Atomic Structure and Periodicity

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The five d orbital shapes are shown below. The d orbitals have two different fundamental shapes.

The Boundary Surfaces of All of the 3d Orbitals

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Energy Diagram for Hydrogen Atom

The energy of a particular orbital is determined by its value of n. All orbitals with the same value of n have the same energy and are said to be degenerate. Hydrogen single electron occupy the lowest energy state, the ground state. If energy is put into the system, the electron can be transferred to higher energy orbital called excited state. Orbital Energy Levels for the Hydrogen Atom

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Pauli Exclusion Principle

In a given atom, no two electrons can have the same set of four quantum numbers (n, l, ml, ms).

Therefore, an orbital can hold only two electrons, and they must have opposite spins.

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Polyelectronic Atoms

For polyelectronic atoms in a given principal quantum level all orbital are not in same energy (degenerate). For a given principal quantum level the orbitals vary in energy as follows:

Ens< Enp < End < Enf

In other words, when electrons are placed in a particular quantum level, they prefer the orbital in the order s, p, d and then f.

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Aufbau Principle

As protons are added one by one to the nucleus to build up the elements, electrons are similarly added to these hydrogen-like orbitals.

H : 1s1, He : 1s2, Li : 1s2 2s1, Be : 1s2 2s2

B : 1s2 2s2 2p1, C : 1s2 2s2 2p2.

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Hund’s Rule

The lowest energy configuration for an atom is the one having the maximum number of unpaired electrons allowed by the Pauli principle in a particular set of degenerate orbitals.

N : 1s2 2s2 2p3, O : 1s2 2s2 2p4,

F : 1s2 2s2 2p5, Ne : 1s2 2s2 2p6,

Na : 1s2 2s2 2p63s1 OR [Ne] 3s1

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The Electron Configurations in the Type of

Orbital Occupied Last for the First 18 Elements

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Valence Electrons

The electrons in the outermost principle quantum level of an atom.

Valence electron is the most important electrons to us because they are involved in bonding. Elements with the same valence electron configuration show similar chemical behavior. Inner electrons are called core electrons.

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