Chem 103 Exam 3

Explain why it is that atoms don’t have any macroscopic properties such as melting points, boiling points, color, etc ,while macroscopic materials do (and can be identified by them)

an atom doesn’t have these properties because the specific interactions of multiple atoms on the microscopic level is what allows the for materials to have these macroscopic properties?

Explain the force and energy changes that occur when two atoms approach each other and form a bond

When two atoms approach each other, attractive forces between their nuclei and electrons pull them closer, lowering the system’s potential energy. When the balance between attraction and repulsion is just right, the atoms form a stable bond and energy is released to the surroundings.

What determines wether a bond or interaction is stable?

The stability of a bond or interaction is determined by the balance between the attractive forces. Bonds or interactions are stable when the attractive forces are stronger than the repulsive forces, resulting in a low-energy, stable arrangement of atoms or molecules.

Explain how temperature influences the stability of bonds and interactions

At higher temperatures, the increased motion can overcome the attractive forces holding atoms or molecules together, making the bonds less stable.

Explain on the molecular level why metals are ductile (you can draw them out into wires and malleable

The atoms can move with respect with one another and because they’re held together by the sea of delocalized electrons they’re prevented from breaking.

Explain on the molecular level why metals are shiny?

Metals are shiny because the free electrons can absorb energy from light which promotes the electron to a higher energy level but when it immediately falls back down it emits a photon allowing the meal to shine.(remission) The many wavelengths it interacts with allows the metal to appear white or colorless.

Explain why a third body (another atom or molecule) is almost always needed to form a stable bond

A third body is almost always needed to form a stable bond because when two atoms come together, energy is released as the bond forms — and that energy must go somewhere. The third body helps absorb or carry away this excess energy, allowing the new bond to stay stable instead of breaking apart from the leftover energy.

Explain on the molecular level why metals conduct electricity

Electrons conduct electricity due to the ability of delocalized electrons to move around freely allowing the electric current to flow

Explain why relative melting points and boiling points for substances that exist as molecules such as H2 differ from those that exist as continuous extended networks (diamonds or metals)

Substances like H2 have low melting and boiling points because the forces between molecules (like London dispersion forces) are weak, so it's easy to separate them. In contrast, substances with continuous extended networks, like diamonds or metals, have strong covalent or metallic bonds throughout, requiring much more energy to break these bonds and change the state.

Explain why resonance must be invoked to describe the bond lengths and valence electron distributions in some molecules and molecular ions but not others

This happens when the bonding can be represented by two or more valid structures. In contrast, molecules with a clear, fixed bonding pattern do not need resonance because their electrons are localized in specific bonds.

Use formal charge to explain why the relative contribution of each resonance structure in the overall bonding of a molecule

closer to zero are more stable and contribute more to the overall structure

Explain differences in melting and boiling points in terms of forces and energy

When a substance melts some of the interactions between particles must be overcome so they can move relative to one another (liquid state), when a substance boils all the interactions between the particles must be overcome.

Relationship between Bond order, strength and length

BO inc, Bond strength Inc, Bong length Dec

Bond order formula for Molecular orbitals

1/2(Bonding-antibonding)

What properties do metals have?

Shiny, conduct electricity and heat, malleable and ductile, most are grey but some colored or colorless

Explain bonding in metals as it related to MOs

atomic orbitals combine to form molecular orbitals and as more MOs forms the energy gap between them decreases, forming bands of MOs . These energy bands allow electrons to move freely between MOs and across the metal as a whole.

Explain the process of phase changes: boiling

Energy is absorbed as bonds are broken(which requires energy input) and this energy comes from the surroundings through collisions. This leads to attraction being overcome between particles.

Explain the process of phase changes: freezing and condensing

Energy is released because bonds are formed and this energy is transferred to the sorroundings. During this attractions are being formed between particles.

Difference between a covalent bond and LDF

Covalent: much stronger, caused by attraction of electrons from one atom to the nucleus of another, and is present within molecules, and only when Atomic orbitals interact constructively. LDFs: caused by fluctuating charge distribution and inc predictably with the size of electron cloud and are present between all molecules.

What does the temperature at which a material undergoes a phase change tell you about the strength of the interactions between its particles?

The temperature at which a material undergoes a phase change indicates the strength of the interactions being overcome. Materials with low melting points have weak interactions (e.g., London dispersion forces between discrete molecules), while materials with high melting points have stronger interactions (e.g., metallic bonding in metals).

Properties of diamond vs graphite

Diamond: high MP, hard, brittle, translucent, doesn’t conduct electricity. Graphite: high MP, soft, slippery, grey/shiny, conducts electricity.

Define valence bond theory

In this model we say that atomic orbitals overlap to form bonds, and these bonds are localized between the bonded atoms

Define localized electrons

Restrict e- to between nuclei of atoms forming the bond

MO theory

atomic orbitals from bonded atoms combine to form molecular orbitals, which are delocalized over the entire molecule. Each orbital contains up to two electrons. Electrons in bonding molecular orbitals lower the system's energy and stabilize the molecule, while electrons in antibonding molecular orbitals raise the energy and destabilize it.

How to sigma bonds overlap

End to end

How do pi bonds overlap

side by side

Define isomers

They have the same formula but different structures

Why cant pi bonds rotate freely

Because of their shape and the way they overlap if you wanted to rotate them you’d have to break the bonds.

Formal charge formula

#Ve-#e in lone pairs-1/2#e- in bonds

Bond order formula (non MO theory)

#bonds/#bonding regions

VESPR model

Models adompt geometries to minimize electron electron repulsion

Hybridization when EG: Linear

Sp, 180 degrees

Hybridization when Eg: Trigonal planar

Sp2, 120 degrees

Eg: Tetrahedral

Sp3, 109 degrees

Mg when Eg: Tetrahedral with 3 bonding regions

Trigonal pyramidal <109

Eg: tetrahedral with 2 bonding regions

Bent, <109

Eg: trigonal planar with 2 bonding regions

bent <120

Trend of electronegativity

Inc across a period, bc ENC increases, causing a stronger interaction between the nucleus and outermost electrons, and down a group, bc atomic radius increases, which increases the distance between the valence electrons and nucleus, dec the ESF.

Using MO explain why He2 doesnt form covalent bonds compared to H2 for example

In MO theory a bond forms when there are 2 electrons in a stabilizing bond orbital but not when there are also two electrons in a destabilizing antibonding orbital

VB theory

The orbitals overlap to form a bond; the greater the overlap stronger the bond. Each bond has two electrons, and electrons are localized.

What determines which substance has a higher boiling point where LDFS are involved

Surface area- the electron cloud being larger meaning that the LDFs are stronger requiring more energy to overcome the interaction

How to know if a molecule is polar

The bond dipoles add together to create a molecular dipole

Evidence and reasoning for elements being more electronegative as you go up a group

Evidence: The electron cloud is smaller allowing the bonding electrons to be closer to the nucleus meaning the ESF is stronger, and more electronegative.

Evidence and reasoning for elements being more electronegative across a period

The ENC increases across a period which then inc the ESF of attraction.

Bonding in diamonds

In diamond, each carbon atom uses sp³ hybrid orbitals to form covalent bonds with four other carbon atoms, arranged in a tetrahedral structure. The bond angles are approximately 109.5°, creating a rigid three-dimensional network

Model of graphite bonding

In graphite, each carbon atom uses sp² hybrid orbitals to form covalent bonds with three other carbon atoms, arranged in a planar, hexagonal sheet. The bond angles between these atoms are approximately 120°, creating a two-dimensional layered structure.

Model of metallic bonding

In metals, atoms are arranged in a lattice structure where each metal atom donates its valence electrons to form a 'sea of delocalized electrons.' These free-moving electrons allow the metal atoms to bond through metallic bonding, which holds the positively charged metal ions together in a flexible and conductive structure

Why are only valence electrons involved in bonding

Valence bonds are not held as tightly to the nucleus allowing them to be easier to bond

Explain what causes the attraction between atoms in covalent bonds

The ESF between the two charged the nuclei and the shared pair of electrons

What happens when an electron is promoted to an antibonding orbital

The bond order decreases, making the molecule less stable as the bond strength weakens. If the bond order reaches zero or less, the molecule can no longer be held together and will break apart

Differences between resonance structures and isomers

Isomers are compounds with the same molecular formula but different arrangement of atoms, resonance structures are different lewis structures for the same molecule and show the delocalization of electrons not movement of atoms.

How to determine which resonance structure contributes more

Full octets and minimal formal charges

Is energy absorbed or released when an electron is promoted to an antibonding orbital

Energy is absorbed as it moves to a more excited state

Whats the relationship between melting and boiling points of element relative to the types of bonding and interactions that are overcome during a phase change (structure wise)

Elements with higher boiling points tend to exist in extended networks where the atoms are held together by metallic or covalent bonds. Ones with lower boiling points tend to be small molecules or discrete atoms held together by LDFS.

Explain how the high mp property of graphite can be explained in terms of the bonding that occurs in valence bond theory/hybrid orbital model

The carbon atoms in graphite are bonded in strong covalent networks within layers, which require a large amount of energy to break, giving it a high melting point.

Explain how the soft and slippery property of graphite can be explained in terms of the bonding that occurs in valence bond theory/hybrid orbital model

The layers in graphite are held together by weak London dispersion forces that are between the layers, so the layers can easily slide over each other, making it soft and slippery

Explain how the shiny grey property of graphite can be explained in terms of the bonding that occurs in valence bond theory/hybrid orbital model

It can absorb and re-emit photons of many wavelengths

Explain how the conductive property of graphite can be explained in terms of the bonding that occurs in valence bond theory/hybrid orbital model

Carbon is made up of 3 C-C bonds (Sp2 hybridization) in flat layers. The unhybridized p orbitals overlap to form pi bonds, creating a delocalized π molecular orbital system across the entire layer. These delocalized π electrons are free to move within the plane of the layer, allowing graphite to conduct electricity."

Explain how the hard and brittle properties of diamonds can be explained in terms of the bonding that occurs in valence bond theory/hybrid orbital model

Diamonds from a tetrahedral structure with 4 c-c bonds (Sp3 hybridization) with sigma bonds. The 3d network of covalent bonds is very strong and would require breaking the bonds for them to melt, hence why they’re hard and brittle.

Explain how the mp of diamonds can be explained in terms of the bonding that occurs in valence bond theory

Diamonds from a tetrahedral structure with 4 c-c bonds (Sp3 hybridization) with sigma bonds. The 3d network of covalent bonds is very strong and would require a lot of energy to overcome

Explain how the properties that diamonds aren’t conductive can be explained in terms of the bonding that occurs in valence bond theory

Electrons are localized in the bonds between atoms and aren’t free to move. There is a large band gap between the bonding and antibonding orbitals.

Explain how the properties that diamonds are translucent can be explained in terms of the bonding that occurs in valence bond theory

the large band gap between the valence and conduction bands, prevents the absorption of visible light. Instead, light either passes through or is reflected.