AP Chem Turn in Hydrate Lab Today: Atoms Review; back page = HW Lab Thursday You’re welcome to start coming in before school/during MM to do test corrections Next test in ~ 2 weeks

Evolution of the Atomic Model Since atoms are too small to see even with a very powerful microscope, scientists rely upon indirect evidence and models to help them understand and predict the structure of an atom.

Democritus & Leucippus (~400 BC) Greek philosophers first to propose that matter made up of tiny, indivisible particles called atomos, the Greek word for atoms

John Dalton (1803) Billiard Ball Model: Dalton theorized that the basic unit of matter is a tiny particle called an atom Dalton’s Theory: All elements are composed of indivisible (can’t be broken down) atoms All atoms of a given element are identical Atoms of different elements are different; Compounds are formed by the combination of atoms of different elements Billiard Ball Model: An atom is represented by a hard sphere

JJ Thomson (1897) Used a cathode ray tube to show one of the smaller units that make up an atom Because the cathode ray deflected towards the + charged plate when an electric field was applied, Thomson concluded that the ray was formed by particles and the particles were negatively charged

JJ Thomson (1897) Thomson discovered that the atom is made up of small, negatively charged particles which he called electrons Developed the Plum Pudding Model of the atom

Robert Millikan (1909) Millikan’s Oil drop experiment allowed him to determine the mass and charge of an electron

Rutherford: Gold Foil Experiment

Ernest Rutherford (1911) Conducted the gold-foil experiment: Directed alpha particles (small, positively charged particles), at a thin piece of gold foil Most of the alpha particles passed straight through the foil and a few were slightly deflected Some of the alpha particles were greatly deflected and bounced back  

Ernest Rutherford (1911) Rutherford concluded that atoms have a dense, central core called the nucleus, while the remainder of the atom is essentially empty space Positively charged particles known as the protons are found in the nucleus

James Chadwick First to prove the existence of the neutron Provided explanation as to why the positively charged protons in the nucleus stayed intact and did not repel each other.

Neils Bohr (1913) Bohr Model The nucleus contained the protons and neutrons The electrons orbited around the nucleus (like planets orbiting the sun)   Electrons are shown in concentric circles or shells around the nucleus The 1st shell can hold a maximum of 2 electrons The 2nd shell can hold a maximum of 8 electrons The 3rd shell can hold a maximum of 18 electrons Electrons in the outermost shell are called the valence electrons

Wave-Mechanical/Cloud Model (modern, present-day model) Developed after the famous discovery that electrons can be viewed as both a wave and particle Like planetary model, atom is pictured as having dense, positively charged nucleus The difference in this model is how the electrons are pictured

Wave-Mechanical Model Electrons have distinct amounts of energy and move in areas called orbitals (clouds) An orbital is a region in which an electron is most likely (most probable) to be located Value of l Orbital (subshell) Orbital Shape Name* s sharp 1 p principal 2 d diffuse 3 f fine

What is important about the atomic number? Unique for each element = # protons in nucleus

How do you figure out the number of: Protons in an atom? # protons = the atomic number Electrons in an atom? # electrons = # protons FOR NEUTRAL ATOMS, only! Neutrons in an atom? # neutrons = mass – # protons

What are Isotopes? Isotopes are different forms of the same element that have a different mass Isotopes have the same # protons but different # neutrons

What are Ions? Ions are atoms that have a positive or negative charge In an Ion, the number of protons and electrons are not equal

Standard Nuclear Notation Note: for ions, the charge (+ or -) is indicated in the upper right corner

What is the ONE THING that determines the identity of an atom? # of protons (atomic #)

Practice

Average atomic mass on the periodic table is a weighted average of the masses of all naturally occurring isotopes

Average Atomic Mass 28.085 amu , 14 28 𝑆𝑖 151.9641 amu, 63 152 𝐸𝑢 75.91 % Cl-35, 24.08% Cl-37

Electromagnetic radiation - a form of energy that has wavelike properties - all forms found in the electromagnetic spectrum Examples of electromagnetic radiation: Xrays Microwaves Light waves Radio waves – when you hear static on the radio when you aren’t getting a signal, that is the leftover microwaves from the big bang 15 billion years ago (it is cooling off so now the waves are the energy and wavelength of radio waves)

WAVELENGTH AND FREQUENCY ARE TWO IMPORTANT PROPERTIES OF WAVES. DIFFERENT FORMS OF EMR TRAVEL IN WAVES. WAVELENGTH AND FREQUENCY ARE TWO IMPORTANT PROPERTIES OF WAVES.

Wave Characteristics Amplitude Wavelength Wavelength 2 sec 0 sec Amplitude Wavelength Wavelength Frequency Amplitude The shortest distance between two equivalent Points (meters) How many waves pass a certain point per sec. (s-1 OR Hz) Wavelength – the shortest distance between two equivalent points on a wave Frequency – how many wave pass a certain point per second Amplitidude –the height of the wave from crest to origin State: Light travels as photons; photons have no mass but have energy The energy a photon has is based on the equation E = hv Energy is directly proportional to the frequency. Frequency has a unit of Hz which is = 1 𝑠 or s-1 or per second. Wavelength is also related to frequency; how many people know the speed of light? Yes, what is it? EA 3 x 108 m/s The speed of light is a constant that is known. Speed of light is based on the speed it travels over time. Wavelength is in meters and frequency is per second, so c=vλ The height of a wave from crest to origin

THE HIGHER THE FREQUENCY, THE SHORTER THE WAVELENGTH FREQUENCY = THE NUMBER OF WAVES THAT PASS BY A FIXED POINT PER SECOND. THE HIGHER THE FREQUENCY, THE SHORTER THE WAVELENGTH

THE HIGHER THE FREQUENCY, THE HIGHER THE ENERGY FREQUENCY = THE NUMBER OF WAVES THAT PASS BY A FIXED POINT PER SECOND. THE HIGHER THE FREQUENCY, THE HIGHER THE ENERGY

THE HIGHER THE ENERGY, THE SHORTER (SMALLER) THE WAVELENGTH

Practice

Photons - released during electromagnetic radiation – tiny particles that have no resting mass which carry a quantum of energy All photons travel at the speed of light in a vacuum Photons behave like particles quantum – the minimum amount of energy that can be absorbed or released from an atom - cannot be any value but are in discrete energy levels (like bundles or packets of energy)

𝑐=𝑣λ 3.00× 10 8 =𝑣×4.9× 10 −7 𝒗=𝟔.𝟏× 𝟏𝟎 𝟏𝟒 𝑯𝒛

3.17 m 1.70 x 10-22 J 8.28 x 10-16 J 1.0 x 10-5 m

Electrons and Energy Levels Each electron has a distinct amount of energy that is related to the energy level (shell) it is in Electrons with the lowest energy are found in the shell closest to the nucleus Electrons with the highest energy are found in the shell furthest from the nucleus The greater the distance from the nucleus, the greater the energy of the electron

Ground vs. Excited State Ground state=when the electrons occupy the lowest energy levels possible Excited state= on or more electrons gain energy and move to a higher energy level or shell (leaving the lower energy levels to be not completely full)

Ground and Excited State When an electron gains energy, it jumps to a higher energy level or shell This is a very unstable condition We call this condition the excited state Very rapidly, an electron in the excited state will lose energy and move back to a lower energy level or shell When excited electrons fall from an excited state to a lower energy level, they release energy in the form of light

Ground  Excited State Electron gains energy from heat, light, electricity Electron “jumped” to a higher energy level (shell)

Excited  Ground State Electron releases energy in the form of light Electron “falls” back and returns to ground state (normal position)

Bright Line Spectrum Electrons falling from an excited state down to the ground state give off visible light Different elements produce different colors of light or spectra These spectra are unique for each element (just like a human fingerprint is unique to each person)