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**The Quantum Mechanical Picture of the Atom**

The allowed energy states of atoms and molecules can be described by sets of numbers called quantum numbers n m ms

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**Quantum Numbers - the angular momentum quantum number**

n - the principal quantum number n = 1, 2, 3, 4, “shells” - the angular momentum quantum number = 0, 1, 2, 3, … , n – 1 “subshells” = s, p, d, f, m - the magnetic quantum number m = –, – + 1, … , – 1, ms - the spin quantum number ms = –½, +½

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**Quantum Numbers and Orbitals**

Define the energy of the electron Defines the shape of the orbital Orbital The volume around the nucleus where the electron appears 90-95% of the time The Pauli principle No two electrons in an atom may have identical sets of four quantum numbers

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s orbital ( = 0) There is only one s orbital per n level

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**p orbital ( = 1) There are 3 p orbitals per n level**

They are named px , py , and pz

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d orbital ( = 2) There are 5 d orbitals per n level

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f orbital ( = 3) There are 7 f orbitals per n level

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**Order of Orbital Energy**

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Building Atoms Build the nucleus by adding the required # of protons and neutrons according to the atomic # and mass # of the atom Fill energy levels (orbitals) with the required # of electrons starting from the lowest available energy level and following Pauli and Hund rules (this is called Aufbau principle)

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**Building Atoms - Period 1**

n m ms H He

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**Building Atoms - Period 2**

n m ms Li Be

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**Building Atoms - Period 2**

n m ms B C

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Hund’s Rule Electrons occupy all the orbitals of a given subshell (with the same ) singly before pairing begins. These unpaired electrons have parallel spins (the same sign of ms). n m ms C

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**Building Atoms - Period 2**

F Ne

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**Building Atoms - Period 3**

Na P Mg S Al Cl Si Ar

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**Building Atoms - Period 4**

K Ca Sc – 4p or 3d ?

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