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Questions and Answers
What is the molecular shape of ammonia based on its nuclei arrangement?
What is the molecular shape of ammonia based on its nuclei arrangement?
Which molecule has a linear shape according to its atomic arrangement?
Which molecule has a linear shape according to its atomic arrangement?
How many sigma bonds are present in water based on its atomic arrangement?
How many sigma bonds are present in water based on its atomic arrangement?
Identify the hybridization of the central atom in boron trifluoride.
Identify the hybridization of the central atom in boron trifluoride.
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Which statement is true regarding the shape of methane?
Which statement is true regarding the shape of methane?
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What is the total number of electron pairs at the central atom in ammonia?
What is the total number of electron pairs at the central atom in ammonia?
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Which statement regarding isotopes is accurate?
Which statement regarding isotopes is accurate?
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What is the total number of protons and neutrons in an atom with an atomic number of 8 and a mass number of 16?
What is the total number of protons and neutrons in an atom with an atomic number of 8 and a mass number of 16?
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Which element would most likely form a covalent bond with nitrogen?
Which element would most likely form a covalent bond with nitrogen?
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What describes the role of electronegativity in chemical bonding?
What describes the role of electronegativity in chemical bonding?
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What is the primary characteristic of ionic bonds?
What is the primary characteristic of ionic bonds?
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According to the octet rule, how many electrons do most elements strive to obtain in their valence shell?
According to the octet rule, how many electrons do most elements strive to obtain in their valence shell?
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In a Lewis structure, how are double bonds represented?
In a Lewis structure, how are double bonds represented?
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Which is true about the valence electrons of an alkali metal?
Which is true about the valence electrons of an alkali metal?
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When constructing a Dot & Cross diagram for MgCl2, what is essential to remember?
When constructing a Dot & Cross diagram for MgCl2, what is essential to remember?
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What is the primary reason that rotation about a double bond has a higher energy barrier compared to rotation about a single bond?
What is the primary reason that rotation about a double bond has a higher energy barrier compared to rotation about a single bond?
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In the context of hybridization, what is the result of combining one s orbital with two p orbitals?
In the context of hybridization, what is the result of combining one s orbital with two p orbitals?
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Which of the following configurations represents a cis isomer?
Which of the following configurations represents a cis isomer?
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What characterizes the geometry of an sp hybridized carbon atom?
What characterizes the geometry of an sp hybridized carbon atom?
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Among the following, which statement accurately describes a sigma bond?
Among the following, which statement accurately describes a sigma bond?
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What is the strength of the pi bond in a C=C double bond typically measured at?
What is the strength of the pi bond in a C=C double bond typically measured at?
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Which hybridization describes the theoretical structure of the molecule acetylene?
Which hybridization describes the theoretical structure of the molecule acetylene?
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What is a key characteristic of trans isomers compared to cis isomers?
What is a key characteristic of trans isomers compared to cis isomers?
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What is the correct Lewis structure representation of water (H2O)?
What is the correct Lewis structure representation of water (H2O)?
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In the Valence Shell Electron Pair Repulsion model, what is the primary consideration when determining molecular shape?
In the Valence Shell Electron Pair Repulsion model, what is the primary consideration when determining molecular shape?
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How many total bonds are present in a molecule of ethyne (H-C≡C-H)?
How many total bonds are present in a molecule of ethyne (H-C≡C-H)?
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How many total valence electrons are there in the Lewis structure for sulfur trioxide (SO3)?
How many total valence electrons are there in the Lewis structure for sulfur trioxide (SO3)?
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In the Lewis structure for methane (CH4), what type of bonds are formed between the carbon and hydrogen atoms?
In the Lewis structure for methane (CH4), what type of bonds are formed between the carbon and hydrogen atoms?
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What does the Lewis structure for the sulfate ion (SO4-2) indicate about the formal charge of the sulfur atom?
What does the Lewis structure for the sulfate ion (SO4-2) indicate about the formal charge of the sulfur atom?
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What can be concluded about the formal charge calculation for bromine in the bromate ion (BrO3-)?
What can be concluded about the formal charge calculation for bromine in the bromate ion (BrO3-)?
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What distinguishes constitutional isomers from other types of isomers?
What distinguishes constitutional isomers from other types of isomers?
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In the Lewis structure for dichlorine (Cl2), how many total valence electrons are involved?
In the Lewis structure for dichlorine (Cl2), how many total valence electrons are involved?
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Which of the following statements about the Lewis structure for ethane (C2H6) is true?
Which of the following statements about the Lewis structure for ethane (C2H6) is true?
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When adjusting lone pairs in the Lewis structure for SO3, what is the likely outcome for sulfur?
When adjusting lone pairs in the Lewis structure for SO3, what is the likely outcome for sulfur?
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What is the main distinction between resonance structures and equilibrium?
What is the main distinction between resonance structures and equilibrium?
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Which statement regarding resonance stabilization is true?
Which statement regarding resonance stabilization is true?
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What role do curved arrows play in resonance theory?
What role do curved arrows play in resonance theory?
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Which of the following is NOT a proper resonance structure?
Which of the following is NOT a proper resonance structure?
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When drawing resonance structures, which action is permissible?
When drawing resonance structures, which action is permissible?
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Which characteristic is true for resonance contributors?
Which characteristic is true for resonance contributors?
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What does the resonance hybrid indicate about a molecule's structure?
What does the resonance hybrid indicate about a molecule's structure?
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Why can resonance structures not resonate onto an sp3 carbon atom?
Why can resonance structures not resonate onto an sp3 carbon atom?
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In the example of NO3, which factor contributes to its resonance stability?
In the example of NO3, which factor contributes to its resonance stability?
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Which of the following examples illustrates proper resonance structures?
Which of the following examples illustrates proper resonance structures?
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Study Notes
Atomic Structure
- The atomic number (Z) equals the number of protons in an atom.
- The atomic number also corresponds to the group number on the periodic table.
- Electrons are the only moving particles within an atom and have negligible mass.
- Protons have a positive charge and a mass of 1 atomic mass unit (amu).
- Neutrons are neutral and also have a mass of 1 amu.
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
- For example, Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons.
- The atomic number represents the number of protons in an atom.
- The mass number represents the total number of protons and neutrons in an atom.
- The number of protons in an atom is always equal to the number of electrons.
- Atoms with the same atomic number are the same element despite having different numbers of neutrons.
- The mass number minus the atomic number equals the number of neutrons in an atom. For example, Lithium with a mass number of 8 has 5 neutrons (8-3=5).
Valence Electrons and the Octet Rule
- Atoms have up to 7 electron shells.
- Valence electrons occupy the outermost shell of an atom.
- Atoms strive to gain or lose electrons to achieve a full valence shell, often with 8 electrons, according to the octet rule*.
- A full valence shell leads to greater stability for an atom.
- Hydrogen and helium only need 2 valence electrons for a full shell, as they only have one electron shell.
Ionic Bonds
- Ionic bonds are formed through the transfer of one or more electrons between atoms, creating ions.
- For example, sodium (Na) needs to lose one valence electron to become stable.
- Chlorine (Cl) needs to gain one valence electron to become stable.
- Na + Cl → Na+ + Cl-
Covalent Bonds
- Covalent bonds occur when atoms share electrons.
- For example, nitrogen (N) needs to gain 3 electrons to achieve a full octet.
- Hydrogen (H) only needs to gain one electron to achieve a full shell.
- N + 3H → NH3
Ionic Bonds and Ions
- Ions are formed when atoms lose or gain electrons.
- For example:
- Na → Na++ 1e-.
- Cl + 1e- → Cl-.
- Opposite charges attract each other through electrostatic forces, forming ionic bonds.
Dot and Cross Diagrams
- Dot and cross diagrams illustrate the transfer of electrons in ionic bonding.
- For example, in Magnesium Chloride (MgCl2):
- Mg2+: two dots represent two valence electrons transferred.
- Cl-: One cross represents one gained valence electron, and one dot represents the initial 7 valence electrons of Chlorine.
Electronegativity
- Electronegativity increases from left to right across a horizontal row of the periodic table.
- It also increases from bottom to top along a vertical column of the periodic table.
- Electronegativity measures how strongly an atom attracts electrons in a covalent bond.
- The number of protons in the nucleus of an atom influences electronegativity.
- Atoms with more protons exert a greater force on shared electrons, increasing electronegativity.
Noble Gases
- Noble gases do not have electronegativity values.
- This is due to their already full valence shells, making them very stable and unreactive.
Periodic Table
- Electronegativity trends allow us to determine the relative electronegativity of elements.
- For example, nitrogen (N), with 5 valence electrons, needs 3 more electrons.
- Fluorine (F), with 7 valence electrons, needs 1 more electron.
- Bromine (Br), also with 7 valence electrons, needs 1 more electron.
- Fluorine is more electronegative than bromine because it has more protons, meaning a stronger force on the shared electrons.
- In the list: Si, Ar, Cs, Si, P, O, Oxygen (O) is the most electronegative, while Argon (Ar) is a noble gas and therefore does not have electronegativity.
Lewis Structures
- Lewis structures are diagrams that represent covalent bonds in molecules and ions.
- Atoms share valence electrons to achieve a full octet (except for hydrogen).
- One shared electron pair represents a single bond.
- For example, in a hydrogen molecule (H2), the shared pair of electrons is represented as H-H.
- In an oxygen molecule (O2), the shared pair of electrons is represented as O=O.
Steps to Draw Lewis Structures
- Count all the valence electrons in the molecule.
- Identify the central atom, typically the element present only once in the molecule.
- Draw single bonds between the central atom and other atoms.
- Place remaining valence electrons as lone pairs on atoms.
- If necessary, convert lone pairs into double or triple bonds to give every atom an octet.
Examples of Lewis Structures
-
H2O (Water)
- H (2) + O (6) = 8 valence electrons
- H-O-H
-
SO3 (Sulfur Trioxide)
- S (6) + O3 (6 x 3 = 18) = 24 valence electrons
- O-S-O
- O
- Convert one lone pair on oxygen into a double bond to give sulfur an octet.
-
CH4 (Methane)
- C (4) + H4 (1 x 4 = 4) = 8 valence electrons
- H
- |
- H - C - H
- |
- H
-
Cl2 (Chlorine)
- Cl2 (7 x 2 = 14) valence electrons
- Cl - Cl
-
C2H6 (Ethane)
- C2 (4 x 2 = 8) + H6 (1 x 6 = 6) = 14 valence electrons
- H - C - C - H
- H - C - H
-
SO4-2 (Sulfate Ion)
- S (6) + O4 (6 x 4 = 24) + 2 (charge) = 32 valence electrons
- [ O - S - O ]-2
- O - O
- BrO3- (Bromate Ion)
- Br (7) + O3 (6 x 3 = 18) + 1 (Charge) = 26 valence electrons
- [ O - Br - O ]- - O
Formal Charge
- Formal charge (FC) helps determine the most stable Lewis structure.
- FC = Valence electrons - (Bonding electrons + Lone pair electrons)
-
Example: H - C - H
- FC (central carbon atom) = 6 - (4 + 2)/2 = +1
- FC (oxygen atom) = 6 - (2 + 6)/2 = 0
- FC (hydrogen atoms) = 1 - (1 + 0)/2 = 0
-
Example: Br- with BrO3-
- FC (Bromine) = 7 - (6 + 2)/2 = 0
- FC (Oxygen with single bond) = 6 - (1 + 6)/2 = -1
- FC (Oxygen with double bond) = 6 - (2 + 4)/2 = 0
- Note that the formal charges on the oxygen atoms with single bonds add up to -2 to balance with the -2 charge of the BrO3-.
Isomers
- Isomers are molecules with the same chemical formula but different structures.
- Constitutional isomers have different connections between atoms, leading to different physical and chemical properties.
- For example, C4H10 (butane) has two constitutional isomers:
- H3C - CH2 - CH2 - CH3 (n-butane)
- H3C
- |
- CH
- |
- CH3
- CH3 (isobutane)
Structural Formulas
- Dash Formula: Shows the connections between atoms.
- Condensed Formula: Groups atoms together, such as CH3CH2OH.
- Bond-Line Formula: Represents carbon atoms as corners and hydrogen atoms as implied; other atoms are explicitly shown.
3-D Formulas
- These formulas use wedges and dashes to represent the arrangement of atoms in 3D space:
- X(-): Atom lies in the plane of the paper.
- X(4): Atom projects out of the plane/towards you.
- X(III): Atom projects behind the plane/away from you.
Resonance Theory
- Resonance theory describes molecules with multiple Lewis structures, each representing a possible arrangement of electrons.
- These structures differ only in the positioning of electrons, not in the arrangement of atoms.
- Resonance structures are not realistic representations but contribute to the overall structure of the molecule.
- The actual molecule is a hybrid of the resonance contributors.
- Resonance structures are connected with a double-headed arrow, indicating an average of the contributing structures.
- For example, the resonance structures of NO3-:
- [ N = O ]-
- O
- [ O = N - O ]-
- O
Differences Between Resonance and Equilibrium
- Equilibrium: Different structures with moving atoms.
- Resonance: Structures exist only on paper. Atoms do not actually move.
Resonance Stabilization
- Resonance structures that are not equivalent contribute to the stabilization of a molecule.
- The resonance hybrid (average) is more stable than any single contributing structure.
- Equivalent resonance structures lead to significant resonance stabilization.
Curved Arrows in Resonance Structures
- Curved arrows represent the movement of two electrons.
- The arrowhead points towards where the electrons are moving.
- The tail of the arrow indicates the origin of the electrons.
Rules for Resonance Structures
- Only move electrons (lone pairs or electrons in double or triple bonds), not atoms.
- Never resonate onto sp3 hybridized carbon atoms, as it would result in too many bonds.
- Sp2 hybridized carbon atoms with an adjacent unhybridized p-orbital can participate in resonance.
Restricted Rotation and Double Bonds
- There is a larger energy barrier for the rotation of groups connected by a double bond compared to single bonds.
- This restricted rotation is due to the strength of the pi bond.
Cis-Trans Isomerism
- Cis-trans isomerism is a type of stereoisomerism that occurs in molecules with double bonds.
- Trans: Groups are on opposite sides of the double bond.
- Cis: Groups are on the same side of the double bond.
Structure of Ethyne (Sp Hybridization)
- Ethyne (H - C≡C - H), also known as acetylene, is a linear molecule with two sigma bonds and two pi bonds between the carbon atoms, forming a triple bond.
- Carbon in ethyne is sp-hybridized.
Hybridization
- Hybridization is the process of mixing atomic orbitals to form new hybrid orbitals.
- sp3 hybridization: Mixing one s orbital with three p orbitals, resulting in 4 sp3 hybrid orbitals with tetrahedral geometry.
- sp2 hybridization: Mixing one s orbital with two p orbitals, resulting in 3 sp2 hybrid orbitals with trigonal planar geometry.
- sp hybridization: Mixing one s orbital with one p orbital, resulting in 2 sp hybrid orbitals with linear geometry.
Sigma and Pi Bonds
- Sigma bond (σ): A type of single bond.
- Pi bond (π): A type of bond formed by the sideways overlap of two adjacent p orbitals, creating electron density above and below the bonding axis. Found in double and triple bonds.
Valence Shell Electron Pair Repulsion (VSEPR) Model
- VSEPR theory explains the shapes of molecules based on the repulsion of electron pairs around the central atom.
- Electron pairs (both bonding and nonbonding) repel each other and try to stay as far apart as possible.
- Nonbonding pairs occupy a larger space than bonding pairs.
-
Steps to predict molecular geometry:
- Count all valence electron pairs (bonding and nonbonding) around the central atom.
- Electron pairs repel each other, leading to specific shapes.
- Nonbonding pairs exert more repulsion.
- Describe the shape based on the positions of the atoms, not the electron pairs.
Example of VSEPR applications
-
Methane (CH4):
- 4 sigma bonds
- Tetrahedral shape.
-
Ammonia (NH3):
- 3 sigma bonds and one lone pair
- Trigonal pyramidal shape.
-
Water (H2O):
- 2 sigma bonds and two lone pairs
- Angular shape.
-
Boron Trifluoride (BF3):
- 3 sigma bonds and no lone pairs
- Trigonal planar shape.
-
Beryllium Hydride (BeH2):
- 2 sigma bonds and no lone pair
- Linear shape.
Table of Molecular Shapes and Hybridization
| Electron Pairs at Central Atom | Bonding | Nonbonding | Total | Hybridization | Shape of Central Atom or Ion | Examples |
|---|---|---|---|---|---|---|
| 2 | 2 | 0 | 2 | sp | Linear | BeH2 |
| 3 | 3 | 0 | 3 | sp2 | Trigonal planar | BF3, CH3 |
| 4 | 4 | 0 | 4 | sp3 | Tetrahedral | CH4, NH4 |
| 3 | 3 | 1 | 4 | sp3 | Trigonal pyramidal | NH3, CH3 |
| 2 | 2 | 2 | 4 | sp3 | Angular | H2O |
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Test your knowledge of atomic structure fundamentals, including concepts like atomic number, mass number, and isotopes. This quiz will challenge your understanding of protons, neutrons, and electrons, helping you grasp the essential building blocks of matter.
