Chemistry

• Elements each have an atomic number that corresponds to the number of protons. He has number 2, so every helium has two protons. The mass number is the number of protons + neutrons.
• The periodic table is arranged in columns and rows. 

  Columns have a roughly “similar” chemistry because they have the same number of electrons in the outermost shell.  

  Rows correspond to the number of shells. The second row has two shells. There can be up to 8 electrons in each shell. Elements in the same row have a similar size.
• The innermost shell can hold 2 electrons at most. 
• Electrons in a shell are organized in something called orbitals. One orbital can have up to 2 electrons. 

  When it has two → the orbital has a full and stable structure. 

  If only one electron → very unstable structure, it will try to fill it with another element.
• The standard covalent bond is nonpolar → electrons are equally shared, they spend exactly the same amount of time surrounding each nucleus.
• Covalent bonds that are polar → electrons are attracted to one nuclei more than the others like the water molecule H2O, electrons spends more time around O 

  • \_ this results in a partial negative charge ẟaround O and a partial negative charge ẟ+around the H atoms. This leads to electrostatic attraction (positive and negative charges can attract each other)
• A hydrogen bond is the electrostatic attraction between the oxygen and a neighboring hydrogen. 
• Ionic bonds are found in elements at the edge of the periodic table (that have large differences in electronegativity)like Cl and Na: the Cl atom grabs the electron of the Na atom such that the outermost shell of Cl is completely filled, and Na also generates a very stable ion which has now a positive charge because it has one electron less than the protons.
• Hydrophilic stands for water lover, substances dissolve easily in water. Generally polar molecules like salt, and ions dissolve easily in water. 

  Hydrophobic molecules are usually nonpolar molecules, they do not dissolve in water and stay away from the water. They are mostly made up of carbon and hydrogen atoms.
• The hydrophobic interaction describes the separation of hydrophobic molecules. When hydrophobic molecules are individually dissolved in water they disrupt a lot of the hydrogen bonds in the water, whereas when they are all stuck up together it minimizes the disruption of the surrounding hydrogen bonds and that is thermodynamically favorable. 
• The Van der Waals interactions are the weakest interactions, it describes the interaction of all the molecules but is only relevant for the nonpolar molecules. You have a very transient dipole mediating a very short term electrostatic attraction. These interactions describe the attraction of nonpolar molecules to each other because of transient dipoles (caused by the random localization of electrons in different areas of their orbitals).
• -The covalent bonds are by far the strongest bonds (sharing of electron pairs). 

  -The hydrogen bonds are considered considerably weaker but are still strong and important (attraction of opposite charges). 

  -The ionic bonds are also the attraction of opposite charges and are as strong as hydrogen bonds. We’ll refer only to those who are relevant in biology, mostly between the amino acids and the carboxyl groups in the interior of proteins. 

  -The Van der Waals are the weakest bonds. (interactions due to fluctuating electrical charges)