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learning goals
At the end of this part you can:
- Write Lewis -S -Symbols to neutral atoms and ions
- Draw Lewis structures that display the bond in simple molecules
Until now we have discussed the different types of bonds formed between atoms and/or ions in this chapter.
Lewis -Symbolen
We use Lewis symbols to describe valence electron configuration of atoms and monatomic ions.Lewis symbolConsists of an elementary symbol surrounded by a dot for each of its valence electrons:
Image \ (\ Page index {1} \): Displays the Lewis symbols of the elements of the third period of the periodic system.
Lewis symbols can also be used to illustrate the formation of cations from atoms, as shown here for sodium and calcium:
Similarly, they can be used to show the formation of anions from atoms, as shown here for chlorine and sulfur:
Image \ (\ Page index {2} \) demonstrates the use of Lewis symbols to display the transfer of electrons during the formation of ionic connections.
Lewis structures
We also use Lewis symbols to indicate the formation of covalent bindingsLewis structures, drawings that describe the binding in molecules and polyatomical ions.For example, when two chloratomas form a chlormolecule, they share a few electrons:
The Lewis structure indicates that each CL - atom has three pairs of electrons that are not used for binding (calledLonely) and a shared few electrons (written between the atoms).
A single shared few electrons called oneSome bond.
The octet rule
The other halogues molecules (F2, Br2, I2and on2)) Form bonds such as those in the chloros: a single bond between atoms and three lonely pairs of electrons peratom.This enables each halogen atom to have a noble gas electron configuration.OCTS -regel.
The number of bonds that can often be formed can be predicted an atom from the number of electrons needed to reach an octet (eight valence electrons);n, o and f).For example, each atom in a group of 14 element has four electrons in the outer scale and therefore requires four electrons to reach one octet.For carbon in CCL4(carbon tetrachloride) and silicon in sih4(Silaan).TA hydrogen only needs two electrons to fill its valence scale, it is an exception to the octet rule.
Group 15 elements, such as nitrogen, have five valence electrons in Atomic Lewis symbol: a lonely pair and three unpaired electrons.3(Ammonia).
Double and triple bonds
As mentioned earlier, when a few atoms share a few electrons, we call this a single binding.In the time that a few atoms may have to share more than a few electrons to reach the required Oct.Double bondForms when two pairs of electrons are divided between a few atoms, such as between carbon and oxygen atoms in CH2O (formaldehyde) and between the two carbon atoms in C2H4(Ethylene):
INTriple bondForms when three electron pairs are divided by a few atoms, such as in carbon monoxide (CO) and the Cyanidion (CN-):
Lewis structures write with the octet rule
Too simple molecules and molecular ions we can write Lewis structures by simply linking the unpaired electrons on the constant atoms.
For more complicated molecules and molecular ions, it is useful to follow the step -by -step procedure described here.
Lewis Structures Write
- Determine the total number of valence electrons (outer scale).For cations, drag an electron for every positive charge.
- Draw a skeleton structure of the molecule or ion and place the atoms around a central atom..
- Divide the remaining electrons as lonely pairs on the terminal atoms (except hydrogen) and complete an octet around each atom.
- Place all remaining electrons on the central atom.
- Reduce the electrons of the outer atoms to create more bonds with the central atom to reach octettes where possible.
Let's determine lewis structures of \ (\ ce {sih4} \), \ (\ ce {cho2^{-} \ \), \ (\ ce {no^{+}}} \) and \ ce {or2} \) as examples by following this procedure:
- Determine the total number of Valence (outer scale) electrons in the molecule or ion.
- For a molecule we add the number of valence electrons to every atom in the molecule:
\ [\ start {altign*} \ ce {sih4} & \\ [4pt]
\ Text {si: 4 Valence Electrons/Atom} \ Times \ Text {1 Atom} & = 4 \\ [4pt]
+ \ quad \ text {h: 1 valentie elektron/atom} \ times \ text {4 atoms} & = 4 \\ [4pt]
\ HLINE \ TEXT {Total Valence Electrons} and = 8
\ End {align*} \ nonumber \] For anegative, such as \ (\ ce {cho2^{ -}} \), we add the number of valence electrons to the atoms to the number of negative loads on ion (an electron obtained for any negative charge):
\ [\ \ {JUSTER *} \ CE {CHO2 {-}} & \\ [4PT]
\ text {c: 4 valentie elektronen/atom} \ times \ text {1 atom} & = 4 \\ [4pt]
\ Text {h: 1 valentie elektron/atom} \ times \ text {1 atoms} en = 1 \\ [4pt]
\ Tekst {O: 6 Valentie Elektron/Atom} \ Times \ Text {2 Atoms} & = 12 \\ [4pt]
+ \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ quad \ tekst {1 elekton} & = 1 \\ [4pt]
\ Hline \ text {Total valence electrons} and = 18
\ End {align*} \ nonumber \]
\ [\ start {altign*} \ ce {nr^{+}} & \\ [4pt]
\ text {n: 5 valentie electron/atom} \ Times \ text {1 atom} & = 5 \\ [4pt]
\ Text {o: 6 valentie elektron/atom} \ times \ text {1 atoms} en = b6 \\ [4pt]
+ \ quad \ text {-1 electron (positive charge)} \ Times \ Text {1 electron} and = -1 \\ [4pt]
\ hline \ text {TOTAL VALENCE -Electrons} and = 10
\ End {align*} \ nonumber \]
\ [\ Begin {align*} \ ce {of2} & \\ [4pt]
\ Text {O: 6 Valentie electron/atom} \ Times \ Text {1 atom} & = 6 \\ [4pt]
+ \ quad \ text {f: 7 valentie elektron/atom} \ times \ text {2 atoms} & = 14 \\ [4pt]
\ hline \ text {TOTAL VALENTEGE -ELECTRONS} and = 20
\ End {align*} \ nonumber \]
- For a molecule we add the number of valence electrons to every atom in the molecule:
- Draw a skeleton structure of the molecule or ion, place the atoms around a central atom and connect each atom with a single (an electron pair) binding.Outside the brackets :?
When different events of atoms are possible, such as for \ (\ Ce {Cho2^{-} \), we must use experimental evidence to choose the right one.atoms.
\ (\ This {cho2^{ -}} \)
,, The smaller electronegative carbon atom absorbs the central position of oxygen and hydrogen atoms that surround it.I \ (\ ce {so2} \), and \ (\ ce {cl} \) in \ (\ ce {clo4^{-}} \).Elementalgative element can also be a central atom.
- Divide the remaining electrons such as lonely pairs on the terminal atoms (except hydrogen) to complete their valence scales with an octet electrons.
- There are no remaining electrons on \ (\ Ce {sih4} \) so it is unchanged:
- There are no remaining electrons on \ (\ Ce {sih4} \) so it is unchanged:
- Place all remaining electrons on the central atom.
- For \ (\ ce {sih4} \), \ (\ Ce {Cho2^{ -}} \) and \ CE {no^{+}} \), there are no remaining electrons;The electrons determined in step 1.
- Force {from 2} \) We had 16 electrons over in step 3 and we placed 12 and placed 4 placed on the central atom:
- Reduce the electrons of the outer atoms to create more bonds with the central atom to reach octettes where possible.
- \ (\ Ce {sih4} \): si already has an octet, so nothing needs to be done.
: - \ (\ Ce {no^{+}} \): For this ion we have added eight valence electrons, but none of the atoms has one octet 1 so we have to move electrons to form a multiple bond:
This still does not produce an octet, so we have to move another pair and form a triple bond:
- ICE {of2} \) Each atom has drawn one octet like this so that nothing changes.
Example \ (\ Page index {1} \): Lewis structures write
Nasa's Cassini-Huygens-Mission discovered a large cloud of toxic hydrogenity3CCH3), Acetylene (HCCH) and Ammonia (NH3).What are Lewis structures of these molecules?
Solution
Trin 1. Calculate the number of valence electrons.
HCN: (1 × 1) + (4 × 1) + (5 × 1) = 10
H3CCH3: (1 × 3) + (2 × 4) + (1 × 3) = 14
HCCH: (1 × 1) + (2 × 4) + (1 × 1) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 1010) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10)= 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 1010) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10) = 10
NH3: (5 × 1) + (3 × 1) = 8
Trin 2Draw a skeleton and connect the atoms with a few bindings.Remember that H is never a central atom:
Trin 3. Where necessary, divide electrons to the terminalatomas:
HCN: Six electrons posted on N
H3CCH3: There are no electrons left
HCCH: No terminal atoms that can accept electrons
NH3: No terminalatomas that can accept electrons
Trin 4. Where necessary, remaining electrons on the central atom:
HCN: There are no electrons left
H3CCH3: There are no electrons left
HCCH: Four electrons posted on carbon
NH3: Two electrons placed on nitrogen
Trin 5. Where necessary, electrons have been re -arranged to form multiple bonds to reach an octet on each atom:
HCN: Form two more C - N -Bonds
H3CCH3: All atoms have the correct number of electrons
HCCH: Form a triple bond between the two carbon atoms
NH3: All atoms have the correct number of electrons
Exercise \ (\ PAGEANDEX {1} \)
Both carbon monoxide, CO and carbon dioxide, CO2, are products from burning fossil fuels.Both gases also cause problems: co is toxic and co is2are involved in global climate change.What are Lewis structures in these two molecules?
- Answer
-
How Scientific Incects: The Fuller -Chemistry
Carbon, in various forms and connections, has been known since the prehistoric times.is the central additive of iron in the steel production process and diamonds have a unique place in both culture and industry.From the element, researchers began to reveal to reveal the potential for even more varied and extensive carbon structures.
As early as the sixties, chemists started to observe complex carbon structures, but they had few indications of supporting their concepts, or their work did not come to the mainstream.Eiji Osawa predicted a spherical form based on observations of a similar structure, but his work was not generally known outside of Japan.In a similar way, the most extensive progress was probably the calculation of chemist Elena Galperns, who predicted a very stable molecule with 60 carbon in 1973;To reveal the nature of long carbon chains that were discovered in the interstellar space.
Kroto tried to use a machine developed by Richard Smalley's team at Rice University to find out more about these structures.Intensive series of experiments that have led to a great discovery.
In 1996, the Nobel Prize in Chemistry was awarded RichardSmalley(Image \ (\ page index {3} \)), Robert Curl and Harold Kroto for their work in detecting a new form of carbon, C60The Buckminsterfullers -Molecule.60.This type of molecule, called a fuller, shows promise in a number of applications.also have unique electronic and optical properties that are used for good use in devices with solar energy and chemical sensors.
Exceptions to the octet rule
Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures.These molecules fall in three categories:
- ODD electron molecules have a strange number of valence electrons and therefore have an unpaired electron.
- Molecules of electronefonic molecules have a central atom that has fewer electrons than is necessary for a noble gas configuration.
- Hypervalent -Molecules have a central atom that has more electrons than is necessary for a noble gas configuration.
Odd-electron molecules
We mention molecules that contain an odd number of electronsFree radicalsNitrogen oxide, no, is an example of an odd electron molecule;It is produced in internal combustion engines when oxygen and nitrogen react at high temperatures.
To draw the Lewis structure for an odd electrons molecule as no, we follow the same five steps that we would do for other molecules, but with a few small changes:
- Determine the total number of electrons of valence (outer scale)Of.
- Draw a skeleton structure of the moleculeWe can easily draw a skeleton with a n -o -binding:
Nee - Divide the remaining electrons as lonely pairs on the terminal atoms.
- Place all remaining electrons on the central atom.If there are no remaining electrons, this step does not apply.
- Reduce the electrons to create more bonds with the central atom to achieve octettes where possible.We know that an odd electron molecule may not have an octet for every atom, but we want to get every atom as close as possible to an octet.Nitrogen We take one of the lonely pairs of oxygen and use it to form a no double bond (we can't absorb another lonely pair of electrons and form a triple bond because nitrogen would have nine electrons :)
Molecules for electroneficiency
We will also come across a few molecules that contain central atoms that do not have a full valence peel.In general, these are molecules with central atoms from groups 2 and 13, external atoms that are hydrogen or other atoms that do not form multiple bonds.For example, in Lewis structures of Beryllium dihydride, beep2, and a homififluuride, bf3, The Beryllium and Bor atoms each have only four and six electrons.3This meets the octet rule, but experimental evidence suggests that the binding lengths are closer to the expected B -F -Tabletbinder.Connection is also in accordance with an electroneficiency, the B - F - B - The bindings are slightly shorter than what is actually expected for B - F -Single bindings, indicating that there is a certain double binding character in the actual molecule.
An atom that Boratomet in BF3, who does not have eight electrons, is very reactive.3responds with BF3Because the lonely pair can be shared in nitrogen with the Bor -atom:
Hypervalent molecules
Items in the second period of the periodic table (N= 2) can only house eight electrons in their valence shell -orbitals because they only have four valence or bitals (a 2Sand three 2SOrbals).N≥ 3) has more than four valence or bitals and can share more than four pairs of electrons with other atoms because they are emptyDOrbitals in the same shell.molecules formed from these elements are sometimes mentionedHypervalent molecules.5a sf6.
In some hyper -falling molecules, such as if it5in chef4, some of the electrons in the outer scale in the central atom are lonely pairs:
When we write Lewis structures for these molecules, we find out that we have electrons over after filling the outer atoms with eight electrons.These extra electrons must be assigned the central atom.
Example \ (\ page index {2} \): Lewis -Structures Write - Octte Rule Violations
Xenon is a noble gas, but it forms a number of steady connections.We have investigated XEF4Earlier.What are Lewis structures in XEF2in chef6?
Solution
We can draw the Lewis structure in any covalent molecule by following the six steps discussed earlier.In this case we can condense the last few steps as not all apply.
Step 1. Calculate the number of valence electrons:
Kok2: 8 + (2 × 7) = 22
Kok6: 8 + (6 × 7) = 50
Step 2. Draw a skeleton that accompanies the atoms with a few bindings.Xenon will be the central atom because Fluor cannot be a central atom:
Step 3. Divide the remaining electrons.
Kok2: We place three lonely pairs of electrons around each F -atom, good for 12 electrons and give each F atom 8 electrons.Is acceptable, because Xe -atoms have an empty valence -shellDOrbitals and can be suitable for more than eight electrons.Lewis structure of XEF2Two bind pairs and three lonely pairs of electrons around the XE atom show:
Kok6: We place three lonely pairs of electrons around each F -atom that will access 36 electrons.
Exercise \ (\ PageDex {2} \): Interhals
The halogens form a class of connections called Interhales, where halogen atoms bind covalent together.
- Answer
-
