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Atomic Structure

WHAT IS THE STRUCTURE OF AN ATOM?

 

In the 1800's  John Dalton used three laws to develop his Atomic Theory , which consists of five basic principles:

      1.  All matter is made of indivisible and indestructible atoms.

      2.  Atoms of a given element are identical in their physical and chemical properties.

      3.  Atoms of different elements have different physical and chemical properties.

      4.  Atoms of different elements combine in simple, whole number ratios to form chemical compounds.

      5. Atoms cannot be subdivided, created or destroyed when they are combined, separated, or rearranged in chemical reactions.

 

FINDING THE STRUCTURE OF THE ATOM

     

      In 1897, Thomson discovered the electron, by experimenting with cathode ray tubes.  He concluded that the small negatively charged electron had a very small mass in comparison to the rest of the atom.  Using this information and evidence that an atom had a neutral charge,

He proposed a model of the atom named the "Plum Pudding Model" which showed the atom as a positive ball imbedded with negatively charged electrons.

 

      Rutherford later discovered that the atom has a dense central nucleus, which contains positively charged protons.

 

      In 1913 Bohr proposed that electrons reside in certain energy levels in the atom.

 

      Chadwick's experiments lead the discovery of a third particle, the neutron.  The neutron is a neutral particle that is located in the nucleus, and has the same mass as a proton.

 

QUANTUM THEORY A MODERN PICTURE OF THE ATOM.

      The discovery of the wave property of electrons led to the development of Quantum Theory, which states that electrons can be found in regions called orbitals.   The quantum model is often called the electron cloud model.

 

      The modern atomic model describes the atom as a central cluster of neutrons and protons surrounded by electrons that travel in orbitals.

 

 

LOCATING ELECTRONS IN ATOMS.

      The exact location of an electron is described in quantum numbers.  The first quantum number tells which energy level the electron is located in.

  

Within each energy levels are orbitals :

      The S orbital is shaped like a sphere and is the lowest energy level.  The S orbital can contain up to two electrons.

  

      The P orbital is shaped like a dumbbell, it has three areas called sublevels.  The P orbital can hold up to six electrons.

 

       The D orbital has five sublevels and can contain up to ten electrons.

  

Wolfgang Pauling determined that no two electrons can have the same location (quantum numbers) in an atom.  This rule is called the Pauli Exclusion Principle.

 

ELECTRON CONFIGURATION

 

Electron configurations are models for electron arrangements around the nucleus of an atom.

 

Each orbital is filled with two electrons, spinning in opposite directions.  Each orbital is fill so that each sub level gets an electron spinning in one direction.  The second atom is then added to the sublevels in opposite spins.

 

This type of electron configuration follows Hund's Rule  which states that orbitals of equal energy are filled with one electron before the pairing occurs with the second electron.

 

Use the following steps to determine electron configuration.

 

1.  Use the atomic number to determine the number of electrons for the atom.

 

2.  Use chart to fill orbitals with electrons in accordance with Hunds Rule.

 

3.  Check to see if the total number of electrons in the configuration equals the atomic number of the element.

 

**Electron configurations are sometimes abbreviated using the noble gasses.