Atomic Structure And Interactions Review.

Compared to He, the London Dispersion Forces between Xe atoms are expected to be...?

Stronger (the larger electron cloud of Xe is easier to distort it is more polarizable - has a larger separation of charge allowing for stronger LDFs).

How do the He atoms "know" the temperature is rising?

The energy is transferred from other atoms that have collided with the walls of the container, that were directly heated (thermal energy is transferred through collisions).

Here is Katie again, the ball is falling down. What causes the ball to fall down?

The force causes the ball to fall down. The force also causes the change in PE!

What happens when you add thermal energy (raise the temperature) to the Helium atoms? Why?

The kinetic energy of the atoms increases, they move faster, collide and/or vibrate with more energy. Eventually they vibrate enough (have enough kinetic energy) to overcome the attractive force between two He atoms and they fly apart.

Here is Katie again, the ball is falling down. As the ball move towards the ground what happens to the PE of the system (ball + earth).

The potential energy decreases.

Why do you think it takes a temperature of 6000K to break the interaction between two hydrogen atoms - but only 4K to break the interaction between two helium atoms?

The strength of the interaction is much stronger for H2 because it is a covalent bond. Not just an LDF between two atoms. The Potential Energy well for Hydrogen is far deeper than helium so it requires far more energy to break.

As the helium atoms approach what happens to : - Total energy - Kinetic energy - Potential energy

The total energy stays the same because this is a closed system so energy is not gained or lost. The KE increases as the atoms are attracted to each other and they start to move faster. The PE decreases because the two charges in the electrostatic field are closer together in attractive field.

What can the position on the x axis of the PE (potential energy) well tell you?

The x-axis is labeled as internuclear distance (the distance between two atoms). A point along the x-axis must be a distance. At this point the interaction is most stable, it takes a lot of energy to disrupt. The attractive and repulsive force balance and total force is zero.

When the atoms are a far distance apart they move closer together, which force is strongest?

There is an electrostatic attraction between atoms.

As the helium atoms get very close (overlapping electron clouds) what happens to : - Total energy - Kinetic energy - Potential energy

Total energy still won't change because it is an isolated system. The kinetic energy was high but decreases as the electron clouds start to overlap and the atoms slow down. Once they begin to move apart it will increase again. The PE increases sharply because of the repulsion between the electron clouds and the nuclei.

What is force?

We usually define it as a push or a pull. In chemistry you will hear the word "interaction" as another word for force.

Which is the substance with the highest boiling point?

X has the higher BP because the potential well is deeper. WHY? It requires more energy to separate the atoms (or molecules).

Which is the substance with the largest atomic radius?

X.

Which do you think would have a larger melting and/or boiling point?

Xe - Stronger LDFs.

How big do you think an atom (H) is?

toms are about 0.1-0.2 nm. Cell are 1-100E-6 m (1-100 micrometers). Smallest objects that can be see by eye ~0.1 mm

Hydrogen melts at 10K and boils at around 20K, but the hydrogen molecules do not form hydrogen atoms until the temperature reaches over 6000K.

5K: solid, molecule. Draw an oval with 2 dots (nuclei) packed in an orderly manner. 15K: liquid, molecule. Draw an oval with 2 dots still touching but not organized. 30K: gas, molecule. Draw an oval with 2 dots not touching, further apart from each other. Above 6000K: gas, atoms. Draw circles with 1 dot, far apart and not touching. At this temperature, there is enough energy to break the covalent bond so there are only atoms present.

Here is Katie, she has thrown a ball in the air. What forces are acting on the ball?

After the ball leaves her hand, the only force acting upon the ball is gravity.

How would you explain the difference between an atom and an element?

An atom is the smallest unit of an element. When we talk about atoms we are talking about the properties of those individual nanoscale entities. When we talk about an element we are referring to a macroscopic sample which has bulk properties. The scales of these are vastly different as are the properties.

As the He atoms approach : This is an attractive interaction but... if they get too close, the electron clouds start to repel each other.

As the atoms approach each other, the KE initially increases (and PE decreases). Then, the KE starts to decrease until the atoms stop and then start moving away from each other.Remember our electrostatics. When the atoms get too close together, the electron clouds start to overlap and there is repulsion by the electrons in different atoms. This is a closed system so the total energy is constant. If we leave it alone it will oscillate forever (not realistic though since we don't really have isolated systems like this.

What happens when two atoms of He approach each other?

Atoms start at a distance where they are just starting to feel the interactions. As they get closer, the PE decreases and the KE increases. When they get too close together, the PE spikes.

What is energy?

Changes in energy are caused by changes in forces.

As the atoms approach each other the potential energy?

Decreases.

At the potential minimum, which force is in strongest?

E/M attraction = repulsion.

As the atoms move even closer, which force is in strongest?

Electromagnetic attraction

As the atoms move closer, which force is in strongest?

Electromagnetic repulsion.

Which statement about the breaking of a single chemical bond is true?

Energy is absorbed.

Here is Katie again, the ball is falling down. As the ball move towards the ground what happens to the total energy of the system (ball + earth).

Energy is conserved. The total energy will not change while PE is converted into KE.

Which statement about the forming of a single chemical bond is true?

Energy is required to break bonds (energy is absorbed).

Newton's Law of Universal Gravitation - F - M1 - M2 - R - G -

F = G(m1m2/r2). F - Gravitational force between 2 objects. M1 - Mass of 1 object in Kilograms. M2 - Mass of 2 object in Kilograms. R - Distance between object. G - Gravitational constant.

Which statement about the breaking of a single chemical bond is true?

Forming of a bond is the reverse of breaking a bond. If energy is required to break a bond, then the reverse process must release energy. We saw that in the simulation when a third atom comes in (panel 6). Energy is released from the two atom system and transferred to the third atom.

Which is biggest?

Frame of reference is important and it depends on what you mean by "biggest". There are molecules folded up in cells that when unfolded (extended) are much longer than the largest dimension of a cell.

How do you think the gravitational force of the earth is able to influence the ball even though the ball and the earth are not in contact?

Gravity is an example of forces acting at a distance. The gravitational force exists between any two objects with mass or between an object with mass and energy.

Here is Katie, She has a ball in her hand. What forces are acting on the ball?

Gravity. There is an electrostatic force pushing up.

Which has atoms in it?

Heat is not matter. It is thermal energy (actually the transfer of thermal energy).

What can the depth of the PE (potential energy) well tell you?

How much energy would be needed to overcome the interaction (once trapped in well, there must be a transfer of energy through collisions to escape). Lower energy = most stable.

As the atoms get very close what happens to the potential energy?

Increases.

Thermal Energy

Is a measure of the average kinetic energy of the atoms.

Temperature T

Is directly related to average kinetic energy (1/2mv^2).

Kinetic energy.

KE = 1/2mv^2. The energy associated with motion.

Here is Katie again, the ball is falling down. As the ball move towards the ground what happens to the KE?

Kinetic energy increases as the velocity of the ball increases.

London Dispersion Force - How many molecules must be present for LDFs to exist?

LDF operates between EVERY SINGLE ATOM, MOLECULE, ETC. It underpins everything. YOU MUST HAVE AT LEAST TWO ATOMS/MOLECULES IN AN ELECTROSTATIC FIELD FOR INTERACTION TO OCCUR.

Which type of interaction is stronger?

LDFs are extremely weak compared to covalent bonds. Also notice that when two atoms are interacting through LDFs they are farther apart than when they are covalently bonded together.

London Dispersion Forces.

LDFs are stronger for bigger atoms/molecules because they have more electrons. LDFs are stronger for objects with more surface area because they have more area to interaction. Long thin objects have larger surface area than spherical objects with same volume.

Coulomb's Law.

Look how similar the models for the gravitational and electrostatic forces are! Both interactions follow an inverse square law. Don't get them mixed up though! Remember the differences. The electrostatic force is MUCH STRONGER and can be ATTRACTIVE or REPULSIVE. The gravitational force is too weak to be detected at the atomic/molecular level.

Compared to Ne you think the London Dispersion Forces between Xe atoms are...?

Ne is smaller - LDF is stronger as the masses increases.

- Was the Greek idea of atoms a scientific theory? Was it based on experiments, evidence, data?

Not really, emerged from philosophy.

Here is Katie again, the ball is falling down. What causes the potential energy to decrease?

PE is the energy due to the position in a field. As the position in the field changes due to the force, the PE changes.

- What properties ascribed by the Greeks to atoms do we still consider to be valid? - If atoms are in constant motion, what do you think keeps them moving?

Properties ascribed by Greeks to matter still considered valid-Matter composed of atoms that are in constant motion. Kept in motion by thermal energy

Example of inverse-square law.

R - Increases, F - Decreases, Factor of 1/r2.

What happens to the force of attraction as : - The mass of the interacting objects increases? - The distance between the objects increases?

- As mass increases, force increases. - As distance increases, force decreases.

Where on the curve is? - 200pm -120pm -150pm

- C - A The electron clouds are right on top of each other and overlapping. The repulsive force makes the PE increase. - B Here, the two electron clouds are the perfect distance apart. This is a PE minimum or well. Most matter exists in some sort of PE well.

At 5K, what holds the nuclei together within an H2 molecule? At 5K, why do molecules of H2 stick to other molecules of H2.

- Covalent bond (nuclei within a molecule). - LDF's (between molecules).

The four fundamental types of forces?

- Gravity - Electromagnetic - Strong - Weak

- Why do the atoms move toward each other? - Why do they oscillate (go back and forth)? - What does the third atom do? - What does it take to from a stable interaction?

- Initially, the atoms move together due to the attractive LDFs. - The atoms oscillate because this is an isolated system with only enough total energy to allow the atoms to separate back to the starting distance. There is not enough energy to move them further apart. - Energy is transferred to the third atom through collision. It removes energy from the two-atom system and allows the other two atoms to stick together (now they don't have enough energy to move apart). In real systems, atoms are colliding with other atoms all of the time and transferring energy. What is the mechanism of energy transfer? Collision! - To form a stable interaction, we must remove energy from the system (watch the total E meter in the simulation)! This is equivalent to cooling down the system.

- Initially, what prevents the He atoms from flying apart? - Why do they fly apart when the T is raised?

- LDFs. The system of 2 atoms doesn't have enough energy to escape the energy well. - When energy is transferred to the system by collision, the atoms have enough energy to escape the well.