Atoms

Dalton’s Atomic Theory 1) All elements are composed of tiny individual particles called atoms. Element = A pure substance containing 1 type of atom. All of the substances listed on the periodic table are elements (K, O, Br, C, etc.). 2) Atoms of the same element are identical. Atoms of the same element have the same number of protons, neutrons, and electrons.

Dalton’s Atomic Theory 3) Atoms can mix together or chemically combine with one another in simple, whole-number ratios to form molecules and compounds. Molecule – A combination of two or more atoms that are chemically bound to one another. (ex: H20, Br2, O2) Compound – A molecule containing two or more different elements that are chemically bound to one another. (ex: H20, CO2, C6H1206) ie: All compounds are molecules, but not all molecules are compounds.

Compound or Molecule? Carbon Monoxide (CO) Liquid Bromine (Br2) Compound (& a molecule) Liquid Bromine (Br2) Molecule Hydrochloric Acid (HCl) Chlorine Gas (Cl2)

Atomic Structure Nucleus Orbitals Protons – positive (+1) charge Neutrons – neutral (no) charge Orbitals Electrons – negative (-1) charge

Subatomic Particles Protons and electrons have equal and opposite charges. Protons and neutrons have equal masses (nearly), while electrons are 2,000x smaller in mass.

History of the Atom As we’ve discussed, no one has “seen” an atom so it’s structure is theoretical. We’ll walk through how the theory of the atom has changed over time based on different lab evidence.

Early Atomic Models – Plum Pudding Anyone? Cathode Ray: deflection of electrons using negatively charged plates. - First evidence that of negatively charged particles existing. - If matter has (-) particles in it, but isn’t negative overall, there must be (+) charge present to balance it out!

Plum-Pudding Model Negatively charged particles, spread out through a positively charged sphere

Advances in the Atomic Model: Rutherford’s Atom Gold Foil Experiment: Alpha particles (2p/2n) are “Red Rovered” at a thin piece of gold foil, just a couple atoms thick. +2 +2 +2

Gold Foil Experiment Hypothesis If the charges in an atom are spread out (Plum-Pudding), the positively charged alpha particles should be able to pass right through. Gold-Foil Experiment

Gold Foil Experiment Results The fact that some particles deflected shows there must be a concentrated region of positive charge in the atom, with negative particles found elsewhere. Almost all of the particles passed through, showing the positive area was a very, very small core (nucleus) and almost all of the atom was a less dense negative area (electron cloud)

Rutherford Model Electrons don’t follow a defined, circular path around the nucleus. Electrons can be found anywhere within a defined “cloud” region. The “cloud” is a graphical depiction of the area around the nucleus where an e- is likely to be found at any given moment.

Lab Support for Rutherford’s Model Mapping Electron Clouds

Bohr Model Electrons follow defined pathways around the nucleus called energy levels. They can jump up to higher energy levels if they absorb energy = excitation When they fall back down to their original energy level, they release the energy back out as a light wave = emission

Lab Support for Bohr’s Model Light Emission: electron transitions between the same energy levels always produce the same color photon (light) emission. Emission Animation

What does Bohr’s experiment show us? Stay with me here… Different colors of visible light waves have unique wavelengths (ex: red light = 700nm, blue = 475nm) The energy of a light wave is dependent on its wavelength (we’ll talk about the equation to calc this later) Short Wavelength = High Energy Long Wavelength = Low Energy Therefore, a wave’s energy is directly related to it’s color (wavelength) Blue = high energy Red = low energy

Quantized Energy Levels If we get the same color every time an electron drops down between the same energy levels, the levels must be a specific amount of energy apart!

Building Elements As we build atoms (add protons/neutrons/electrons) notice what happens to the atoms’ 1) Stability 2) Element Name (symbol) 3) Overall Charge http://phet.colorado.edu/en/simulation/build-an-atom

Building Atoms Summary Changing the number of protons was the only thing that changed the element. The number of protons in the nucleus determines the element Changing the ratio of protons to neutrons influences the stability of the nucleus. Changing the ratio of protons and electrons causes an overall charge to form on the atom. More protons → + More electrons → -

Why is it that certain combinations of nucleons are stable in a nucleus while others are not? A complete answer to this question cannot yet be given, largely because the exact nature of the forces holding the nucleons together is still only partially understood The ratio between the neutrons and protons appears critical.

Columns = Groups (1-18) Rows = Periods (1-7) Know Your Group Names! - Alkali Metals - P-block Metals/Non-metals - Halogens - Semi-Metals - Alkaline Earth Metals - Transition Metals - Noble Gases

How to Read the Periodic Table Atomic Number : # of protons Element Symbol: Unique abbreviation for that element Atomic Mass: Total number of protons and neutrons