The s orbital is spherical, as seen below. Hydrogen normally only has one electron and it resides in an s orbital, shown to the right. This is a complete electron density diagram of Hydrogen, but only one s-type orbital is seen because there is only one electron, and it occupies a single s orbital.

Electron probability density for Hydrogen

(partial courtesy Zumdahl) Energy Levels
The many solutions to Schrodinger's equation can be classifed by the shape their probability distributions take, called orbitals, like s, p, and d-type, as shown above. Most orbital types have several possible orientations too, as seen with the vertical and horizontal p-type orbitals shown with Carbon above.

In addition to orbital type and orientation, each orbital can exist at several energy levels. Orbitals of different energy levels have the same general shape, in terms of boundary surface representation, but are bigger at higher energy levels.

In addition, the probablity distributions for orbitals at higher energy levels show "dead" spots, called nodes. This doesn't change the overall shape we imagine the orbital has, but is an interesting phenomenon.

Note: Not all orbitals exists at all energy levels. There is no 1st-level p orbital, or 1st- and 2nd-level d orbital.

2
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Cool goIITian

Joined: 20 Nov 2008
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23 Nov 2008 18:15:29 IST
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Blazing goIITian

Joined: 8 Oct 2008
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28 Feb 2011 01:30:54 IST
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hi

Structure of Atom

## Introduction

A story unfolds - Structure of atom : Let us right away meet those who scripted a "mighty story" of a "miniscule" atom.
John Dalton (1766 - 1844) : Born in a poor family of a weaver in England, he rose to become a teacher and later a Principal of a school. In 1793, he left for Manchester to teach physics and chemistry in a college.

## Fundamentals of atomic structure

Table consists of some important formulae which are frequently used in solving numerical problems related out atomic structure.

## Atomic structure

Although Dalton's theory of the existence of atoms was published in 1808 there was no clear idea of what an atom might look like until the early part of the twentieth century. With the advent of electricity and phenomenon of spectrum, experimentation underwent a change and a flurry of research in the structure of matter has put forth the modern concept of the structure of atom.

## Discovery of Neutrons

Although Rutherford's model was enormously successful at explaining the scattering of alpha particles, there was a problem when the experimental data were used to calculate the mass of the nucleus. From the results of the scattering experiments, it was possible to calculate the charge on the nucleus. However the mass of these protons was only about half the overall mass of the nucleus. In 1920, William Draper Harkins an American physicist suggested that the missing mass could be accounted for if the nucleus contained other particles with mass similar to that of a proton but no charge. He named this particle neutron. James Chadwick finally discovered it in 1932. He bombarded the element beryllium with alpha-particles.

## Discovery of Electrons

In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which, make other substances emit light too. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles.

## Discovery of Protons

Since the atom is electrically neutral there must be positively charged particles present in the atom to neutralize the negative charges of the electrons. Goldstein experimentally proved the existence of protons in the atom.

## Atomic Models

'Raisin Pudding' Model - Thomson's model of atom : J. J Thomson the discoverer of the electron believed that the atom is a uniform sphere of positive charge with electrons (raisins) embedded in it.

## Characteristics of Electrons, Protons and Neutrons

Charges present in the nucleus are termed as electrons, protons and neutrons hence Electrons are negatively(-1) charge, present outside of the nucleus, Protons are positive(+1) charges and neutrons with neutral(0) charge, present inside of the nucleus.

## X-Rays

In 1895, Wilhelm Conrad Roentgen, a German physicist observed, while doing some experiments with the discharge tube, that when cathode rays are allowed to fall on a metal target called anticathode placed in their path, a new kind of rays are produced. These radiations are called X-rays.

## Bohr's Model of an Atom

Rutherford's model had a major drawback. It could not explain why ultimately electrons did not fall into the nucleus by taking a spiral path. This was in concurrence with the electromagnetic theory that states 'if a charged particle undergoes accelerated motion then it must radiate energy (lose) continuously'.

## Atomic Number and Atomic Mass

The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z.

## Arrangement of Electrons in an Atom (Bohr-Bury Scheme)

Electrons revolve around the nucleus in different energy levels or shells and each shell is associated with definite energy. The energy of the K shell is the least while those of L, M, N and O shells increases progressively. We also know that any system that has least energy is the most stable.
1st energy level is K shell
2nd energy level is L shell
3rd energy level is M shell
4th energy level is N shell and so on.

## Valence Electrons

The outermost shell of an atom is known as the valence shell. The electrons present in the valence shell are called valence electrons.

## Valency

It is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom has 4 valence electrons.
K L
C(6) 2  4

## Isotopes

It is interesting to note that atoms of a given atomic number can have different number of neutrons.

The discovery of the electron towards the end of the nineteenth century was the starting point of new avenues of research in science, which were to give physicists an insight into the structure and nature of the atoms of matter.

## Numerical Problems on Atomic Structure

Few problems to solve related to atomic structurewhich involves numericals on tranisition of electrons, wavelength and energy required to move any electron.

## Summary

Atoms are made up of three fundamental particles: electrons, protons and neutrons.
Discharge tube experiments by William Crookes and J.J. Thomson led to the discovery of electron !!!!!

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