Notes On Correct Model Of Atom And Why We Care

• For a high school chemistry learner, the most correct explanation of the atom is called Charged Cloud Model or the Quantum Mechanical Model.
  

• Based principles:
  
  • Don't know "Exactly" where the electron is located but just know the highest probability to find the electron. It is a region of space outside the nucleus. Electrons can be anywhere and move in any direction but most often are in that defined region. Therefore, we say that is "where the electron is".
  • To paraphrase Bohr, you need to cut the region outside the nucleus into sub regions (he called them energy level). But Bohr did not cut enough, we now know you need to cut Bohr's energy levels into even smaller regions (thereby having more regions than 7).
  • For all atoms, there will be the same specific line up of regions outside the nucleus where the electrons will be "located" starting from the region closest to the nucleus (electrons with the lowest amount of energy will be located here) thru the region farthest away from nucleus (electrons with the highest energy will be located here). We call this listing of regions (with electrons included) the Electron configuration of the atom.

• Why do they call it a Charged Cloud Model.
  • The only particle in the region of space outside the nucleus are electrons that are negatively charged particles.
  • Electrons are constantly moving at approximately the speed of light (3.00E8m/s). The diameter of an atoms is about 1nm wide (closer to 10 nm but for this calculation 1nm is easier). So, if the electron is moving back and forth on the diameter of the atom (that is not actually how it moves), the electron will move across 1.00E17nm/s or approximately 1 billion billion times across the diameter of the atom in a second. So when you "see" the atom (we can't actually see it because it is so small), it would look like a "cloud" or haze.
    

• Why do they call it Quantum Mechanical Model
  • Electrons act like quantum particles that deal with the universe in a different set of rules than you and I. More later on this point.
    

• Why do we care where the electrons are "located" outside the nucleus.
  • Answer: Coulombic force (CF). The strength of the CF is determined by the distance the two charged particles are away from each other (in this case the nucleus and electrons) and how many charged particles there are.
    
  • The actual equation is CF = (Q₁*Q₂)/r²
    
    • where Q₁ is charged particle 1 (usually an electron), Q₂ is charged particle 2 (usually the nucleus) and r = distance between the two charged particles.
      
  • Also, remember same charged particles repel and dissimilar charged particles attract.

• So looking at CF in an individual atom and its subatomic particles (proton, electron, neutrons).
  • There is an attractive force between the nucleus (positively charged) and the electron (negatively charged) where the magnitude of the force is partially determined by the distance the electrons is from the nucleus. The farther away the electron is located, the less CF or pull to the nucleus.

• Why would this be important.
  
  • Answer: Helps in understand that the radii or diameter of that atom would be smaller than an atom whose outermost electrons are farther away from the nucleus.
    
  • For that electron, there would be less CF so it would take less energy to pull (overcome CF) and allow that electrons to be removed from the atom (chemist call that Ionization energy).

• So looking at CF between 2 atoms. When the atoms get close enough together, not only are each atom's electrons are being attracted to their own nucleus, the electrons will be attracted to the nucleus of the other atom. There is also a repulsive force between electrons of each atom. So depending on how far the electrons are away from their own nucleus will influence the overall CF (sum of all attractive/repulsive force) of the electrons of the two atoms.
  
  • There are 3 possible scenarios:
    
    • Electrons are tightly held to both atoms, we call it a covalent bond (an actual chemical bond).
      
    • Electrons from one atoms are pulled to another atoms, called "ionic bond, actually not a chemical bond but just creates two different ions (cation - positively charged ion that lose electrons and anion - negatively charged ion that gain electrons).
    • Electrons are not tightly held by any atom and they are constantly pulled between atoms (called a sea of free flowing electrons). This is called a metallic bond.