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Introduction to the Atom and Atomic Models Atomic - Molecular Theory of Matter The Atomic - Molecular Theory of Matter states that all matter is composed of small, fast moving particles called atoms. These atoms can join together to form molecules. This theory is really thousands of individual ideas and models that provide evidence for the whole theory. Matter Since the atom is too small to be seen even with the most powerful microscopes, it or not this is a scientists rely upon Believe microscope. Even with the models to help us to world’s best microscopes we cannot see the structure or behavior of the atom. understand the atom. DEMOCRITUS 400 BC Democritus believed all things consisted of tiny indivisible units. He called these tiny units he called atomos. The Greek word for “can not be cut” or “indivisible” Ancient philosopher: Father of the Atom John Dalton (1799) Developed what is considered to be the 1st Atomic Theory Was born into a modest Quaker family in England Began lecturing in public at the age of 12 Dalton’s Model (1799) • Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were. Dalton’s Model Dalton’s model of the atom was similar to a tiny billiard ball. Dalton’s model of the atom was solid and had no internal structure. Dalton’s Atomic Theory • elements consisted of tiny particles called atoms. 1. all atoms of an element are identical 2. atoms of each element are different from one another; they have different masses. 3. compounds consisted of atoms of different elements combined together. 4. chemical reactions involved the rearrangement of combinations of those atoms. Flaws in Dalton’s Model • Dalton’s falsely believed that the atom was the most fundamental particle. – We now know the atom is made up of even smaller particles we call the proton, neutron and electron. • Dalton’s theory could also not account for the formation of ions (charged particles) Daltons Atomic Model Summary • Called: Billiard Ball Model • Could account for – Atoms of different element have different atomic masses – Elements consists of tiny particles called atoms • Could NOT account for – Atom being the smallest particle – Atoms having an internal structure – Atoms having charged particles John J. Thomson (1897) • Discovered the electron using the Cathod Ray Tube (CRT) • Thomson found that the beam of charge in the CRT was attracted to the positive end of a magnet and repelled by the negative end. Thomson’s Hypothesis • Concluded that the cathode beam was a stream of negative particles (electrons). • He tested several cathode materials and found that all of them produced the same result. • He also found that the charge to mass ratio was the same for all electrons regardless of the material used in the cathode or the gas in the tube. • Thomson concluded that electrons must be part of all atoms. Thomson’s atomic model • Called the “plum-pudding” it was the most popular and most wildly accepted model of the time. Thompsons atomic model could account for….. • the atom having an internal structure • Light given off by atoms • Atom with different atomic masses Thompsons atomic model could NOT account for….. • Empty space (had atom filled with positive pudding) • Formation of ions Gold Foil Experiment • Conducted by students of Rutherford. • Proved that all atoms had a tiny, positively charged center. • Confirmed that atom’s were mostly empty space. Rutherford ~ early 1900s • α-particle interaction with matter studied in gold foil experiment Rutherford's Nuclear Model • 1. The atom contains a tiny dense center – the volume is about 1/10 trillionth the volume of the atom • 2. The nucleus is essentially the entire mass of the atom • 3. The nucleus is positively charged – the amount of positive charge of the nucleus balances the negative charge of the electrons • 4. The electrons move around in the empty space of the atom surrounding the nucleus Rutherford’s atomic model (1911) • Could account for: – Empty space – Ions – Internal structure – Light given off when heated to high temperature. • Could not account for: – Stability Planetary model • Planetary model used to explain electrons moving around the tiny, but dense nucleus • Nucleus contains – Protons- existence proposed in 1900s – Neutrons- existence proposed in 1930s Bohr • Questioned ‘planetary model’ of atom – Electrons located in specific levels from nucleus (discontinuous model) • Proposed electron cloud model based on evidence collected with H emission spectra Bohr’s Atomic Model (1913) • Bohr was a student of Rutherford. • Improved Rutherford’s model by proposing electrons are found only in specific fixed orbits. • These orbits have fixed levels of energy • This explained how electrons could give off light (gain or lose energy) BOHR MODEL • Electrons are placed in energy levels surrounding the nucleus 8e8eNucleus (p+ & n0) 2e- Bohr’s Atomic Model • Could account for – Internal structure – Atoms of different masses – Atom being mostly empty space – Light given off – Formation of positive ions • Flaws – Only really worked for Hydrogen Chadwick (1932) • Discovered the neutron by bombarding Be with beta radiation. • Nuclear fission released a neutron. chart Review • Describe each of the 6 different atomic models. Give the – Scientist Name – Name of model – What they could account for – What they could not account for (flaws) Subatomic particle summary Particle Discovery by Year Proton Rutherford 1911 Gold Foil Experiment Electron Thompson 1897 The response of cathode ray tube to a magnetic and electric fields 1932 Bombarded Be with beta radiation and a neutron was released Neutrons Chadwich experiment Subatomic Particles Name Relative Symbol Charge mass Actual mass (g) Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0 0 1 1.67 x 10-24 Subatomic Particles (cont.) • All atoms of an element have the same # of protons protons identify an atom atomic # • Atoms are electrically neutral #p = #e- • Only neutrons and protons contribute to an atoms mass #n + #p = atomic mass Subatomic Particles ATOM ATOM NUCLEUS NUCLEUS ELECTRONS ELECTRONS PROTONS PROTONS NEUTRONS NEUTRONS POSITIVE CHARGE NEUTRAL CHARGE NEGATIVE CHARGE NEGATIVE CHARGE equal in a Atomic Most Number of the atom’s mass. neutral atom equals the # of... QUARKS ISOTOPES = atoms with the same number of protons but DIFFERENT numbers of neutrons Atomic Number Mass Number Element Symbol Ex. Na-23 or Sodium-23 C-14 or Carbon-14 F-19 or B. Isotopes • Atoms of the same element with different mass numbers. Nuclear symbol: Mass # Atomic # 12 6 Hyphen notation: carbon-12 C. Johannesson C B. Isotopes C. Johannesson © Addison-Wesley Publishing Company, Inc. B. Isotopes • Chlorine-37 – atomic #: – mass #: – # of protons: – # of electrons: – # of neutrons: 17 37 17 17 20 C. Johannesson 37 17 Cl C. Relative Atomic Mass • 12C atom = 1.992 × 10-23 g atomic mass unit (amu) 1 amu = 1/12 the mass of a 12C atom 1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu C. Johannesson © Addison-Wesley Publishing Company, Inc. D. Average Atomic Mass • weighted average of all isotopes • on the Periodic Table • round to 2 decimal places Avg. Atomic Mass (mass)(% ) (mass )(% ) 100 C. Johannesson D. Average Atomic Mass • EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Avg. (16)(99.76 ) (17)(0.04) (18)(0.20) 16.00 Atomic 100 amu Mass D. Average Atomic Mass • EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Avg. Atomic Mass (35)(8) (37)(2) 35.40 amu 10 C. Johannesson