Models of Atom

THOMPSON MODEL

Thompson proposed an atomic model which visualized the atom as a homogenous sphere of positive charge inside of which are embedded negatively charged electrons.

He also determined the ratio of  the charged to mass,  e/m , of electrons, and found e/m to be identical for all cathode rays particles, irrespective of the kind of gas in the tube or the metal the electrons are made of.

RUTHERFORD MODEL

He proposed a planetary model of the atom which suggested that the atom consists of positively charged heave core called the nucleus where most of the mass of the atom was concentrated .around this nucleus, negatively charged electrons circle in orbits much as planets move around the sun. Each nucleus must be surrounded by a number of electrons necessary to produce an electrically neutral atom

LIMITATION OF RUTHERFORD MODEL

1. It predicts that light of a continuous range of frequencies will be emitted whereas experiment show line spectra instead of continuous spectra.
2. It predict that  atoms are unstable-electrons quickly spiral into the nucleus- but we know that atoms in general are stable, since the matter around us is stable.

Clearly Rutherford’s model was not sufficient to explain experimental observations. Some sot of modification was needed and this was provided by Neils Bohr.

EVALUATION

What are the limitations of Rutherford’s model?

THE BOHR’S MODEL

He suggested a model of hydrogen atom in which

1. the electron moves around the nucleus in certain specific circular orbits called energy level and that the centrifugal force  due to this motion counter balances the electrostatic attraction between the electron and the  nucleus.  The electrons can move without losing or radiating energy in such orbits. He called the possible orbits stationary states.  Only orbits of particular radii were possible. In general, the higher the energy of the electron, the farther its orbits from the nucleus.

ii.   the energy of an electron in an atom cannot vary continuously to a limited number of discrete or individual values. The energy of the electron is said to be quantized ( i.e can have only discrete values). He thus postulated that electrons in an atom cannot lose energy continuously but mist do so in quantum ‘jumps’.  He postulated that light is emitted only when an electron jumps from one stationary state to another of lower energy.

When such a jump occurs, a single photon of light  would be emitted whose energy is given by

hf = E_u – E_l

E_u         = Energy of upper state

E_l         =  energy of lower state

h = planck constant ( h = 6.67 x 10^-34 Js)

f = frequency of emitted light

Bohr was able to account for the appearance of line spectrum rather than continuous spectrum.

An electron absorbs energy when it transfers to  higher energy level ( excitation) Photon An electron emits a photon when it moves to a lower energy level.

iii. The third postulate of Bohr’s model was that angular momentum values of the electron in an atom are quantized, that is, they are restricted to a limited number of discrete values that are integral 9n0 multiples of a  constant, planck’s constant (h) divided by 2 π. That angular momentum

L =n (h)

          2π    n = 1,2,3,4…

The integral n is called a quantum number

Bohr model is also known as the Bohr – Rutherford model since it was an extension of Rutherford planetary model.  The great success of Bohr theory is that;

1. it gives a model for why atoms emits, line spectra and accurately predicts, for hydrogen, the wave lengths of emitted lights or the frequencies of the lines in the hydrogen spectrum.
2. It offers an explanation for absorption spectra; photons of just the right wavelength can knock an electron from one energy level to a higher one. To conserve energy, the photon must have just the right energy. This explains why a continuous spectrum passing through a gas will have dark (absorption) lines at the same frequencies as the emission line.
3. It ensures the stability of atoms by stating that the ground state is the lowest state for an electron and there is no lower energy level to which it can go and emit more energy.
4. It accurately predicts the ionization energy of 13.6ev for hydrogen.

EVALUATION

What are the successes of Bohr’s model?

THE ELECTRON CLOUD MODEL

This model visualizes the atom as consisting of a tiny nucleus of radius of the order of  10^-10 – 10^-15m. The electron is visualized as being in rapid motion within a relatively large region around the nucleus, but spending most of its time in certain high probability regions.  Thus, the electron is not considered as a ball revolving around the nucleus but as a particle or wave with a specified energy having only a certain probability of being in a given region in the space outside the nucleus. The electron is visualized as spread out around the nucleus in a sort of electron – cloud.

Chemists prefer to consider the electron in terms of a cloud of negative charges (electron cloud), with a cloud being dense in regions of high electron probability and more diffuse in region of low probability.                               

The probability of finding the electron inside the spherical  boundary is high. The probability then decreases rapidly as the distance of the thin shell from the nucleus increases.

ATOMIC STRUCTURE AND CHEMICAL BEHAVIOUR

Today we consider the atom as made up of tiny but massive nucleus at the centre and outside the nucleus is a cloud of electrons which move in wave-like orbits or shells around the massive nucleus. The nucleus consists of protons which carry positive changes and neutrons which carry no charge. The neutron and proton together constitute the nucleon. All the mass of an atom is concentrated in the central nucleus. The protons, neutrons and electrons are the fundamental sub atomic particles of the atom.

The electron is the lightest particle of an atom, with a mass (Me) of 9.10^-31kg and an electronic charge e- = 1.6 x 10^-19 C.

The proton has a mass of 1.67 x 10^-27kg which is over 1836 times heavier than the mass of an electron. It carries a positive charge, e+ = 1.67 x 10-29 c ( i.e e+ = e- = 1.6 x 10 -10). There are the same number of protons in the atoms of different elements.in a neutral atom, the number of protons equals the number of electrons.

We denote the atom of an element X by  ^A_zX

      A  = mass number

     Z = atomic number

The atomic number or proton number (Z) is the number of protons in the nucleus of an element. The mass number or nucleon number (A) is the total number of protons and neutrons in an atom of an element

ISOTOPES

Isotopes are atoms of the same element which have the same atomic number (X) but different mass number. Isotopes are thus atoms with the same number of protons, but different number of neutrons. Isotopes have similar chemical properties because they have the same number of electrons round the nucleus. Chemical combinations is due to an exchange of outer or valence electrons between elements.

Examples of isotopes

1. i.    ³⁵ ₁₇Cl (17 protons , 17 electrons, 18 neutrons)
   1. ³⁷₁₇Cl (17 protons, 17 electrons, 20 neutrons)

b)   ¹²₆Cl  (6 protons, 6 electrons, 6 neutrons)

¹³₆Cl  (6 protons, 6 electrons, 7 neutrons)

c)   ¹⁶₈O  (8 protons, 8 electron 8 neutrons)

¹⁷₈O  (8 protons, 8 electrons, 9 neutrons)

¹⁸₈O   (8 protons, 8 electrons, 10 neutrons)

• ²³⁸₉₂U (92 protons, 92 electrons, 146 neutrons)

^238\5₉₂U (92 protons,92 electrons, 143 neutrons)

²³⁴₉₂U (92 protons, 92 electrons, 142 neutrons)

GENERAL EVALUATION

1. Mention four effect of thermal expansion of solid
2. Mention four the advantages and disadvantages of thermal expansion of solid

READING ASSIGNMENT

New School Physics pgs 464-471

WEEKEND ASSIGNMENT

1.  Which of the following statement is not true of the isotope of an element? They A. Are atoms of the same element B. Have the same chemical properties C. Have the same atomic mass D. Have the same mass number

2.  Which of the following representation is correct from an atom X with 28 electrons and 30 neutrons? A.  ³⁰₂₈X       B   ²⁸₃₀X      C ⁵⁸₃₀X   D. ⁵⁸₂₈X   E. ³⁰₂X

3.  Bohr theory provides evidence for the A structure of the atom   B. positive charge of an electron     C existence of energy level in the atom     D. positive charge on a proton

4.  Which of the following particles determine the mass of an atom? A. protons and neutrons

B. Neutrons only C. protons and electrons D. Neutrons and electrons E.  Protons only

5.   Which of the following names is not associated with the models of the atom.

      A. Isaac Newton   B. Neils Bohr   C. J.J. Thompson   D. Ernest Rutherford   E. John Dalton

THEORY

1. Describe the essential feature of the Bohr- Rutherford Model of the atom. What are its

successes and its failures.  How does it account  for line spectra.

2. What are the essential features of the Electron –Cloud Model of the atom. Illustrate with a diagram.