structure and bonding

CHEM1302

What is the plum pudding model?

Who came up with it?

Homogenous positive mass with electrons inside it

Thomson

Limitations of the plum pudding model

Negatively charged things attract positively charged things = the protons and electrons would attract each other and the atom would collapse

What was Rutherford’s model of the atom?

Protons in the nucleus, electrons moving which stop atom from collapsing. Neutrons in the nucleus, gluing protons together

What does the gold foil experiment show?

Gold foil confirms protons are stuck together in nucleus, some alpha particles bounced back showing interactions with nucleus. Most went through showing atom is mainly empty space.

What is Bohr’s model of the atom?

What happens when energy is put in?

Electrons that circulate the nucleus can only take certain values of energy (energies are quantised) = energy levels.

When energy is put in, electrons move from one energy level to another. When they drop from high to low EL, they emit radiation. Energy can be emitted/absorbed according to fixed frequencies, equivalent to the energy difference between levels.

Limitation of Bohr’s model

Electrons are modelled as behaving as particles. Electrons are not particles

What does it mean that electrons are superpositions?

They can be particles or waves depending on how you look at them

What are probability clouds?

Electrons have probability densities of being in a certain place around the nucleus

What is n?

Principle Quantum Number = it determines which electron shell an orbital occupies r

• which energy level

What is l?

Orbital Angular Momentum Quantum Number = determines the shape of an atomic orbital

• it gives the number of nodal planes

What do the different values of l mean?

l = 0 is s orbital

l = 1 is p orbital

l = 2 is d orbital

What is m_l?

Magnetic Quantum Number = determines the direction of an atomic orbital in space

• orientation

What are the possible values for m_l?

l = 0 (s orbital) = m_l is 0

l = 1 (p orbital) = m_l is -1, 0 or 1

l = 2 (d orbital) = m_l is -2, -1, 0, 1 or 2

What is m_s?

Electron spin

• electrons can spin either clockwise or anticlockwise, either +1/2 or -1/2

What is an orbital?

A region of space that can be occupied by an electron

What is an orbital node?

A region of zero probability density

What is the limitation of this graph of electron probability density?

The probability of finding the electron at the nucleus is highest which would suggest the electron is in the nucleus

What is a radial probability plot?

It shows the probability of finding the electron in any given shell at a particular distance from the nucleus

• probability is zero at the nucleus

What happens as Principle Quantum Number increases for a given type of orbital?

Orbital shape stays the same

• Radius of max probability increases (size of orbital)

• Radius gives us size of an atom

• Orbital develops radial nodes

What is the Aufbau principle?

Orbitals are filled with electrons in order of increasing energy

What is Pauli’s Exclusion Principle?

No two electrons in the same electrons in the same atom can have the same set of 4 quantum numbers

• must have a different spin quantum number = spin pairs

What is Hund’s Rule of Maximum Multiplicity?

In a set of degenerative orbitals, electrons prefer to occupy them singly before pairing

What is Valence Orbital Energy?

• the energy of an electron in a particular atomic orbital

  • the negative value of ionisation energy

What is the equation for VOE?

What is continuum?

Which VOE is more stable?

Continuum = the energy zero point; energy of an electron not interacting with anything in a vacuum (no kinetic or potential energy)

VOE₂ is more stable as it is more negative

Order of VOE for the orbitals for same value of n (same EL)

s < p < d < f

What does a low Valence Orbital Energy mean?

The lower (more negative) the VOE, the more stable the orbital

How does VOE change with n?

How does VOE change across a period?

How does VOE change as you go down a group?

• VOE gets less negative as n increases

• VOE gets more negative across a period as Z (charge) increases = more protons

• As you go down a group, n increases so VOE gets less negative

What is the first ionisation energy of an element?

What conditions?

The energy needed to remove the first valence electron at 0K and 1 atm from a gaseous atom

What is ionisation energy proportional to?

How does IE change across the periodic table?

• Increases across a period

• Increases up a group

What is Z_eff?

Why is it lower than Z?

Effective nuclear charge

The amount of positive charge every electron feels from the nucleus taking into account shielding from other electrons

Explain changes in IE in terms of Z_eff

Increasing Z_eff with constant n means increase across a period

Smaller n and similar Z_eff means smaller and tightly bound orbital = increase up a group

Highest and lowest values of IE

Highest = noble gases, filled shells

Lowest = alkali metals (single electrons outside full shell)

What is electron pairing energy and how does it affect IE?

Electron-electron repulsion means lower than expected IE

What is first electron affinity?

The internal energy released when an electron is added to a neutral gas phase atom

X (g) + e^- → X^- (g)

What is electronegativity?

The ability of an atom to attract electrons to itself in a molecule

What is electronegativity an indicator of?

• general chemical behaviour

• electron distribution in a bond

• whether a compound will be ionic or covalent

What is the Pauling electronegative scale?

The difference between the average homonuclear bond energies and heteronuclear bond energy (in eV)