Modern Atomic Theory Chapter 4.3

1913  Nucleus surrounded by a large volume of space  like Rutherford's model of 1911  Focused on the arrangement of electrons.  Electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun.  Each electron in an atom has a specific amount of energy. Bohr’s Model of the Atom

 Bohr believed  Individual orbitals could only contain so many electrons  Orbitals are filled from the inside out Bohr’s Model of the Atom OrbitalNumber of Electrons

Bohr’s Model of the Atom 2 e - ’s 8 e - ’s 18 e - ’s

Atom gains or loses energy  Electron(s) moves between energy levels Electron Nucleus Bohr Model Electrons gain or lose energy when they move between fixed energy levels Electron Nucleus Bohr Model Electrons gain or lose energy when they move between fixed energy levels Bohr’s Model of the Atom

Analogy: electron energy levels like a staircase - The landing at the bottom of the staircase is the lowest level. Each step up represents a higher energy level. The step height represents an energy difference between levels. You can only move in whole numbers – you can’t go up half a step Bohr’s Model of the Atom

Electron can:  Move up or down (i.e. gain or lose energy)  Move one or more energy levels at a time  if it gains or loses the right amount of energy.  Size of jump  amount of energy change (gained or lost) Each element has unique set of energy levels Bohr’s Model of the Atom

Light is a form of energy  It can be observed/measured.  It can be emitted or absorbed by electrons Absorb energy  move to a higher energy level Release energy  move to a lower energy level Bohr’s Model of the Atom

Ground state:  All the electrons in an atom have the lowest possible energies  Most stable electron configuration Excited State:  Electron(s) have absorbed energy & are in now in an higher energy orbital  Very unstable  Electron(s) will return to its original state. Electron Configuration

Lithium atom (Atomic # = 3) has three electrons. Ground state:  Two electrons in lowest energy orbital;  Third electron in next (second) energy level. Excited state: Atom absorbs energy  Electron moves to higher orbital, leaving “hole” in lower orbital Excited state less stable than ground state, so Electron in higher energy level loses energy  falls into “hole” Atom returns to the ground state. Electron Configuration

The ground state of a person is on the floor. A gymnast on a balance beam is like an atom in an excited state—not very stable. When she dismounts, the gymnast will return to a lower, more stable energy level. Electron Configuration

The movement of electrons between energy levels explains the light you see when fireworks explode. Heat  electrons to move to higher energy levels. When those electrons move back to lower energy levels, they release energy.  Some of that energy is released as visible light. Different elements emit different colors of light because each element has its own unique set of energy levels. Bohr’s Model of the Atom

Bohr’s model was improved  Electron “orbits” do have specific energy levels. [Bohr got this part right]  Electrons do not move like planets in a solar system. [Bohr got this part wrong] Electron cloud model: Visual model based on probability (statistical equations) Electron Cloud Model

 Heisenberg’s Uncertainty Principle Electron Cloud Model

 Heisenberg’s Uncertainty Principle  The position and momentum of a particle cannot be simultaneously measured with arbitrarily high precision. Translation:  We cannot pin point the exact location or speed of an electron at any given point and time.  We can only approximate based on statistics Electron Cloud Model

When propeller stopped, you see location of blades. When propeller is moving, you see only a blur.  Blur is similar to drawing of electron cloud. Electron Cloud Model

Imagine a map of our school.  Mark your exact location with a dot once every 10 minutes for one week.  Dots on map are model of your “orbital”  Shows most likely locations to find you. Places visited most  highest concentration of dots. Places visited least  lowest concentration of dots.  Everybody’s dots together = school’s “electron cloud” Electron Cloud Model

Orbital:  Region of space around the nucleus where an electron is likely to be found. Electron cloud:  All the orbitals in an atom. Electron cloud = good approximation of how electrons behave in their orbitals. Electron Cloud Model

Electron cloud model replaced Bohr's model of electrons moving in predictable paths. The nucleus contains protons and neutrons The electron cloud is a visual model of the probable locations of electrons in an atom. The probability of finding an electron is higher in the denser regions of the cloud. Electron Cloud Models Electron Cloud Model

The level in which an electron has the least energy—the lowest energy level—has only one orbital. Higher energy levels have more than one orbital. Electron Cloud Model

Electron Configuration Orbitals