Chapter 8 Flashcards
How a chemical bond occurs?
When a chemical reaction occurs between two atoms, some of their electrons are reorganized, resulting in a net attractive force – a chemical bond – between the atoms.
What covalent bonding involves?
Covalent bonding involves a sharing of valence electrons between each atoms.
How an ionic bond is forming?
An ionic bond forms when one or more valence electrons is transferred from one atom to another, creating positive and negative ions
In molecules or polyatomic ions made up only of non-metal atoms, the atoms are attached by ____________ bonds
covalent bonds
Does the core electrons are involved in bonding reaction?
no, only valence electrons are involved
What are the letters of the valence electrons of the main group element (A groups) in the periodic table
For main group elements (elements of the A groups in the periodic table), the valence electrons are the s and p electrons in the outermost shell.
For main group element (A Group), the number of valence electrons is equal to ______________
the group number
For transition elements what are the letter of the valence electrons?
Valence electrons for transition elements include the electrons in the ns and (n-1)d orbitals.
What is the octet rule?
The tendency of molecules and polyatomic ions to have structures in which eight electrons surround each atom is the basis for the octet rule
What are the steps to draw Lewis dot structure?
Step 1: determine the arrangement of atoms within a molecule. The central atom is usually the one with the lowest affinity for electrons
Step 2: determine the total number of valence electrons in the molecule or ion. In a neutral molecule, this number will be the sum of the valence electrons for each atom. The number of valence electron pairs will be half to total number of valence electrons.
Step 3: place on pair of electrons between each pair of bonded atoms to form a single bond.
Step 4: use remaining pairs as lone pairs around each terminal atom (except H) so that each terminal atom is surrounded by eight electrons. If, after this is done, there are electrons left over, assign them to the central atom.
Step 5: if no valence electron pairs remain after forming single bonds and completing the octets of terminal atoms, and the central atom does not have an octet of electrons, then multiple bonds can be created by sharing one or more pairs of electrons between terminal atoms and the central atoms.
How de we choose the central atom in Lewis dot structure?
Generally the central atom is the one of lowest electronegativity.
How do we draw Lewis dot structure with a multiple bond?
Problem: draw the Lewis dot structure for CO2
Step 1: designate C as the central atom
Step 2: calculate the valence electrons
C= 1 X 4= 4
O= 2 X 6= 12
total valence electron= 16
Step 3: Two electron pairs form single bonds between C and O
Step 4: distribute three lone pairs on the terminal O atoms to complete their octets.
Step 5: none of the original 8 electron pairs remain ti be used, but the C atom does not yet have an octet of electrons. Therefore, we use lone pairs of electrons on the O atoms to form additional CO bonds, one on each side of the molecule.
to see the drawing: http://f.tqn.com/y/chemistry/1/W/N/f/1/CO2-Lewis.png
or student book p.290
How do we draw Lewis dot structure for polyatomic ions?
Problem: Draw Lewis dot structure for the ClO3^- and the NO2^+
For ClO3^-
Step 1: designate C as the central atom
Step 2: calculate the valence electrons Cl: 1 X 7= 7 O: 3 X 6= 18 negative charge (-1)= + 1
total valence electron= 26
Step 3: three lone pairs forms single bond between Cl and O
Step 4: distribute three lone pairs on each of O atoms to complete the octet of electrons around each of these atoms.
Step 5: one pair of electron remains, and it is placed on the central Cl atom to complete its octet.
* remember that you should put brackets with the charge at the drawing when its an ionic compound*
to see the drawing:
http://www.chemeddl.org/resources/models360/files/104770/clo3_.png
or student book p.291
For NO2^+
Step 1: designate N as the central atom
Step 2: calculate the valence electrons N: 1 X 5= 5 O: 2 X 6=12 positive charge (+1)= -1 5+12-1= 16
total valence electron= 16
Step 3: two electron pairs form single bonds form the N to each O
Step 4: distribute the remaining six pairs of electron on the terminal O atoms.
Step 5: the central N atom is two electron pairs short of an octet. A lone pair of electrons on each oxygen atom is converted to a bonding electron pair to give 2 N=O double bond. Each atom in the ion now has 4 electrons pairs. N has 4 bonding pairs, and each O atoms has 2 lone pairs and shared 2 bond pairs
* remember that you should put brackets with the charge at the drawing when its an ionic compound*
to see the drawing: http://reocities.com/MotorCity/highway/8497/Image8.jpg
or student book p.291
What is the definition of atom formal charge in covalent molecules and ions?
The formal charge is the electrostatic charge that would reside on an atom in a molecule or polyatomic ion if all bonding electrons are shared equally.
How the formal charge for an atom in a molecule or ion is calculated?
formal charge= NVE – (LPE + 1/2 (BE))
NVE: number of valence electrons in the uncombined atom
LPE: number of lone pair electrons on an atom
BE: number of bonding electrons around an atom
What a positive formal charge means?
A positive formal charge means that an atom in a molecule or ion “contributed” more electrons to bonding than it “got back”
Did the formal charge in a molecule must be zero?
YES, The sum of the formal charges on the atoms in a molecule must be zero
Didi the formal charge in an ion must be zero?
NO, the sum for atoms in an ion equals the charge on the ion.
What is resonance in Lewis dot structure?
Resonance structures are used to represent bonding in a molecule or ion when a single Lewis structure fails to describe accurately the actual electronic structure
Draw the resonance structure for ozone (O3)
to see it: https://i.ytimg.com/vi/Nam8Szygg-g/maxresdefault.jpg
or student book p.298
Who proposed the concept of resonance?
Linus Pauling
What is a coordinate covalent bond?
A bonding pair of electrons originates on one of the bonded atoms. like H3F→BF3. The boron atom can accommodate a fourth electron pair when that pair is provided by another atom, and molecules or ions.
Which periods in the periodic table form compounds and ions which the central element can be surrounded by more than 4 valence electron pairs?
3rd or higher periods
What is the valence shell electron-pair repulsion (VSEPR) model ?
is a method for predicting the shapes of covalent molecules and polyatomic ions. It is based on the idea that bond and lone electron pairs in the valence shell of an element repel each other and seek to be as far apart as possible
In the VSEPR model for molecules with two pairs of electrons around the central atom, what is the geometry type expected and the angle?
linear geometry is expected
with a bond angle of 180 degrees.
In the VSEPR model for molecules with three pairs of electrons around the central atom, what is the geometry type expected and the angle?
trigonal planar geometry
with 120 bond angles.
In the VSEPR model for molecules with four pairs of electrons around the central atom, what is the geometry type expected and the angle?
tetrahedral geometry
angle of 109.5 degrees
In the VSEPR model for molecules with five pairs of electrons around the central atom, what is the geometry type expected and the angle?
trigonal bipyramidal configuration
with angles of 120 degrees or 90 degrees.
In the VSEPR model for molecules with six pairs of electrons around the central atom, what is the geometry type expected and the angle?
Octahedral geometry
with 90 degrees angles
What are the effect of lone pairs on bond angles ?
Lone pairs of electrons seem to occupy a larger volume than bonding pairs, and the increased volume of lone pairs causes bond pairs to squeeze closer together.
Predicting the shapes of molecules and polyatomic ions EXERCICE
If you have the Chemistry and chemical reactivity book 9th edition, go on p 310 and try example 8.9
The answer is not on the flash cards
Answer in Chemistry and chemical reactivity book 9th edition, go on p 310 and try example 8.9
Which molecular geometries (structures) shows a central atoms with more than four valence electron pairs ?
trigonal-bipyramodal and octahedral
In a molecular geometry (structure) which have a central atom with more than four valence electron pairs, where do we draw the remaining lone pairs of electron ?
The lone pairs is often drawn in the top or bottom position to make it easier to visualize the molecular geometry, which in this case is square pyramidal.
predicting molecular shape
What is the shape of the ICl4^- ion?
Exercice 8.10: Chemistry and chemical reactivity book 9th edition p.312
The central atom is Iodine (I) with 6 valence electrons pairs.
2 of these are lone pairs
Placing the lone pairs on opposite sides leaves the 4 chlorine atoms in a square-planar geometry.
to see the shape : https://i.ytimg.com/vi/yMdbXO8M3p8/hqdefault.jpg
Do the electron pairs in double bonds and triple bonds have an effect on the overall molecular shape?
Double and triple bonds involve more electron pairs than single bonds, but this has little effect on the overall molecular shape
finding the shapes of molecules and polyatomic ions
What is the shape of the nitrate ion, NO3^- ?
Exercice 8.11: Chemistry and chemical reactivity book 9th edition p.313
The NO3^- ion is isoelectronic (having 32 electrons)
The electron-pair geometry and molecular shape of NO3^- are trigonal planar
to see it : http://image.slidesharecdn.com/vsepr-100319132250-phpapp01/95/vsepr-32-728.jpg?cb=1269005229
What is a pure covalent bonding?
atoms share an electron pair equally
In which condition a pure covalent bonding occurs?
A pure covalent bonding occurs only when two identical atoms are bonded.
What is a polar covalent bond?
A bond in which the two atoms have partial electrostatic charges
In which condition bonds are polar?
Bonds are polar because not all atoms hold onto their valence electrons with the same force or take on additional electron with equal ease
How the polarity is indicated for a polar covalent bond?
the polarity is indicated by writing the symbols δ^+ and δ^- alongside the atom symbol, where δ stands for a partial charge.
Who proposed a parameter called: atom electronegativity?
Linus Pauling
How the electronegativity of an atom is defined?
The electronegativity, χ, of an atom is defined as a measure of the ability of an atom in a molecule to attract electrons to itself.
Electronegativity values for the metalloids are ________(around, less than or greater than) __#___
around 2
Electronegativity values for the non-metals are __________(around, less than or greater than) __#___
greater than 2
estimating bond polarity
For each of the following bond pairs, decide which bond is more polar and indicate the negative and positive poles.
a) B–F and B–Cl
b) Si–O and P–P
Exercice 8.12: Chemistry and chemical reactivity book 9th edition p.316
a) B and F lie relatively far apart in the periodic table. B is a metalloid, and F is a nonmetal.
Here, X for B=2.0, and X for F= 4.0. Similarly, B and Cl are relatively far apart in the periodic table, but Cl is below F in the periodic table (X for Cl= 3.2) and is therefore less electronegative than F.
The difference in electronegativity for B–F is 2.0, and for B–Cl is 1.2.
Both B–F and B–Cl bonds are expected to be polar, with B positive and the halide atom negative, but a B–F bond is more polar than a B–Cl bond.
b) The P–P bond is nonpolar because the bond is between two atoms of the same kind.
In the case of Si–O, the O atom has a greater electronegativity (3.5) than Si (1.9), and the bond is highly polar (deltaX=1.6), with O the more negative atom.
What is the geometry of an H3O atom ?
tetrahedral
to see the shape: http://chemistry.elmhurst.edu/vchembook/images/205hydronium.gif
How is called the way electrons are distributed in a molecule or ion?
is called its charge distribution.
What is electroneutrality?
electrons will be distributed in such a way that the charges on all atoms are as close to zero as possible
In the concept of electroneutrality, if a negative charge is present where the negative charge should reside?
it should reside on the most electronegative atoms.
In the concept of electroneutrality, if a positive charge is present where the positive charge should reside?
positive charges are expected on the least electronegative atoms.
Calculating and using formal charges
Boron-containing compounds often have a boron atom with only three bonds (and no lone paire). Why not form a double bond with a terminal atom to complete the boron octet?
To answer this, consider possible resonance structure of BF3, and calculate the formal charges of the B and F atoms. Are the bonds polar in BF3? If so, which is the more negative atom?
Exercice 8.13: Chemistry and chemical reactivity book 9th edition p.316
The 2 possible structures for BF3 are illustrated here.
http://chemwiki.ucdavis.edu/@api/deki/files/43167/IMG00019.GIF?revision=1
Calculate the formal charges
F= 7-(6+ (1/2X2))= 0
B= 3-(0+(1/2X6))=0
http://media1.shmoop.com/images/chemistry/chembook_bonds_graphik_7c.png
Calculate the formal charges
F=7-(4+(1/2X4))= +1
B= 3-(0+(1/2X8))= -1
The first structure is strongly preferred because all atoms have a zero formal charge.
What describes the term polar?
The term “polar” described a bond in which one atom has a partial positive charge and the other a partial negative charge.
How do we predict if a molecule is polar?
To predict if a molecule is polar, we need to consider how the atoms are positioned relative to one another.
Does diatomic molecules composed of two atoms with different electronegativities are polar?
Diatomic molecules composed of two atoms with different electronegativities are always polar there is one bond, and the molecule has a positive and a negative end.
Does water molecule is polar?
Because O has a larger electronegativity (χ=3.5) than H (χ=2.2), each of the O—H bonds is polar, with the H atoms having the same δ^+charge and oxygen having a negative charge (δ^-).
How can you predict that a molecule will not be polar?
You can predict that a molecule will not be polar if
- all the terminal atoms are identical
- all the terminal atoms are arranged symmetrically around the central atom.
except: trigonal-bipyramidal electron-pair geometry
What is the order of a bond?
the order of a bond is the number of bonding electron pairs shared by two atoms in a molecule.
When a fractional bond orders will occur?
Fractional bond orders occur in molecules and ions having resonance structures
How can we calculate the bond order?
bond order=(number of shared pairs in all X—Y bonds) / (number of X—Y links in the molecule or ion)
What is the definition of bond length?
Bond length is the distance between the nuclei of two bonded atoms, is clearly related to the sized of the atoms, for a given pair of atoms, the order of the bond plays a role.
What is the definition of bond dissociation enthalpy?
Bond dissociation enthalpy is the enthalpy change for breaking a bond in a molecule with the reactant and products in the gas phase.
The process of breaking bonds in a molecule is exothermic or endothermic?
always endothermic
Suppose you wish to break the carbon-carbon bonds in ethane (H3C-CH3), ethylene (H2C=CH2), and acetylene (HC≡CH).
Which bond will requires the most energy to break it?
The carbon-carbon bond orders in these molecules are 1,2, and 3, respectively, and these bond orders are reflected in the bond dissociation enthalpies.
Breaking the single C—C bond in ethane requires the least energy in this group, whereas breaking the C—C triple bond in acetylene requires the most energy.
Does the formation of bonds from atoms in the gas phase is endothermic or exothermic?
The formation of bonds from atoms or radicals in the gas phase is always exothermic
Whats is the average bond dissociation enthalpy of H-H molecule (in kJ/mol)?
436
Whats is the average bond dissociation enthalpy of H-C molecule (in kJ/mol)?
413
Whats is the average bond dissociation enthalpy of H-N molecule (in kJ/mol)?
391
Whats is the average bond dissociation enthalpy of H-O molecule (in kJ/mol)?
463
Whats is the average bond dissociation enthalpy of H-F molecule (in kJ/mol)?
565
Whats is the average bond dissociation enthalpy of H-Si molecule (in kJ/mol)?
328
Whats is the average bond dissociation enthalpy of H-P molecule (in kJ/mol)?
322
Whats is the average bond dissociation enthalpy of H-S molecule (in kJ/mol)?
347
Whats is the average bond dissociation enthalpy of H-Cl molecule (in kJ/mol)?
432
Whats is the average bond dissociation enthalpy of H-Br molecule (in kJ/mol)?
366
Whats is the average bond dissociation enthalpy of H-I molecule (in kJ/mol)?
299
Whats is the average bond dissociation enthalpy of C-C molecule (in kJ/mol)?
346
Whats is the average bond dissociation enthalpy of C-N molecule (in kJ/mol)?
305
Whats is the average bond dissociation enthalpy of C-O molecule (in kJ/mol)?
358
Whats is the average bond dissociation enthalpy of C-F molecule (in kJ/mol)?
485
Whats is the average bond dissociation enthalpy of C-S molecule (in kJ/mol)?
272
Whats is the average bond dissociation enthalpy of C-Cl molecule (in kJ/mol)?
339
Whats is the average bond dissociation enthalpy of C-Br molecule (in kJ/mol)?
285
Whats is the average bond dissociation enthalpy of C-I molecule (in kJ/mol)?
213
Whats is the average bond dissociation enthalpy of N-N molecule (in kJ/mol)?
163
Whats is the average bond dissociation enthalpy of C-O molecule (in kJ/mol)?
201
Whats is the average bond dissociation enthalpy of C-F molecule (in kJ/mol)?
283
Whats is the average bond dissociation enthalpy of C-Cl molecule (in kJ/mol)?
192
Whats is the average bond dissociation enthalpy of O-O molecule (in kJ/mol)?
146
Whats is the average bond dissociation enthalpy of O-Si molecule (in kJ/mol)?
452
Whats is the average bond dissociation enthalpy of O-P molecule (in kJ/mol)?
335
Whats is the average bond dissociation enthalpy of O-Cl molecule (in kJ/mol)?
218
Whats is the average bond dissociation enthalpy of O-Br molecule (in kJ/mol)?
201
Whats is the average bond dissociation enthalpy of O-I molecule (in kJ/mol)?
201
Whats is the average bond dissociation enthalpy of F-F molecule (in kJ/mol)?
155
Whats is the average bond dissociation enthalpy of F-Si molecule (in kJ/mol)?
565
Whats is the average bond dissociation enthalpy of F-P molecule (in kJ/mol)?
490
Whats is the average bond dissociation enthalpy of F-S molecule (in kJ/mol)?
284
Whats is the average bond dissociation enthalpy of F-Cl molecule (in kJ/mol)?
253
Whats is the average bond dissociation enthalpy of F-Br molecule (in kJ/mol)?
249
Whats is the average bond dissociation enthalpy of F-I molecule (in kJ/mol)?
278
Whats is the average bond dissociation enthalpy of Si-Si molecule (in kJ/mol)?
222
Whats is the average bond dissociation enthalpy of Si-S molecule (in kJ/mol)?
293
Whats is the average bond dissociation enthalpy of Si-Cl molecule (in kJ/mol)?
381
Whats is the average bond dissociation enthalpy of Si-Br molecule (in kJ/mol)?
310
Whats is the average bond dissociation enthalpy of Si-I molecule (in kJ/mol)?
234
Whats is the average bond dissociation enthalpy of P-P molecule (in kJ/mol)?
201
Whats is the average bond dissociation enthalpy of P-Cl molecule (in kJ/mol)?
326
Whats is the average bond dissociation enthalpy of P-I molecule (in kJ/mol)?
184
Whats is the average bond dissociation enthalpy of Ci-Ci molecule (in kJ/mol)?
242
Whats is the average bond dissociation enthalpy of Ci-Br molecule (in kJ/mol)?
216
Whats is the average bond dissociation enthalpy of Ci-I molecule (in kJ/mol)?
208
Whats is the average bond dissociation enthalpy of Br-Br molecule (in kJ/mol)?
193
Whats is the average bond dissociation enthalpy of Br-I molecule (in kJ/mol)?
175
Whats is the average bond dissociation enthalpy of I-I molecule (in kJ/mol)?
151
Whats is the average bond dissociation enthalpy of N=N molecule (in kJ/mol)?
418
Whats is the average bond dissociation enthalpy of C=N molecule (in kJ/mol)?
615
Whats is the average bond dissociation enthalpy of O=O (in O2) molecule (in kJ/mol)?
498
Whats is the average bond dissociation enthalpy of C=C molecule (in kJ/mol)?
610
Whats is the average bond dissociation enthalpy of C=O molecule (in kJ/mol)?
745
Whats is the average bond dissociation enthalpy of C=O (in CO2) molecule (in kJ/mol)?
803
Whats is the average bond dissociation enthalpy of N≡N molecule (in kJ/mol)?
945
Whats is the average bond dissociation enthalpy of C≡N molecule (in kJ/mol)?
887
Whats is the average bond dissociation enthalpy of C≡C molecule (in kJ/mol)?
835
Whats is the average bond dissociation enthalpy of C≡O molecule (in kJ/mol)?
1046
