Atomic Structure Democritus 460 BC Greek Philosopher Suggested

Atomic Structure

Democritus 460 BC ► Greek Philosopher ► Suggested world was made of two things – empty space and “atomos” § Atomos – Greek word for uncuttable ► 2 Main ideas § Atoms are the smallest possible particle of matter § There are different types of atoms for each material

2 John Dalton’s Atomic Theory 1804 1. All matter is made of atoms. 2. Atoms of one element are all the same. 3. Atoms cannot be broken down into smaller parts 4. Compounds form by combining atoms

Dalton’s Early Atomic Model ► “Billiard Ball” model ► he envisioned atoms as solid, hard spheres, like billiard(pool) balls, so he used wooden balls to model them

J. J. Thomson 1897 Discovered the electron He was the first scientist to show the atom was made of even smaller things

JJ Thomson ► Used the Cathode ray tube to discover electrons

Thomson’s Experiment - Voltage source Vacuum tube Metal Disks +

Thomson’s Experiment - Voltage source +

Thomson’s Experiment - Voltage source + n Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source ►By adding an electric field

Thomson’s Experiment Voltage source + n By adding an electric field

Thomson’s Experiment Voltage source + - - Adding an electric field cause the beam to move toward the positive plate. n Thomson concluded the beam was made of negative moving pieces. n

Thomson’s “Plum Pudding” Atom Model

Democritus, Dalton, Thomson Video

Eugen Goldstein 1850 -1930 Using a cathode ray tube he discovered canal rays which are beams of positively charged particles. He is credited with the discovery of protons in an atom. Canal Rays discovery

Millikan’s Oil Drop Experiment • Determined the charge on an electron • Used Thomson’s charge to mass ratio to calculate the mass of an electron Millikan’s Oil Drop Experiment – another video

Ernest Rutherford - 1913 • discovered the nucleus of a gold atom with his “gold foil” experiment

Using J. J Thomson’s Plum Pudding atomic model, Rutherford predicted the alpha particles would pass straight though the gold foil. That’s not what happened.

Gold Foil Experiment Results most alpha particles go straight through the gold foil A few alpha particles are sharply deflected

Rutherford’s Conclusion ►The atom is mostly empty space. ►There is a small, dense center with a positive charge. ►Rutherford discovered the nucleus in atoms

Rutherford’s Contribution to the Atomic Theory ►The atom is mostly empty space. ►The nucleus is a small, dense core with a positive charge.

Gold Foil video

Rutherford’s Atomic Model

Structure of the Atom ► Atom – smallest particle of an element that can exist alone § Two regions of an atom ►Nucleus § Center of atom § Protons and neutrons ►Electron “cloud” § Area surrounding nucleus containing electrons

Structure of the Atom ► Proton – Positive charge (+), 1 atomic mass unit (amu); found in the nucleus § amu -Approximate mass of a proton or a neutron ► Neutron – Neutral charge (0), 1 amu; found in the nucleus ► Electron – Negative charge (-), mass is VERY small

Counting Atoms ► Atomic Number § Number of protons in nucleus § The number of protons determines identity of the element!! ► Mass Number (Atomic Mass) § Number of protons + neutrons § Units are g/mol

Counting Atoms ► Isotopes § Atoms of the same element with varying number of neutrons § Different isotopes have different mass numbers because the number of neutrons is different

Composition of the atom video Start at 3: 25

Counting Atoms § Nuclear Symbol Notation

Atoms ► Protons have a positive (+) charge and electrons have a negative (-) charge ► In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero (0) § Example/ Helium, with an atomic number of 2, has 2 protons and 2 electrons when stable

Ions ► In a neutral atom § Atomic number = # of protons = #of electrons ► Sometimes atoms will gain or lose electrons and form IONS ► Because an electron has a negative charge: § When an atom GAINS electrons it becomes NEGATIVE § When an atom LOSES electrons it becomes POSITIVE

Ions Cation = a positive ion Anion = a negative ion

Let’s Practice ► Aluminum (Al) (no periodic table) § Protons = 13 § Electrons = § Neutrons = 14 § Atomic Number = § Atomic Mass =

Let’s Practice w/ nuclear symbol notation ► Nuclear Symbol notation (no periodic table) § Protons = § Electrons = § Neutrons = § Atomic Number = § Atomic Mass = Ag 108 47

Let’s Practice w/ isotopes ► use the periodic table § Protons = § Electrons = § Neutrons = § Atomic Number = § Atomic Mass = Uranium-235

Let’s Practice with Ions Use the periodic table § Charge = § Protons = § Electrons = § Neutrons = § Atomic Number = § Atomic Mass = 39 K 1+