Electronic Structure and Periodic Properties of Elements

Question 1

What is electromagnetic radiation?

Answer 1

Electromagnetic radiation is the energy transmitted by waves that have an electric field component and a magnetic field component.

Question 2

What is a wave?

Answer 2

A wave is an oscillation or periodic movement that can transport energy from one point in space to another.

Question 3

What is the photoelectric effect?

Answer 3

The photoelectric effect is the observation that electrons can be ejected from atoms when light with a threshold frequency collide with them.

Question 4

In what way does light act as a wave?

Answer 4

When light passes through a double slit, it forms interference patterns.

Question 5

In what way does light act as a particle?

Answer 5

Light hits electrons more like a stream of particles (photons).

Question 6

What is the numerical value of Planck’s constant (h)?

Answer 6

h = 6.626 x 10^-34 J•s

Question 7

What is Planck’s formula?

Answer 7

E = hν, where E is energy, h is Planck’s constant, and ν is frequency.

Question 8

Under what circumstances do photons produce a continuous spectrum of light?

Answer 8

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a range of energies and thereby produce a continuous spectrum in which an unbroken series of wavelengths is present.

Question 9

Under what circumstances do photons produce line spectra of light?

Answer 9

Exciting a gas at low partial pressure using an electric current, or heating it, will produce line spectra.

Question 10

What is a line spectrum?

Answer 10

A line spectrum is electromagnetic radiation emitted at discrete wavelengths by a specific atom or atoms in an excited state.

Question 11

What is the energy expression for hydrogen-like atoms?

Answer 11

E_n = -(kZ²) / n², where k is the constant 2.179 x 10^-18 J, Z is the nuclear charge (+1 for hydrogen, +2 for helium, +3 for lithium, etc.), and n is the energy level (1,2,3,4,…).

Question 12

What is the formula for de Broglie wavelength?

Answer 12

λ = h / mv, where h is Planck’s constant, m is mass, and v is velocity.

Question 13

What is the Heisenberg uncertainty principle?

Answer 13

The Heisenberg uncertainty principle states that it is fundamentally impossible to determine simultaneously and exactly both the momentum and position of a particle.

Question 14

What is the equation for the Heisenberg uncertainty principle?

Answer 14

For a particle of mass m moving with velocity v_x in the x direction, the product of the uncertainty in the position, Δx, and the uncertainty in the momentum, Δp_x, must be greater than or equal to ħ/2, where ħ = h / 2π :

Δx × Δp_x = (Δx)(mΔv) ≥ ħ/2

Question 15

What is the principle quantum number (n)?

Answer 15

The principle quantum number the location of the energy level of an electron in an atom.

Question 16

What is an atomic orbital?

Answer 16

An atomic orbital is a general region in an atom within which an electron is most probable to reside.

Question 17

What is the secondary (angular momentum) quantum number (l)?

Answer 17

The secondary (angular momentum) quantum number specifies the shape of the orbital. It can take values 0, 1, 2, 3, … , n-1.

Question 18

To which orbital does the value l = 0 correspond?

Answer 18

l = 0 corresponds to the s orbital.

Question 19

To which orbital does the value l = 1 correspond?

Answer 19

l =1 corresponds to the p orbital.

Question 20

To which orbital does the value l = 2 correspond?

Answer 20

l = 2 corresponds to the d orbital.

Question 21

To which orbital does the value l = 3 correspond?

Answer 21

l = 3 corresponds to the f orbital.

Question 22

What is a radial node?

Answer 22

A radial node is a distance from the nucleus at which the probability density of finding an electron at a particular orbit is zero.

Question 23

What is the expression for the number of radial nodes in an orbital?

Answer 23

n – l – 1

Question 24

What is the magnetic quantum number (m_l)?

Answer 24

The magnetic quantum number specifies the relative spatial orientation of a specific orbital. The total number of possible orbitals with the same value of l is 2l + 1. The values of m_l, in general, are -l, -(l –1),…, 0,…, (l –1), l.

Question 25

What is the spin quantum number (m_s)?

Answer 25

The spin quantum number describes a phenomenon in which an electron acts as a tiny magnet or a tiny rotating object with anglular momentum, or as a loop with an electric current.

Question 26

What is the α state of quantum spin?

Answer 26

In the α state, the z component of the spin is in the positive direction of the z axis, which corresponds to a spin quantum number of m_s = 1/2.

Question 27

What is the β state of quantum spin?

Answer 27

In the β state, the z component of the spin is in the negative direction on the z axis, and has a spin value of m_s = -1/2.

Question 28

What is the Pauli exclusion principle?

Answer 28

The Pauli exclusion principle states that no two electrons in the same atom can have exactly the same set of all the four quantum numbers.

Question 29

What is m_l degeneracy?

Answer 29

The m_l degeneracy is the number of orbitals within an l subshell and so is 2l + 1.

Question 30

What is the Aufbau principle?

Answer 30

The Aufbau primciple states that each added electron occupies the subshell of lowest energy available, subject to the limitations imposed by the allowed quantum numbers according to the Pauli exclusion principle. Electrons enter higher energy subshells only after lower energy subshells have been filled to capacity.

Question 31

What are the orbitals in order from lowest to highest energies?

Answer 31

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p,…

Question 32

What is Hund’s rule?

Answer 32

Hund’s rule states that the lowest energy configuration for an atom with electrons within a set of dengenerate orbitals is that having the maximum number of unpaired electrons.

Question 33

What are valence electrons?

Answer 33

Valence electrons are electrons that occupy the outermost shell orbital(s).

Question 34

What are core electrons?

Answer 34

Core electrons are electrons that occupy the inner shell orbitals.

Question 35

Why do valence electrons play the most important role in chemical reactions?

Answer 35

Valence electrons have the highest energy of the electrons in an atom and are more easily lost or shared than core electrons. Valence electrons are also the determining factor in some physical properties of the elements.

Question 36

Why do elements of the same group have similar chemical properties?

Answer 36

Elements of the same group have similar chemical properties because they have the same number of valence electrons.

Question 37

In what order are orbitals filled or emptied when ions are formed?

Answer 37

For main group elements, the electrons that were added last are the first electrons to be removed. For transition metals and inner transition metals, however, electrons in the s orbital are easier to remove than the d or f electrons, so the highest ns electrons are lost, then the (n–1)d or (n–2)f electrons. When electrons are gained, they fill in the order predicted by the Aufbau principle.

Question 38

What is the covalent radius?

Answer 38

The covalent radius is one-half the distance between the nuclei of two identical atoms when they are joined by a covalent bond.

Question 39

What happens to the covalent radius as you move down a group, and why?

Answer 39

As you move down a group, the covalent radius increases because the principle quantum number n. Thus, the electrons are being added to a region of space that is increasingly distant from the nucleusl. Consequently, the size of the atom (and its covalent radius) must increase as we increase the distance of the outermost electrons from the nucleus.

Question 40

What is effective nuclear charge (Z_eff)?

Answer 40

Effective nuclear charge is the pull exerted on a specific electron by the nucleus, taking into account any electron-electron repulsions.

Question 41

What is shielding?

Answer 41

Shielding is the repulsive force an electron experiences due to repulsions from other electrons that counteracts the attractive force from the nucleus. Core electrons are adept at shielding, while electrons in the same valence shell do not shield each other as efficiently.

Question 42

What happens to the covalent radius as you move across a period, and why?

Answer 42

The covalent radius decreases as you move across a group. Each time you move from one element to the next in a period, the atomic number increases, but the shielding increases only slightly. Thus, the effective nuclear charge increases across a period, which pulls the electrons closer to the nucleus, making the covalent radius smaller.

Question 43

What happens to the covalent radius as electrons are removed from the outer shell of an atom, and why?

Answer 43

As electrons are removed from the outer shell of an atom, the covalent radius decreases because the remaining core electrons occupying smaller shells experience a greater effective nuclear charge and are drawn even closer to the nucleus.

Question 44

What happens to the covalent radius as electrons are gained in an atom, and why?

Answer 44

As electrons are added to the valence shell of an atom, there is a greater repulsion among electrons and a decrease in effective nuclear charge per electron, which causes in an increase in covalent radius relative to the parent atom.

Question 45

What does isoelectronic mean?

Answer 45

Atoms and ions are said to be isoelectronic if they have the same electron configuration.

Question 46

What determines the size of atoms and ions that are isoelectronic?

Answer 46

Since isoelectronic atoms and ions have the same number of electrons, the number of protons determines the size. The more protons there are, the greater the nuclear charge and, thus, the smaller the covalent radius.

Question 47

What is ionization energy?

Answer 47

Ionization energy is the amount of energy required to remove the most loosely bound electron from an atom.

Question 48

What happens to ionization energy as you move across a period, and why?

Answer 48

In general, ionization energy increases across a period. Across a period, the atomic radius decreases, and the electrons are closer to the nucleus. Thus, the attractive forces due to the nucleus are harder to overcome, so more energy is required to remove a valence electron.

Question 49

What happens to ionization energy as you move down a group, and why?

Answer 49

As you move down a group, the ionization energy decreases. Since the atomic radius increases down a group, the outermost electrons are farther away from the nucleus. Therefore, the effective nuclear charge is lower, and it takes less energy to remove an electron.

Question 50

What is the deviation to the ionization energy trend regarding s and p orbitals?

Answer 50

Within any one shell, the s electrons are lower in energy than the p electrons. This means that an s electron is harder to remove than a p electron in the same shell. For example, according to the trend that ionization energy increases across a group, boron (Z=5) should have a higher ionization energy than beryllium (Z=4). However, the electron configurations for boron ([He] 2s²2p¹) and beryllium ([He] 2s²) show that, since boron has an electron in the 2p orbital, it will be easier to remove than beryllium’s 2s electron. Thus, boron actually has a lower ionization energy than beryllium.

Question 51

What is the deviation to the ionization energy trend regarding half-filled orbitals?

Answer 51

Half-filled orbitals are more energetically favorable (due to the electron-electron repulsions caused by pairing in the same orbital). This explains why, for example, oxygen has a lower ionization energy than nitrogen (despite the trend that ionization energy increases across a period). Oxygen’s electron configuration ([He] 2s²2p⁴) shows that if we remove an electron from the 2p subshell, it will eliminate the repulsions caused by electrons sharing an orbital in the 2p subshell, and will create a half-filled subshell.

Question 52

What causes the large jumps in successive ionization energies?

Answer 52

It requires more energy to remove core electrons than valence electrons (as core electrons have lower energies than valence electrons). Thus, the first ionization in which a core electron is removed, it takes much more energy to remove it.

Question 53

What is electron affinity?

Answer 53

Electron affinity is the energy change for the process of adding an electron to an atom.

Question 54

In general, what is the trend in electron affinity as you move across a period, and why?

Answer 54

In general, there is a higher electron affinity as you move across a period due to a higher effective nuclear charge.

Question 55

What are the exceptions to the trend across a period for electron affinity, and why?

Answer 55

• An electron that is added to a Group 2 or Group 18 atom requires placement in a higher-energy orbital, which requires a lot more energy. Thus, these electron affinity values are positive instead of negative, opposite to what the trend predicts.
• An electron that is added to Group 15 atoms must be paired with another electron in the same orbital. The half-filled state is more stable, so disrupting this requires more energy, making the electron affinity value positive.

Question 56

What explains that, in general, the second element in a group has a higher electron affinity than the first element, despite that the first elements have higher effective nuclear charges?

Answer 56

While the first elements in a group have higher effective nuclear charges, the n=2 energy levels are so small that the electron-electron repulsions become significant. Since the outermost orbitals in the second element in the group are larger, there is less influence from electron-electron repulsions and more from the attractive force of the nucleus.