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02/20/13
Resources:
“Electron: name tags (4), Nucleus sign
CountingValence.ppt
Bondo & Bondo.ppt
Homework for 02/21: Sketch the biggest molecule you can (alá
Bondo) using 2 C, 4 H, 1 O (Hint: You will
need 1 double bond)
Homework for 02/22: Molecule diagrams (revised version – stick
diagrams)
Bell Work:
Bond slogans
Play Bondo WITHOUT explaining number of
bonds!
D. Covalent Bonds
1. Slogan: “Pair and share”
2. Valence electrons
a. Skit
1) suppose an atom has three electons (volunteers with
signs)
2) the nucleus (another volunteer) keeps two electrons
locked in the basement, but one electron is free to
move around the atom – even “go outside”
3) Which electron is going to be able to meet electrons
from other atoms (another volunteer)?
b. In real atom, some electrons are “locked in the basement”
and have no contact with any other atoms. But those
electrons on the outside have the “power to combine” and
are called “valence” electrons
3. Atomic radius tells the story …
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a. recall atomic radius plot from Predicting Periodically
(Sketch with first small peak, then 3 increasingly higher
peaks)
1) Electrons in the basement: show that as all baskets on
a rung (orbitals in a period) are filled, atom SHRINKS
and becomes more stable – label NEON
2) But! Add just ONE electron more! Size of SODIUM
(label) jumps WAY up – electron is “out of the
basement” and has “power to combine”
3) Add more until ARGON, jump again
4. E-figs tell same story
a. Compare Ar and Sc
b. Sc inner electrons = those that are same as Ar
c. Sc outer electrons = those outside of “inner Ar” =
VALENCE ELECTRONS
d. So, always count from previous Noble gas = count from
beginning of row!
e. Count valence electrons: Na / Al / P / Cl / Ar
5. Sharing electrons
a. “Magic” Number 8: Periods 2 & 3 — Eight electrons for
complete level/shelf/shell to make smallest, most stable
and compact atom possible [Period 1: only need 2] =
“Octet Rule”
b. Noble gases already HAVE complete shell — they won’t add
or lose ANY electrons. So they have NO POWER TO COMBINE
and react with NOTHING (hence, “noble”)
c. All others “try” to create a “noble gas” configuration
(because it’s the most stable state!)
d. Some atoms with one or two “extra” or one “missing” lose
or steal an electron, and then form ionic bonds (ex: Na
and Cl)
e. BUT! Others have to reach eight by SHARING
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1) Suppose Atom A and Atom B both have 7 valence
electrons and both need one more (skit – 2 Ss with
pitchers w/ 7 golf balls)
2) Atom A GIVES one of its electrons TO Atom B — then B
is happy but A now missing TWO. What to do?
3) Suppose they BOTH SHARE ONE AT SAME TIME: Atom B
shares one with Atom A
4) By sharing a pair, both atoms think they have 8!
5) SHARED PAIR = COVALENT BOND
6) Draw as a single stick: “A—B” … AHA! Bondo de ja vu!
6. Number of bonds
a. Need 8 — already have (number of valence electrons) — get
the rest by sharing
b. How many covalent bonds will the following elements form?
1) Li: has 1 — which it will lose
2) Mg: has 2 — which it will lose
3) B: has 3 — and it’s just weird
4) C: has 4, needs 4 … AHA! Bondo de ja vu!
5) N: has 5, needs 3
6) O: has 6, needs 2
7) F: has 7, needs 1
8) Ne: has 8, needs nothing
c. Example 1: Water
1) Oxygen needs 8, oxygen has ? (6), oxygen needs to
share ? (2)
2) Hydrogen needs 2 (special case), has ? (1), needs to
share ? (1)
3) Share 1 pair: O—H. H is filled, O is not
4) Share second pair: H—O—H. Everybody’s filled
5) Why isn’t it H—H—O?
d. Multiple bonds:
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1) If Atom A and Atom B both need TWO electrons, they can
share TWO pair! DOUBLE BOND
2) Ex: O=O … AHA! Bondo de ja vu!
7. When to “steal & stick” or “share & pair”
a. Steal when you have atoms on opposite ends of the PT: F
will STEAL from Na, but nor from O
b. Share everywhere else (and halogens can share or steal
depending on who they’re with)