Lecture 1.1 Refresh your high school chemistry CS882, Fall 2006

Atoms and molecules

Atoms Protons Neutrons Electrons (much smaller) orbiting around the nucleus

Atom models

Second shell Helium 2 He First shell Third shell Hydrogen 1 H 2 He 4.00 Atomic mass Atomic number Element symbol Electron-shell diagram Lithium 3 Li Beryllium 4 Be Boron 3 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorus 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar Electron-shell diagrams of the first 18 elements in the periodic table

Formation of covalent bond Hydrogen atoms (2 H) Hydrogen molecule (H 2 ) 1 In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus. When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. 2 The two electrons become shared in a covalent bond, forming an H 2 molecule

Water molecule, H 2 O

Electron transfer and ionic bonding Cl – Chloride ion (an anion) – The lone valence electron of a sodium atom is transferred to join the 7 valence electrons of a chlorine atom. 1 Each resulting ion has a completed valence shell. An ionic bond can form between the oppositely charged ions. 2 Na Cl + Na Sodium atom (an uncharged atom) Cl Chlorine atom (an uncharged atom) Na + Sodium ion (a cation) Sodium chloride (NaCl)

Sodium chloride crystal Na + Cl -

Hydrogen bond Water (H 2 O) Ammonia (NH 3 ) –– ++ O H H ++ –– N H H H A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia. ++ ++ ++

Hydrogen bond

Walking on water