5.2 – Quantum Theory and the Atom Chapter 5 5.2 – Quantum Theory and the Atom

Bohr’s Model Danish physicist, Niels Bohr proposed model for H atom Proposed H atom has only certain allowable energy states Lowest allowable energy state is ground state Atom gains energy, it’s in an “excited” state Related H atom’s energy states to electrons w/i atom Suggested electron in H atom moves around nucleus in certain circular orbits Smaller orbit => lower energy state

Bohr’s Model

Bohr’s Model Bohr assigned a # n (quantum #) to each orbit for calculations First orbit is that closest to the nucleus n=1 For H, atom is in ground state when the single electron is in the n=1 orbit and does not radiate energy If outside E (energy) is added, the electron will move to higher orbit Atom is now in excited state

Bohr’s Model Side note: Limit’s of Bohr’s Model: Orbitals are not evenly spaced apart Limit’s of Bohr’s Model: Explained H well Failed to explain other elements Didn’t fully explain chemical behavior of atoms

Quantum Mechanical Model of the Atom French grad student Louis Broglie proposed a fix to Bohr’s Model Compared Bohr’s electron orbits to waves

Quantum Mechanical Model of the Atom Quantum Mechanical Model – model where electrons are treated as waves AKA – “Mechanical Model” Like Bohr’s – limits electrons E to certain values Unlike Bohr’s – does NOT attempt to describe electron path

Hydrogen Atomic Orbitals Boundary of atomic orbital is unclear => orbital does not have exact shape Principal Quantum # - first quantum # of atomic orbitals (n) Indicates relative size and energy of atomic orbitals As n increases, orbital grows and electron spends more time away from nucleus increasing E Each major energy level is called Principal Energy Level

Hydrogen Atomic Orbitals Principal Energy Level Lowest E level is assigned a principal quantum # of 1 Up to 7 E levels for H Energy Sublevels Principal E level 1 – 1 sublevel Principal E level 2 – 2 sublevels Principal E level 3 – 3 sublevels and so on…

Hydrogen Atomic Orbitals Sublevels are labeled s, p, d or f according to shape of atom’s orbital s orbitals are spherical p orbitals are dumb-bell shaped

Hydrogen Atomic Orbitals Sublevels are labeled s, p, d or f according to shape of atom’s orbital d and f orbitals are not all the same

Hydrogen Atomic Orbitals Each orbital will contain at most, 2 electrons Single sublevel in princ. E level 1 corresponds with 1s orbital S sublevel has 1 orbital 2 sublevels in princ. E level 2 are 2s and 2p Note: 2s is larger in size than 1s p sublevels have 3 orbitals of E Princ. E level 3 has 3 sublevels 3s, 3p and 3d d sublevels have 5 orbitals of energy 4th sublevel consists of 4s, 4d, 4p, and 4f f sublevels have 7 orbitals of E

Atomic Orbitals

5.3 – Electron Configuration

Ground State Electron Configuration Electron Configuration – arrangement of electrons in an atom Low E systems are more stable => atoms tend to arrange themselves in such a way to give them lowest E possible (ground state electron configuration). 3 Rules/Principles: Aufbau Principle Pauli Exclusion Principle Hund’s Rule

Aufbau Principle Aufbau Principle: Electrons occupy lowest energy orbital Diagram gives sequence (p. 156) Sequence of energy sublevels go: s p d f

Pauli’s Exclusion Pauli’s Exclusion: Electrons in orbitals are represented by arrows in boxes Each electron has associated spin (like a top) Can only spin in 1 of 2 directions Arrows point up or down to represent spin Empty box indicates unoccupied orbital Box with 1 arrow implies one electron Box with arrow up and down means orbital is full **MAX of 2 arrows can occupy a box (must have opposite spin)

Pauli’s Exclusion

Hund’s Rule Hund’s Rule: Single electron with same spin must occupy each equal- energy orbital before additional electrons with opposite spins can occupy the same orbital

Electron Arrangement 2 ways to represent electron configuration: Orbital Diagrams (ex: C) Electron Configuration Notation Describes principal energy level and energy sublevel associated with each orbital Superscripts represent # of electrons in orbital Ex: C = 1s22s22p2

Electron Arrangement Noble Gas Notation Method of representing electron configuration of noble gases Noble gases are elements in last column of periodic table Usually have 8 electrons in outermost orbital and are unusually stable Notation uses bracketed symbols Ex: Helium: 1s2 or [He] Ex: Neon: 1s22s22p6 or [Ne] Ex: Sodium: [Ne]3s1

Exceptions Use the aufbau diagram to write correct ground-state electron configurations for all elements up to #23 For elements after that, look at a sublevel diagram to see the order in which the orbitals are filled (there is an increased stability of half-filled and filled sets of s and d orbitals) Periodic Table Method

Sublevel Diagram

Valence Electrons Valence Electrons – electrons in atom’s outermost orbitals Determine chemical properties of an element Ex: S [Ne]3s23p4 6 valence electrons Ex: Cs [Xe]6s1 1 valence electron

Electron Dot Structure Electron Dot Structure – Consists of elements symbol and is surrounded by dots to represent valence electrons. These electrons are involved in forming chemical bonds Place one dot on each of the 4 sides of the symbol (any sequence is fine) Once all single spots are filled, begin pairing up the electrons Ex: Carbon

Electron Dot Ex: O Ex: F Ex: N