IVL 4 IVL

Atomic orbital – the region, or volume, of space in an atom within the high probability (95% chance) of finding an electron in an atom.

Types of orbital: s, p, d and f orbital

Core shell – first shell that holds two electrons

Valence shell – the outermost shell

Effective nuclear charge (Nuclear attraction) accounts for (increases from left to right of the periodic table):

• i) attraction to the nucleus
• ii) repulsion from core electrons
• iii) minimal repulsion by other valence electrons

Ionisation energy is influenced by:

• i) nuclear charge (nuclear charge increases, the force of attraction on the electrons becomes stronger and the ionisation energy increases.)
• ii) distance of the electrons from the nucleus (further the outer electrons are from the nucleus, ionisation energies will be lower.)
• iii) screening effect (outermost electrons in an atom are shielded from the attraction of the nucleus by the repelling effect of the inner effect. The higher the screening effect, lower the ionization energies.)

Electronic Structure

Number of electrons in shell = 2(n)²

• Example: Lithium atom.

Nucleus = made up of both neutrons and protons
Core shell = 1^st energy level (electron occupancy of 2)
Valence shell = 2^nd energy level (electron occupancy up to 8 )

Arrangement of electrons in an atom

• Aufbau principle – Electrons occupy orbitals with the lowest energy level first
• Pauli exclusion principle – Each orbital can hold maximum of two electrons withopposite spin
• Hund’s rule – Orbital with the same energy level (degenerate orbitals), electron will occupy different orbital singly/one electron first the parallel spin, before pairing

Electronic Configurations

The electrons are filled according the orbitals (Aufbau principle).

1. Fills the 1s orbital to: 1s²
2. Fills the 2s orbital to: 2s²
3. Fills the 2p then 3s orbitals to: 2p⁶3s²
4. Fills the 3p then 4s orbitals to: 3p⁶4s²
5. Fills the 3d, 4p then 5s orbitals to: 3d¹⁰4p⁶5s²

(Extended knowledge: the process is repeated until all of the electrons have been accounted for. g-, h- and j-orbital exist in theory but the periodic table contains no elements that have electrons in either g-, h- and j-orbitals.)

The first break from numerical sequencing comes when the 4s level is filled before the 3d level, despite the fact that the perimeter of the 3d level is closer to the nucleus than the 4s orbital. The reason is that the energy of the level is based on an average position of the electron, not the extreme position.

Ionising electrons are not removed from the atom in reverse order! However, the outer shell electrons are always removed first when forming cations.

Examples

Example 1: Electronic configuration for manganese.

-> Solution 1:

• Neutral manganese (Mn) atom must contain 25 electrons.
• Electronic configuration of Mn: 1s²2s²2p⁶3s²3p⁶4s²3d⁵

Example 2: Which column of the periodic table is diamagnetic?

-> Solution 2:

• Column 2 (alkaline earth metals) and Column 8 (noble gas). A diamagnetic compound has its entire electron spin-paired. There must be an even number of electrons in the element. Valence electronic configuration for alkaline earth metals is ns². Valence electronic configuration for noble gas is ns²np⁶.
• Column 1 (alkali metals) and Column 7 (halogen) are not diamagnetic.
• Column 6 (chalcogen) are paramagnetic. Valence electronic configuration for chalcogen is ns²np⁴.

Example 3: Electronic configuration for chromium

-> Solution 3:

• Half-filled d-shell stability in chromium: 1s²2s²2p⁶3s²3p⁶4s¹3d⁵ rather than 1s²2s²2p⁶3s²3p⁶4s²3d⁴. (Others half-filled d-shell element are molybdenum and tungsten)

Example 4: Electronic configuration for copper.

-> Solution 4:

• Fully-filled d-shell stability in copper: 1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰ rather than 1s²2s²2p⁶3s²3p⁶4s²3d⁹.

Example 5: Which of the following electronic configuration represents an exited state?
A. He: 1s²
B. Li: 1s²2p¹
C. N: 1s²2s²2p³
D. F: 1s²2s²2p⁶

-> Solution 5:

• B (Li should have 1s²2s¹ as a ground state and the electronic configuration has the last electron in a 2p-orbital that is higher energy than the ground state 2s.)
• An excited state electronic configuration does not follow energetic sequence. An excited state has at least one electron in an energy level higher than the ground state.

Important: Not to confuse an ion (either cation or anion) with an excited state. A cation is an atom that has a deficit of at least one electron and thus carries a positive charge. An anion is an atom that has an excess of at least one electron and thus carries a negative charge.

Periodic Table can be classified into 4 main groups.

1) The s-block elements:

• Group 1 – general electronic configuration ns¹.
• Group 2 – ns².

2) The p-block elements

• Group 13 – ns²np¹.
• Group 14 – ns² np².
• Group 15 – ns² np³.
• Group 16 – ns² np⁴.
• Group 17 – ns² np⁵.
• Group 18 – ns² np⁶.

3) The d-block elements

• Between Group 2 and Group 13 that the d orbitals are partially occupied.

4) The f-block elements

• Lanthanides (15 elements) – 4f orbitals are partially filled and must have a 6s².
• Actinides (15 elements) – 5f orbitals are partially filled and must have a 7s².