- Most of the passed through the gold foil undeflected.

- A small fraction of the was deflected by small angles.

- A very few (1 in 20,000) bounced back, that is, were deflected by nearly 180°.

- The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford.

- The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus, Rutherford’s model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets.

- Electrons and the nucleus are held together by electrostatic forces of attraction.

Mass number (A) = Number of protons (*Z
*) + Number of neutrons (n)

The results observed in this experiment were:

Where n is an integer equal to or greater than 3.

- The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits, stationary states or allowed energy states. These orbits are arranged concentrically around the nucleus.

- The energy of an electron in the orbit does not change with time. However the electron will move from a lower stationary state to a higher stationary state when required amount of energy is absorbed by the electron or energy is emitted when electron moves from higher stationary state to lower stationary state.

- The frequency of radiation absorbed or emitted when transition occurs between two stationary states that differ in energy by , is given by :

- The angular momentum of an electron in a given stationary state can be expressed as:

- The stationary states for electron are numbered n = 1, 2, 3.......... These integral numbers are known as Principal quantum numbers.

- The radii of the stationary states are expressed as:

*
*r_{n
} = n_{2
} a_{0
}

- Where a
_{0 }= 52.9 pm. Thus the radius of the first stationary state, called the Bohr orbit, is 52.9 pm. normally the electron in the hydrogen atom is found in this orbit (that is n=1). As n increases the value of r will increase. In other words the electron will be present away from the nucleus.

- The most important property associated with the electron, is the energy of its stationary state. It is given by the expression.

- Where R
_{H }is called Rydberg constant and its value is 2.18 × 10^{-18 }J.

- The energy of the lowest state also called the ground state is:

- The energy of the stationary state for n = 2 will be:

- Bohr’s theory can also be applied to the ions containing only one electron, similar to that present in hydrogen atom. For example, He
^{+ }, Li^{2+ }etc. The energies of the stationary states associated with these kinds of ions are given by the expression.

Hydrogen

- Where is a mathematical operator called Hamiltonian.

N | 1 | 2 | 3 | 4 |

Shell | K | L | M | N |

Value of l: | 0 | 1 | 2 | 3 | 4 |

Notation for sub-shell | s | p | d | f | g |

m_{l
} = -l, - (l - 1), - (l - 2)…. 0, 1…… (l - 2), (l - 1), l

1s < 2s = 2p < 3s = 3p = 3d <4s = 4p = 4d = 4f <

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s .........

- The maximum number of electrons in the shell with principal quantum number n is equal to 2n
^{2 }.

- s
^{a }p^{b }d^{c }notation

- Orbital diagram

Be = 1s^{2
} 2s^{2
}

B = 1s^{2
} 2s^{2
} 2p^{1
}

C = 1s^{2
} 2s^{2
} 2p^{2
}

N = 1s^{2
} 2s^{2
} 2p^{3
}

O = 1s^{2
} 2s^{2
} 2p^{4
}

F = 1s^{2
} 2s^{2
} 2p^{5
}

Ne = 1s^{2
} 2s^{2
} 2p^{6
}

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