Atomic Structure PDF

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...

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 - +  Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end  Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end  Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end  Thomson’s Experiment Voltage source - + 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 + - By adding an electric field  Thomson’s Experiment Voltage source + - By adding an electric field  Thomson’s Experiment Voltage source + - By adding an electric field  Thomson’s Experiment Voltage source + - By adding an electric field  Thomson’s Experiment Voltage source + - By adding an electric field  Thomson’s Experiment Voltage source + - - Adding an electric field cause the beam to move toward the positive plate. Thomson concluded the beam was made of negative moving pieces. Thomson’s “Plum Pudding” Atom Model 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.  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. Rutherford’s Atomic Model James Chadwick’s Contribution to the Atomic Theory ►English physicist ►proved the existence of a third subatomic particle- the neutron- a subatomic particle roughly the same mass as that of a proton but electrically neutral.  Chadwick’s Contribution to the Atomic Theory ►Nucleons - The protons and neutrons, being located in the nucleus of the atom - with their collective mass comprising the significant mass of any a given atom.  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 Table 1. Subatomic Particles and their Properties Charge, Mass, Particle Symbol coulombs gram Electron e- -1.602 x 10-19 9.108 x 10-28 Proton p+ 1.602 x 10-19 1.673 x 10-24 Neutron n 0 675.10-24  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  Counting Atoms ►Examples of Isotopes  ISOTOPES ►Aside from difference in mass, stability, and possible applications ►vary their abundance on Earth ►Hydrogen, for example is more abundant than deuterium and tritium while Carbon-12 is most abundant among all other carbon isotopes.  ISOTOPES ►But, how do you compute for the average mass? Here is the equation for computation. Average atomic mass= mass1 x % of abundance1 + mass2 x % of abundance2 + …………….  ISOTOPES ► Sample Problem: Carbon has two stable isotopes, carbon-12 and carbon-13. Carbon-12 is more abundant than carbon-13. Carbon-12 contributes 98.90% mass of the whole carbon atoms while carbon-13 contributes 1.10%. If the atomic weight of the carbon-12 is 12.0000 atomic mass unit (amu) and carbon-13 has an atomic weight of 13.0034 amu, compute for the average atomic mass of carbon.  ISOTOPES ► Solution: Average atomic mass= mass1 x % of abundance1 + mass2 x % of abundance2 = 12.0000 amu (98.90%) + 13.0034 amu (1.10%) = 12.01 amu  Counting Atoms ▪ Nuclear Symbol Notation  Atoms ► Protonshave 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