Monday September 16, 2001

Atoms, Electrons, Chemical Bonds

(partially overlaps notes posted for Wed Sept. 12)

Atoms

• tiny particles
• core or nucleus even tinier <1% diameter
• core has almost all the mass >99.9%
• yet "size" of atoms > nucleus

Nucleus

• has a positive electrical charge
• multiples of +1, +2, ... +92, ...
• has 1,2,3 protons
  • protons are + particles
• also has 1-3 times as many neutrons
  • there is more structure-- chemists generally ignore

Proton Count = Atomic Number

• determines the chemical properties
• that is all atoms with 8 protons behave as Carbon

Electrons

• Atoms are electrically neutral
• Contain matching number of electrons
• Electrons are light, negative

All chemistry involves the electrons

• (protons come along for the ride)

Electrons tend to be arranged in pattern

• shells or layers
• repeating groups of 2 - 6 -10 - 14
• matches (produces) our Periodic Table

electrons 1-2: Hydrogen ........ Helium

• last electron in a shell makes atom very inert
• Helium is the first of the inert gases
  • last column in the table

• next 10 elements (another group of 2 and a group of 6)
  • Li
  • Be
  • B
  • C
  • N
  • O
  • F
  • Ne -- again, a very inert species

• Go back to Atomic Number = 9 (Fluorine)
  • very reactive (as active as they come)
  • steals electrons, forms -1 ion
  • "wants" to look like Neon
• The next element
  • Atomic Number =11, Sodium
  • Extremely reactive
  • gives electron away to almost anything
  • forms +1 ion
  • electrons in Na+ look just like Neon
• Go back to Z=3, Lithium
  • very much like sodium
  • reactive, forms Li+
  • Li+ looks just like He
• All the elements in the first row (I-A)
  • very reactive metals
  • form +1 ions
• All the elements in row VII-A
  • halogens
  • form -1 ions
• Elements in II-A
  • have two electrons beyond inert gas
  • reactive
  • tend to give up two electrons
  • form Mg²⁺, Ca²⁺ ions
• Elements in VI-A
  • are two electrons short of inert gas
  • reactive
  • often steal 2 electrons
  • form O^2-, S^2-, Se^2- ions

So far we've really been discussing Ionic Compounds

• electrons move completely
• form + cations and - anions
• compounds hold together
  • mostly due to electrical charges
  • + ions attracted to negative ions

Near the middle of the table, life isn't that simple

• atom of Oxygen -- "wants" two more electrons
• what if nobody is offering electrons?

Covalent bonding, sharing of electrons

• atoms often share one electron with another atom
• this is a pair of electrons-- one from each atom
  • as if each can count shared electrons

• simplest case: H₂
  • H atom-- 1 electron
  • H atom wants 2 electrons (He like)
  • Shares with second H atom
    • H : H
  • each atom sees 2 electrons
  • but now the other atom can't leave
  • chemical bond
  • diatomic molecule

• See in F₂ also
  • F has 9 electrons
  • wants 10, to look like Ne
  • if a second F shares 1 electron
  • form a F-F bond or F:F bond

• Lewis picture-- show each of the electrons in the last shell

• Works with different elements too
  • water H:O:H
    • oxygen needs two electrons
    • can share electrons with 2 hydrogens

• methane CH₄
  • carbon needs 4 electrons
  • can share with four H atoms

• methyl alcohol /methanol

• CH₃OH
• carbon forms four bonds
  • one with oxygen
  • three with hydrogen
  • oxygen forms two bonds
    • one with carbon
    • one with another hydrogen

Sometimes, atoms share more than 1 electron

• ethene (fuel gas)
  • C-- needs four
    • shares with 2 Hydrogen atoms (needs two more)
    • shares two electrons with other carbon
  • C::C double bond or C=C double bond