Quantum Mechanical Atomic Model and Periodic Properties

The Quantum Mechanical Atomic Model

The current atomic model (quantum mechanical) addresses the issues raised by Bohr. It was modified by Heisenberg, who stated that it is impossible to know simultaneously the position and velocity of a particle surrounding the nucleus of an atom. This is known as the Heisenberg Uncertainty Principle. Consequently, a new atomic model emerged, known as the quantum mechanical model, which states that the atom exists in confined areas known as orbitals. These are regions where there is a higher probability of finding electrons, and these orbitals are grouped into different energy levels. To describe the location of each electron, it is necessary to know the so-called quantum numbers, which are used to obtain the electronic configuration of each element in the periodic table.

The Four Quantum Numbers

1. Principal Quantum Number (n): This number indicates the energy level within the atom. It can take integer values of 1, 2, 3, 4, 5, 6, or 7. The higher the value of n, the greater the distance between an electron in orbit and the nucleus, making the orbital larger and less stable.

2. Secondary Quantum Number (l): This number provides information about the sub-levels present in each energy level. These sub-levels are the regions of probability for finding an electron at that level. Its values can be expressed as 0, 1, 2, and 3.

3. Magnetic Quantum Number (m): This number indicates the spatial orientation of orbitals within the sub-levels. The value of m depends on the value of the principal quantum number.

4. Spin Quantum Number (s): This number indicates the direction of rotation of the electron within an orbital. Its value is +1/2 (clockwise rotation) and -1/2 (counter-clockwise rotation).

Pauli Exclusion Principle and Configuration

Pauli Rule: The Pauli Exclusion Principle states that each orbital accepts a maximum of two electrons, which must have opposite spins. Electronic configuration refers to the distribution of electrons within the atom, which is governed by the basic rules of quantum mechanical theory.

Steps for Electronic Configuration:

• 1) Determine the number of electrons.
• 2) Place the electrons in each of the energy levels.
• 3) Respect the maximum capacity for electrons in each sub-level and, therefore, each orbital.

Periodic Properties of Elements

The electronic configuration is a regular feature of atoms. This periodicity is reflected in the arrangement of atoms presented in the periodic table and is determined by the distribution of electrons in their last energy level.

• a) Atomic Radius: This is the actual radius of an atom that is part of a pure element. If we take two atoms of the same element and place them together, we can determine the distance between the nuclei of the two atoms; the atomic radius is half of that distance.
• b) Ionization Potential: This is the minimum energy required to extract an electron from a neutral, ground-state gaseous atom to transform it into a cation (positive ion).
• c) Electron Affinity: This is defined as the energy released when a gaseous atom in its ground state captures an electron to become a negative ion (anion).
• d) Electronegativity: This is the ability of an atom to attract one or more electrons to itself.