Aim: How are electrons arranged in an atom? Warm Up The atomic number of iron is 26 and the mass number is 56, how many protons are present and how many neutrons are present? What element has an atomic number of 2 and mass number of 4?

Modern Models of the Atom Rutherford’s model had a problem… It could not explain the chemical properties of atoms Rutherford’s model fails to explain why objects change color when heated. As the temperature of the horseshoe is increased it changes from black to red to yellow. The observed color change could only occur if the atoms in the iron gave off light in specific amounts of energy.

e- are in specific orbits around the nucleus Niels Bohr (Danish 1885-1962) e- are in specific orbits around the nucleus Each orbit contains a fixed amount of energy Sometimes referred to as the ‘planet’ model.

Energy Levels Like rungs on a ladder Lowest rung = lowest energy level Quantum = the amount of energy needed to move an e- from one energy level to the next Electrons gain or loose energy as they move from one energy level to another. Just like a person on a ladder, an electron can not exist in between rungs or energy levels. Energy levels are not equally spaced. In picture b the rungs are closer together. It takes less energy to climb from one rung of the ladder to another near the top. The Bohr model gave results that fit the hydrogen atom, however it failed to explain the energies emitted and absorbed by atoms with more than one electron.

Drawing Bohr Diagrams Rules: 2e- can occupy the 1st energy level 8e- can occupy all others In all energy levels, e- are 1st placed 2 @ a time starting at 12 o’clock All other e- are placed 1 @ a time at 3, 6 and 9 o’clock Practice drawing Bohr diagrams

Erwin Schrödinger (Austrian 1887-1961) e- are not found in specific orbitals e- are in a ‘cloud’

Quantum (wave) Mechanical Model – Determines an e-’s energy Predicts the probability of finding an e- at various locations around the nucleus Atomic orbital = the region of space where there is a high probability of finding an e-

Quantum Energy Levels Principal quantum number – (n) the energy level where the e- is found n = 1, 2, 3, 4 Energy sublevel – correspond to orbitals of different shapes describe where e- may be found Letter symbols: s, p, f, d

How to fill electron shells

Summary Energy Number of Types of Level Sublevels Sublevels 1 1 s 2 2 s,p 3 3 s,p,d 4 4 s,p,d,f *Principal energy level # = # of sublevels

Energy Level Sublevels # Orbitals Total Orbitals Total Electrons 1 s 2 4 8 p 3 9 18 d 5 16 32 f 7

Practice Problems Write the electron configuration for each atom: Carbon - Argon - Boron - Silicon - 1s22s22p2 1s22s22p63s23p6 1s22s22p1 1s22s22p63s23p2

Periodic Table Arrangement Examine your Periodic Table to see how the electron configurations are written: Formula: Short hand notation lists the number of e- in the each quantum energy level. It does not list the sublevel orbital. These are the ground state (unexcited) electron configurations. 1st-2nd-3rd-4th-5th-6th

Practice Using the Periodic Table Configuration Use the Periodic Table to write the ground state electron configuration for the following elements: Carbon - Argon - Boron - Silicon - 2-4 2-8-8 2-3 2-8-4

Excited State Electrons Ground state Excited state e- are further away from the nucleus e- are at higher energy levels e- are less stable e- have absorbed energy

Sample Problems Which is the electron configuration of an atom in the excited state? 1s22s22p2 1s22s22p1 1s22s22p53s2 1s22s22p63s1

An atom in the excited state can have an electron configuration of? 1s22s2 1s22p1 1s22s22p5 1s22s22p6 Which electron configuration represents a potassium atom in the excited state? 1s22s22p63s23p3 1s22s22p63s13p4 1s22s22p63s23p64s1 1s22s22p63s23p54s2

How Color Tells Us About Atoms Physics and the Quantum Mechanical Model

Prism White light is made up of all the colors of the visible spectrum. Passing it through a prism separates it.

By heating an element we can get it to give off colors. Passing it through a prism does something different.

Atomic Line Spectra Each element gives off its own characteristic color Called Line Spectra The energy emitted is given off as photons of light.

Line-Emission Spectrum excited state ENERGY IN PHOTON OUT ground state

Electron Energy Levels 3rd energy level 2nd energy level Energy absorbed 1st energy level Energy lost nucleus 1999, Addison, Wesley, Longman, Inc.

As e- ‘fall’ from higher orbitals energy is given off. Amount of energy given off = to the distance of the fall 6 5 4 3 2 1 nucleus

‘Atomic Fingerprints’ Each element has a unique atomic line emission spectrum. Used to identify the element

Continuous and Line Spectra 5000 6000 7000 Visible Light Spectrum Na H Ca Hg Spectroscopy is a method of identifying unknown substances from their spectra. Because all materials have unique spectra, you can think of these spectra as being molecular fingerprints. Line spectra of selected elements showing the emission spectra or fingerprint of that element. The unique set of lines produced is due to the fact that electrons are falling from different excited states in the atoms to the ground state. Objects at high temperature emit a continuous spectrum of electromagnetic radiation. A different kind of spectrum is observed when pure samples of individual elements are heated. When the emitted light is passed through a prism, only a few narrow lines, called a line spectrum, are seen rather than a continuous range of colors. Using Planck’s equation, the observation of only a few values of  (or ) in the line spectrum meant that only a few values of E were possible — only states that had certain values of energy were possible or allowed. Any given element has both a characteristic emission spectrum and a characteristic absorption spectrum, which are complementary images. – Emission spectrum: emission of light by atoms in excited states – Absorption spectrum: absorption of light by ground-state atoms to produce an excited state • Emission and absorption spectra form the basis of spectroscopy, which uses spectra to provide information about the structure and composition of a substance or an object

Flame Emission Spectra Photographs of flame tests of burning wooden splints soaked in different salts. methane gas wooden splint strontium ion copper ion sodium ion calcium ion This technique is called emission spectroscopy.

Neon Signs

Fireworks

The chemistry of fireworks – Colors of fireworks due to atomic emission spectra – A typical shell used in a fireworks display contains gunpowder to propel the shell into the air and a fuse to initiate a variety of redox reactions that produce heat and small explosions – Thermal energy excites the atoms to higher energy states, and as they decay to lower energy states, the atoms emit light that gives the familiar colors