Structure of Atom Test - 4

• J. J. Thomson, who discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model.
  
• In Thomson's model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "plums" surrounded by positively charged "pudding".
  
• The 1904 Thomson model was disproved by Hans Geiger's and Ernest Marsden's 1909 gold foil experiment.
  

The emission of free electrons from a metal surface when light is shone on it is called the photoemission or the photoelectric effect. This effect led to the conclusion that light is made up of packets or quantum of energy. Einstein already associated the light quantum with momentum. This strongly supported the particle nature of light and these particles were named photons. Thus, the wave-particle duality of light came into picture. Einstein won the Nobel Prize for Physics not for his work on relativity, but for explaining the photoelectric effect.

Azimuthal quantum number l = 1 is for p and l = 2 is for d

Now Cr has configuration.
1s²,2s²,2p⁶,3s²,3p⁶,3d⁵,4s¹

Hence there are 12, p-electrons and 5, d-electrons

Mass number. The mass number (symbol A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

mass No. = no. of protons + no. of neutrons.

Isoelectronic species refers to the elements that have the same number of electrons.
Mg²⁺ is a 10 electron species (1s²,2s²,2p⁶).Its configuration is like that of Ne.
Thus it is isoelectronic with any element having 10e- or we can say 8e- in its valence shell.
Atomic number of Sodium (Na) is 11 after loosing one electron it became Na+ and have 10 electron.
Thus Mg2⁺ is isoelectronic with Na⁺.

Rutherford's model of an atom: Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil.

On the basis of his experiment, Rutherford put forward the model of an atom, which had the following features:

• There is a positively charged centre in an atom called the nucleus.
  
• Nearly all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in well-defined orbits.
  
• The size of the nucleus is very small as compared to the size of the atom.
  

Cathode rays - In 1897, British physicist J. J. Thomson showed the rays were composed of a previously unknown negatively charged particle, which was later named the electron.

Dalton has developed the theory of the structure of matter and this theory is known as Dalton’s atomic theory. His research was based on experiments and also from law of chemical combination. Dalton’s atomic theory was quickly explained the many heretofore unexplained chemical phenomena. Dalton’s atomic theory quickly became the theoretical foundation in chemistry.

Dalton’s atomic theory stated that:

• All the matter is made of atoms which are tiny particles and indivisible.
  
• All the given atom of the element is identical in mass, size, shape, and in other properties.
  
• All different elements have different types of atoms and also different in their mass, size, shape, and in other properties.
  
• All the atom of an element cannot be made or destroyed.
  

In 1909, Robert Millikan and Harvey Fletcher conducted the oil drop experiment to determine the charge of an electron. They suspended tiny charged droplets of oil between two metal electrodes by balancing downward gravitational force with upward drag and electric forces.

The experiment helped earn Millikan a Nobel prize in 1923

Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number.

The emission spectrum of hydrogen consists of several series of sharp emission lines in the ultraviolet (Lyman series). in the visible (Balmer series). and in the infrared (Paschen series, Brackett series, etc,) regions of the spectrum. These series are named after their discoverer.

The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum.

In an absorption spectrum, portions of a continuous spectrum (light containing all wavelengths) are missing because they have been absorbed by the medium through which the light has passed; the missing wavelengths appear as dark lines or gaps.

Total energy of the electron, E = −3.4 eV
Kinetic energy of the electron is equal to the negative of the total energy.
K = −E = − (− 3.4) = +3.4 eV
Hence, the kinetic energy of the electron in the given state is +3.4 eV.

In an amazing demonstration of mathematical insight, in 1885 Balmer came up with a simple formula for predicting the wavelength of any of the lines in atomic Hydrogen in what we now know as the Balmer series.

Three years later, Rydberg generalized this so that it was possible to determine the wavelengths of any of the lines in the hydrogen emission spectrum. Rydberg suggested that all atomic spectra formed families with this pattern (he was unaware of Balmer's work).

It turns out that there are families of spectra following Rydberg's pattern, notably in the alkali metals, sodium, potassium, etc., but not with the precision the hydrogen atom lines fit the Balmer formula, and low values of n2 predicted wavelengths that deviate considerably.

Rydberg's general equation is as follows: