Electronic Configuration and Shielding

• Cr and Cu have electron configurations that deviate from the Aufbau principle.
• These deviations are attributed to the greater stability of filled or half-filled orbitals.

Anomalous Electronic Configurations of Cr and Cu

• Chromium (Cr, Z = 24) has an observed electron configuration of 1s²2s²2p⁶3s²3p⁶4s¹3d⁵, instead of the expected 1s²2s²2p⁶3s²3p⁶4s²3d⁴.
• Copper (Cu, Z = 29) has an observed electron configuration of 1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹, instead of the expected 1s²2s²2p⁶3s²3p⁶3d⁹4s².
• The stability of having d subshells that are either half-filled (d⁵) or fully filled (d¹⁰) is the underlying cause of these anomalies.
• Chromium's configuration becomes [Ar]4s¹3d⁵ instead of [Ar]4s²3d⁴ by promoting one 4s electron to the 3d subshell.
• Similarly, copper's configuration is observed as [Ar]4s¹3d¹⁰, unlike the expected [Ar]4s²3d⁹.
• Elements like Silver (Ag) and Gold (Au) share similar exceptions, too.
• Exactly filled or half-filled subshells exhibit extra stability.
• The drive to achieve this stability leads to the promotion of an electron into the ns orbital.

Abbreviated Electron Configurations - Noble Gas Notation

• Noble gas notation utilizes noble gas symbols in brackets to abbreviate inner electron configurations.
• The atomic number of the noble gas used must be lower than that of the element.
• This notation represents the core electrons plus the outer shell electrons.
• Noble gas notation emphasizes the outer shell electrons, which affect chemical reactions.
• Examples of noble gas notation are:
• Barium (Ba) = [Xe] 6s²
• Ruthenium (Ru) = [Kr] 4d⁷ 5s¹
• Sulfur (S) = [Ne] 3s² 3p⁴
• Electron configurations for all elements excluding hydrogen and helium can use a noble gas core.
• Potassium (K, Z = 19) has an electron configuration that can be simplified from 1s²2s²2p⁶3s²3p⁶4s¹ to [Ar]4s¹.
• For arsenic (As, Z = 33), the noble gas core is [Ar] with Z = 18.
• The order of filling beyond the noble gas core is 4s, 3d, and 4p.
• [Ar] 3d¹⁰ 4s² 4p³ represents Arsenic's noble gas notation configuration, where the number of electrons beyond the noble gas = 15.
• To determine the noble gas core notation for manganese (Mn), identify the last noble gas filled before Mn and then fill subsequent sublevels.
• The "ns" orbital is filled, followed by the "np" orbital, "(n-1)d" orbital, and "(n-2)f" orbital.
• Examples of complete vs noble electron configurations (elements 11-18)
• Sodium Complete (Na) 1s²2s²2p⁶3s¹
• Sodium Noble (Na) [Ne] 3s¹

Group Electronic Configurations

• Elements from group 2A all possess ns² outer shell electrons.
• Differences between them lie only in the value of n.
• Beryllium (Be): [He] 2s²
• Magnesium (Mg): [Ne] 3s²
• Calcium (Ca): [Ar] 4s²
• Strontium (Sr): [Kr] 5s²
• Barium (Ba): [Xe] 6s²
• Radium (Ra): [Rn] 7s²

Pseudo Noble Gas Electronic Configuration

• Pseudo noble gas electronic configuration has an ending of ns²np⁶nd¹⁰.
• Common with transition metal cations like Ag⁺ and Zn²⁺.
• Results in 18 electrons in valence shell.

Electron Shielding and Effective Nuclear Charge

• Valence shell electrons are crucial for sharing and exchanging during chemical reactions.
• The tightness of electron binding affects the element's chemistry.
• Binding degree arises from two opposing forces: attraction between electrons and the nucleus and repulsions among electrons.

Electron Shielding

• Electrons in atoms are drawn to the nucleus but simultaneously repelled by other electrons.
• Inner shielding electrons weaken the attraction force for outer valence electrons.
• Valence electrons in atoms need less energy for removal compared to inner electrons.

Electron Shielding Trends

• Electron shielding remains constant across a period.
• Electron shielding increases down a group.
• Examples:
• Li: 1s² 2s¹
• Na: 1s² 2s² 2p⁶ 3s¹
• K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
• Rb: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

Effective Nuclear Charge

• Effective nuclear charge is the measure of the attractive force between the positively charged nucleus and negatively charged electrons.

Effective Nuclear Charge Theory

• Three protons are in the nucleus, and two core electrons reside close to the nucleus.
• The outer electron experiences a net positive charge and feels an approximate charge of one proton due to shielding.
• Core electrons diminish the attraction force for valence electrons.
• To calculate effective nuclear charge (Zeff), use the formula Zeff = Z - S, where Z is the atomic number and S is the shielding constant (number of shielding electrons).
• Note: this is an approximation.
• Effective nuclear charge increases going across a period.
• Examples of effective nuclear charge across a period are:
• Na 1s² 2s² 2p⁶ 3s¹ Zeff (Na) = +1
• Mg 1s² 2s² 2p⁶ 3s² Zeff (Mg) = +2
• Effective Nuclear Charge (Zeff) remains constant moving down a group.
• Examples of effective nuclear charge down a group are:
• Li: 1s² 2s¹ Zeff (Li) = +1
• Na: 1s² 2s² 2p⁶ 3s¹ Zeff (Na) = +1
• K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ Zeff (K) = +1
• Rb: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ Zeff (Rb) = +1

Shielding Constant Calculations using Slater's Rules

• Slater's Rules estimate effective nuclear charge (Zeff) considering the effective shielding of orbital electrons.
• Effective nuclear charges (Zeff) experienced by electrons in various atomic orbitals can be estimated using Slater's rules: Zeff = Z - σ
• Z = nuclear charge (atomic number), Zeff = effective nuclear charge, σ = screening (or shielding) constant.

Slater Rule Step 1 & 2

• Step 1: Write electron configuration as follows: (1s) (2s, 2p) (3s, 3p) (3d) (4s, 4p) (4d) (4f) (5s, 5p) ...
• Step 2: Identify the electron of interest, ignore electrons in higher groups to the right, because they do not shield lower groups.

Slater Rule Step 3

• Step 3: If evaluating shielding experienced by an s- or p- electron:
• Electrons within same group contribute 0.35, but 1s electrons contribute 0.30
• Electrons in the n-1 group contribute 0.85
• Electrons belonging to the n-2 or lower groups contribute 1.00
• If evaluating nd or nf valence electrons:
• Electrons that reside within the same group contribute 0.35
• Electrons within the lower groups contribute 1.00

Slater Rule Strategy

• Strategy Steps:
• 1. Determine electron configuration, identify electron of interest
• 2. Apply appropriate Slater's rules
• 3. Determine effective nuclear constant

Nitrogen Shielding Constant Example

• Nitrogen possesses the electron structure 1s² 2s² 2p³.

• The electron configuration is rewritten as (1s²)(2s²,2p³).

• Four electrons are in the same ns, np group so they contribute 0.35 each There are 2 electrons in the n-1 groups and each contributes 0.85

• Shielding constant σ is = 3.10

• Atoms that are in the same subshell do not shield each other, meaning

• The radius to the average distance from nucleus is the same

• They attempt to stay away from each other

• They spend very little time one below another

• Effective nuclear charge (Zeff) is correlated to the difference between charge on the nucleus (Z) and charge on core.

Explore electronic configuration, the Aufbau principle, and valence electrons. Understand electron shielding and its effects on effective nuclear charge. Learn about Slater's rules and their role in calculating shielding constants.