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The Structure of the Atom Chapter 4 The Beginning… Democritus- Greek Philosopher (460-370 BC) Came up with the term “atomos”- now called “atoms” Believed that atoms could not be created, destroyed, or further divided Matter is composed of empty space through which atoms move Atoms are solid, homogenous, indestructible, and indivisible Different kinds of atoms have different sizes and shapes The different properties of matter are due to size, shape and the movement of atoms Aristotle- Greek Philosopher (384-322 B.C.) One of the most influential philosophers Wrote extensively on many subjects Rejected Democritus’ idea that the “nothingness” of empty space could exist His denial went largely unchallenged for two thousand years! Dalton (Schoolteacher- England) (1766-1844) Atomic Theory 1. 2. 3. 4. 5. All matter is made up of atoms. Atoms are indestructible and cannot be divided into smaller particles. All atoms of one element are exactly alike, but they are different from atoms of other elements. Different atoms combine in simple whole number ratios to form compounds In a chemical reaction, atoms are separated, combined, or rearranged. Dalton’s atomic theory explains the conservation of mass when a compound forms from its component elements. Subatomic Particles and the Nuclear Atom Discovering the Electron Sir William Crookes- English Physicist Saw a flash of light within one of the vacuum tubes with metal electrodes at opposite ends Accidental discovery of the cathode ray (ray of radiation originating from the cathode end of the tube) ELECTRONS!! Scientists continued their research using cathode ray tubes, and by the end of the 1800’s were fairly convinced of the following: 1. 2. Cathode rays were actually a stream of charged particles The particles carried a negative charge. (exact value of the negative charge was not known) JJ Thompson Series of experiments to determine the ratio of its charge to mass He concluded that the mass of the charged particles (electrons) was much less than that of a hydrogen atom (lightest known atom) SHOCKING!- this meant Dalton was WRONG and atoms were divisible into smaller subatomic particles Proposed a model of the atom that came known as the plum pudding model. JJ Thomson Robert Millikan Determined the charge of an electron His experimental set up and technique was accurate to within 1% of the currently accepted value. Oil Drop Experiment Ernest Rutherford Studied how positively charged alpha particles interacted with solid matter Thompson’s “Plum Pudding ” Model Rutherford’s results when alpha particles where beamed through foil Rutherford He expected most of the fast moving and relatively massive particles to pass straight through the gold atoms He concluded: an atom consisted mostly of empty space through which atoms move There was a tiny, dense region centrally located within the atom that contained all of an atom’s positive charge and virtually all of its mass (aka NUCLEUS) Eight years later…. (1920) Rutherford refined his concept of the nucleus It contained positively charged particles called PROTONS Few years later… James Chadwick (Rutherford’s coworker) Showed that the nucleus also contained another subatomic particle- a neutral particle called a NEUTRON The neutron has a mass equal to that of a proton How Atoms Differ Section 4.3 Atomic Number and Masses Atomic Number: the number of PROTONS in the nucleus of an atom of an element Also tells you the number of ELECTRONS since elements have no over all charge. The number of PROTONS determines the IDENTITY of the element Isotopes Have different mass numbers because they have a different amount of NEUTRONS *If they had a different amount of PROTONS, they would be a different element completely* Because isotopes have a different number of NEUTRONS, they also have a different mass number Mass Number = Protons + Neutrons Sodium Isotopes Example: NEON Atomic Number:10 Mass Number: 20.180 # of Protons: 10 # of Electrons: 10 # of Neutrons: 10 Mass of Individual Atoms Atomic Mass Unit (amu) is defined as 1/12 the mass of a carbon-12 atom. The atomic mass of an element is the weighted average mass of the isotopes of that element. 75% 25% Average Atomic Mass To calculate the atomic mass of an element 1. Multiply the mass of each isotope by its natural abundance % (expressed as a decimal) 2. Add the products Unstable Nuclei and Radioactive Decay Radioactivity Nuclear Reactions- involve a change in an atom’s nucleus (unlike most chemical reactions) Radioactivity- a process in which substances spontaneously emit radiation Radiation- the rays and particles emitted by the radioactive material Radioactive atoms undergo significant changes that can alter their identities One element can change into atoms of another element Unstable nuclei lose energy by emitting radiation in a spontaneous process (aka radioactive decay) They continue to do this until they form a stable nonradioactive atoms Types of Radiation Alpha Radiation Deflects towards a negatively charged plate Alpha Particles- 2 protons and 2 neutrons (+2 charge) Beta Radiation Radiation that is deflected towards a positively charged plate Consists of fast moving electrons called beta particles Gamma Radiation Gamma Rays are high-energy radiation that possess no mass Because gamma rays are massless, the emission of gamma rays by themselves cannot result in the formation of a new atom