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Properties of Atoms and the Introduction to the Periodic Table Structure of the Atom • Elements are abbreviated in scientific shorthand – first letter or two of element’s name • Atom – smallest piece of matter that still has the properties of the element Hydrogen 1 1 H Protons: 1 Neutrons: 0 Electrons: 1 Atom • • • • Protons have electrical charge of 1+ Neutrons do not have an electrical charge Electrons have electrical charge of 1Protons and neutrons are in the nucleus of an atom; electrons surround the nucleus Example: Lithium atom Lithium has: • 3 protons (red) •4 neutrons (blue) •3 electrons (black) Protons and neutrons are made up of smaller particles called quarks • • Quarks are studied by colliding accelerated charged particles with protons, which leave tracks in a bubble chamber Six quarks are known to exist; the sixth is called the top quark Aristotle • Said all matter consisted of four elements: • • • • Earth Water Air Fire Democritus (Greek) 400BC was first to propose an Atomic Theory • Believed all matter was composed of small indivisible & indestructible particles he called atoms, from Greek word atomos. − Not based on experimentation John Dalton (early 1800’s) – atomic theory based on experimentation • All elements are composed of atoms • Atoms of the same element are identical, atoms of different elements are different • Atoms of different elements can join together in single whole number ratios to form compounds • Chemical reactions occur when atoms are separated, joined or rearranged • Here are some of the symbols Dalton used for atoms of elements and molecules of compounds. He probably used a circle for each because, like the ancient Greeks, he thought of atoms as tiny, round hard spheres. J.J. Thomson (English) 1897 • Discovered electrons using a cathode ray tube. • Sealed tube with vacuum inside and electrodes on each end. When connected to electric current, a beam traveled from cathode to anode. He called the beam “cathode rays.” They were electrons. Thomson’s Cathode Ray Tube Thomson cont. • Proposed “Plum Pudding” model of the atom. • Was a field of evenly distributed positive and negative charge. The 'Plum Pudding' Model of an Atom Rutherford’s Experiments: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the center of the atom, otherwise the heavy alpha particles fired at foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (center of the atom). Compare Thomson’s and Rutherford’s models: Niels Bohr (early 1900’s) The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The nucleus is the centre of an atom, containing protons and neutrons. The energy must be given out when 'excited' electrons fall from a high energy level to a low one. Bohr Atom Model Scientists use scaled-up models to represent atoms. • Early models of atoms used a solid sphere. • Current quantum mechanical model shows electrons traveling in specific energy levels around a nucleus of protons and neutrons Where are protons, neutrons and electrons? • Protons and neutrons are in the center nucleus surrounded by electrons traveling in specific energy levels. Electron Orbitals Masses of Atoms Atomic Mass – composed mostly of the protons and neutrons in the nucleus • Unit of measurement for atomic particles is atomic mass unit (amu) which is one-twelfth the mass of a carbon atom containing six protons and six neutrons. • Atomic number – the number of protons in an atom; number of protons also identifies the element • The sum of the number of protons and neutrons in the nucleus of an atom is the mass number. How many protons, neutrons and electrons are in a neutral atom? Element H Atomic # I Pb Protons 1 O K Mass # Neutrons Electrons 0 16 8 19 20 127 53 82 125 • Isotopes – atoms of the same element with different numbers of neutrons • • • Different isotopes have different properties Numbers of neutrons is equal to mass number minus atomic number Name of element followed by mass number identifies the isotope Isotopes Examples: • Average atomic mass is the weighted-average mass of an element’s isotopes • Average atomic mass is closest to its most abundant isotope How do atomic number and mass number differ? • Atomic number is the number of protons in an atom; mass number is the number of protons and neutrons in an atom. The Periodic Table • Elements are organized in the periodic table by increasing atomic number. • • In the late 1800’s, Dmitri Mendeleev devised the first periodic table based on atomic mass. In 1913, Henry Moseley arranged the elements by atomic number rather than atomic mass. The Periodic Table • Vertical columns in the periodic table are groups of elements with similar properties. • Elements in the same group have the same number of electrons in their outer energy level The Periodic Table • Each of the seven energy levels can have a maximum number of electrons. • • Energy level one can contain at most two electrons Energy level two can contain at most eight electrons • More on this later! (MUCH MORE!) The Periodic Table • Rows are called periods • Each row in the periodic table ends when an outer energy level is filled • Electron dot diagrams use the element symbol and dots to represent outer energy level electrons. Molecular Formula CH4 Electron Dot Diagram H ·· H : C : H ·· H Structural Formula H | H - C - H | H The Periodic Table • Periods – horizontal rows of elements that contain increasing numbers of protons and electrons. • • Elements are classified as metals, nonmetals, or metalloids Elements are synthesized in laboratories all over the world The Periodic Table • The same elements exist all over the universe • • Hydrogen and helium are the building blocks of other naturally occurring elements Supernovas spread heavier elements throughout the universe What do elements in the same group have in common? • The same number of electrons in the outer energy level Electromagnetic Radiation (Light) as a Key to Understanding Electron Paths • Early scientists discovered that Electromagnetic radiation (light) is given off by atoms of an element when they have been excited by some form of energy • Furthermore, atoms of different elements give off different colors of light when they are excited. When salts containing Li, Cu, and Na dissolved in methyl alcohol are set on fire, brilliant colors result. Cu Li Na Spectral Analysis of Emitted Light from Excited Atoms • When the emitted light from excited atoms was passed through a prism, a curious spectrum of discrete lines of separate colors, separate energies, was observed rather than a continuous spectrum of ROY G BIV. • Furthermore, different elements show totally different line spectra. • In fact, line spectra are used to identify the presence of different elements The Continuous Spectrum Continuous Spectrum Atomic Line Spectrum Na Interpretation of Atomic Spectra • The line spectrum must be related to energy transitions in the atom. • Absorption = atom gaining energy • Emission = atom releasing energy • Since all samples of an element give the exact same pattern of lines, every atom of that element must have only certain, identical energy states • The energy of an atom is quantized – limited to discrete values • If the atom could have all possible energies, then the result would be a continuous spectrum instead of lines Interpretation of Atomic Line Spectra in terms of Electron Paths • Electrons may be thought of as traveling in concentric shells or energy levels about the nucleus. • The energy of the shells increase as one proceeds away from the nucleus. • When an atom absorbs energy, electrons are promoted from an inner, low energy, shell to an outer, higher energy shell. • Conversely, when an excited atom emits energy, electrons drop down from an excited outer, higher energy, shells to an inner, lower energy, shells An excited lithium atom emitting a photon of red light to drop to a lower energy state. A sample of H atoms receives energy from an external source The excited atoms (H) can release the excess energy by emitting photons. When an excited H atom returns to a lower energy level, it emits a photon that contains the energy released by the atom. When excited hydrogen atoms return to their lowest energy state, the ground state, they emit photons of certain energies, and thus certain colors. • TEST OVER ALL OF THIS INFOMRATION TOMORROW! • JUST KIDDING