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Chapter 18 Properties of Atoms and the Periodic Table Section 1-Structure of the Atom • Chemical Symbolsconsist of one capital letter or a capital letter plus one or two small letters ex: table 1—pg. 544 Atomic Components • Atoms—the smallest piece of matter that still retains the property of the element • Atoms: Contain protons and neutrons in the nucleus • Electrons—contained in electron cloud • Quarks—smaller particles that comprise protons and neutrons • Scientists have confirmed the existence of six uniquely different quarks • The search for the composition of protons and neutrons is an ongoing effort Models of the Atom • Democritus—”uncuttable” composed of tiny, solid particles that could not be subdivided—(atomos) **Dalton—solid sphere **Thomson Model—”cookie dough” charged particles were evenly embedded throughout a positively charged sphere (more models) • Rutherford Model— proposed almost all the mass of an atom (& + charges) were concentrated in a central nucleus surrounded by electrons • Bohr Model— – Hypothesized electrons traveled in fixed orbits Electron Cloud Model (Quantum Mechanical) • 1926—current model • Electron cloud—area around the nucleus of an atom where its electrons are most likely found • Energy levels are areas of the cloud where electrons are more likely 2 B found Section 2—Masses of Atoms • AMU—unit of measurement used for atomic particles • Mass of 1 proton or 1 neutron is almost 1 amu. • Atomic number—the number of protons in an atom • Mass number—the sum of the number of protons and the number of neutrons in the nucleus • Atomic mass = decimal number under the symbol on the periodic table • NOT THE SAME THING AS ATOMIC NUMBER • It is the average weight of all atoms of the element (including isotopes) • Mass number-does not account for isotopes Calculating neutron number • If you know the mass number and atomic number of an atom, you can find the number of neutrons as well,. • Neutron # = mass # atomic # Isotopes • Carbon-12 is the most common form of carbon • However, Carbon-14 is a radioactive form of carbon • WHY? • Not all the atoms of an element have the same number of neutrons ISOTOPES • Isotopes—atoms of the same element that have different numbers of neutrons • Average atomic mass—the weighted average mass of an element’s mixture of isotopes (used because most elements have more than one isotope) The Periodic Table • Dimitri Mendeleevarranged all the elements known in order of increasing atomic masses and discovered a pattern • Today’s Periodic Table— elements are arranged by increasing atomic number and by changes in physical and chemical properties • Mendeleev-left blank spaces to keep elements in line according to chemical properties • He predicted the existence of two elements not yet discovered Groups • The vertical columns in the periodic tablealso called families • Periodic Table—pg. 556 and 557 Electron Cloud Structure • In neutral atoms, the electron number = the proton number • SO: Atomic number tells how many protons AND how many electrons the element has • Elements in the same group have the same number of electrons in their outer energy levelcalled VALENCE ELECTRONS • This number determines properties of the elements Number of Atoms in Energy Levels • • • • Energy Level 1—holds 2 e Energy Level 2—holds 8 e Energy Level 3—holds 18 e Energy Level 4—holds 32 e • Octet Rule—explains that atoms are most stable with an outer valence holding 8 electrons Periods/Rows • Horizontal groups across the periodic table • Each row ends with an element with a full outer valence (8 electrons) • Periods increase by one proton and one electron going L to R Metals • Metals are on the lefthand side of the periodic table • Most are shiny, ductile, malleable and are good conductors • Ductile—drawn into wire • Malleable-can be hammered into sheets Non-metals • Non-metals are on the right side of the periodic table • Most are gases, brittle, and poor conductors Metalloids • Metalloids-run along the middle zig-zag line • Metalloids have some of the properties of both metals and non-metals • Carbon-14 has 6 protons and 8 neutrons Isotopes—atoms of the same element that have different numbers of neutrons Radioactive isotopes—those isotopes that are unstable and become radioactive Identifying Isotopes • Average atomic mass—the weighted-average mass of the mixture of its isotopes • Ex: 4 out of 5 atoms of B are boron-11 and 1 out of five is boron-10 • Weighted average = 4/5 (11) + 1/5(10) = 10.8 AMU Periodic Table websites: • • • • www.chemicool.com www.Ptable.com www.periodictable.com www.webelement.com