Elements, Atoms and Ions (Ch. 3) 3.1 The Elements The alchemists tried to create gold from other metals, but failed. They discovered new elements (mercury, sulfur) and purified acids. The Elements The most abundant element in the earths crust and atmosphere is oxygen. Oxygen is also the most abundant element in the human body. Oxygen is found as An element (O 2 ) in the air A compound in water and in rocks. The elements The names of elements came from Latin and Greek places famous scientists. Chlorine = color in Greek Californium discovered in California. Element symbols are abbreviations for the elements. C = carbon, F = fluorine Practice questions p55 3.2 John Dalton (1808) First true scientist to study atoms The law of constant composition a compound always has the same proportion of elements. In water, there is always 8g Oxygen : 1g Hydrogen Daltons Atomic Theory 1.All elements are made of atoms atoms are indestructible 2.All atoms of an element are the same 3.Compounds are made by combining 2 or more different kinds of atoms 4.In chemical reactions, atoms just rearrange to make new chemicals. Formulas Formulas show how many atoms are in a compound Example: 4H 2 SO 4 Subscript Tells us how many of the atom there are. Only for Atom directly to its left 3.3 Joseph John (J.J.) Thomson (1897) Thomsons experiment J.J. Thomson pumped electricity through a glass tube. Cathode rays shoot out from the metal cathode. Observation 1: The glowing ray was repelled by a negatively charged plate Conclusion: Rays are negatively charged particles. Observation 2: Any metal could be used as as electrode with the same result. Conclusion: all elements must have these particles. Electrons are small, negatively charged particles. Plum Pudding Model JJ Thomson and William Thomson thought that electrons were stuck in a positively charged ball. Ernest Rutherford (1911) Gold foil experiment Setup Gold foil experiment -particles ( = alpha) are relatively massive (heavy) and positively (+) charged Hypothesis: If an atom had a spread out positive charge, then -particles could go straight through the atom. Planetary model Observation Most alpha rays passed straight through Some pushed away Some bounced straight back Conclusion Most of the atom is empty space Small area of positive charge Small area of huge mass (high density) Discovered the Nucleus Gold foil experiment on the atomic level Nucleus is really small If we built a scale model of an atom, with a baseball as the nucleus, how far away would the outer edge of the atom be located? The Nuclear Atom The dense, positive center is called the nucleus. The nucleus contains protons and neutrons. In atoms, # protons = # electrons (# is my symbol for number) Nucleus The Nuclear Atom ProtonsNeutronsElectrons MassHigh Very small (1/1836 th of a proton) Charge+10 Isotopes Isotopes are atoms with the same number of protons but different numbers of neutrons. Example: Carbon can have 6 neutrons or 7 neutrons. The atomic number = # protons The mass number = # protons + neutrons Isotopes A = mass number Z = atomic number Mass number = # protons + # neutrons Electrons have very little mass, so we ignore them. Isotope Notation Mass number Atomic number Charge Element Symbol Practice problems p67 #3, 4, 7 3.4 Using the Periodic Table The periodic table shows the atomic number above the symbol of each element. What is the atomic number of carbon? Yes, 6 The columns (up and down) are called groups. They have similar chemical properties. 1A is called group 1. Group 1 23 Periodic Table Etc Groups 1A is called group 1. Group 1: alkali metals Group 2: alkaline earth metals Group 7: halogens Group 8: Noble gases metals, metalloids and non-metals Stair-step line Metals to left Non-metals to Right Metalloids Touch Stair-Step Groups Metals are: Good conduction of electricity Malleable (can be hammered flat) Ductile (can be pulled into wires) Lustrous (shiny) Nonmetals are: Usually poor conductors. Brittle (break easily). Dull (not shiny) appearance. Metalloids have properties of both metals and nonmetals. Groups Some elements exist in the pure form. Ex: gold (Au). Most elements exist in a combined form, not the pure form. Ex.: Iron is found as Iron Oxide Fe 2 O Atoms in group 8 (the noble gases) usually do not combine with other elements. Groups Some gases are diatomic molecules, which have two atoms. Ex: O 2 Remember: HOFBrINCl (H, O, F, Br, I, N and Cl form diatomic molecules). Gas, liquid or solid? H, N, O, F, Cl and group 8A are gases at room temperature (25C). Bromine (Br 2 ) and mercury (Hg) are liquids at 25 C. All other elements are solid at 25C. 3.5 Ions Atoms are neutral. Example: sodium NameSymbol# protons# electrons SodiumNa11 3.5 Ions If sodium loses an electron, it will have a positive charge (+1) because there is one more proton than electrons. If chlorine gains an electron, it will have a negative charge (-1). NameSymbol# protons# electrons SodiumNa ChlorineCl 3.5 Ions Cations have a (+) positive charge. Anions have a (-) negative charge. If the charge is only 1+ or 1-, dont write 1. Ex: Na + 3.5 Ions Group1A2A3A6A7A Charge ExampleNa + Mg 2+ Al 3+ O 2- F-F- Some groups form one type of ion: Ionic compounds Ionic compounds contain a cation and an anion. The attraction between the positive and negative charges holds them together Ionic compounds have high boiling points. cannot conduct electricity as a solid. can conduct electricity when melted because the electrons are free to move around. 3.5 Ions Ionic compounds form when a cation (+) combines with an anion (-). Ex. Na + + Cl - NaCl Compounds will ONLY form if the overall charge is zero. 3.5 Ions What is the charge of a Mg ion? +2 What is the charge of a Cl ion? If we mixed a beaker Mg with a beaker of Cl, what would the formula of the compound? Mg 2+ + Cl - + Cl - MgCl 2 Charge: 2+ 2(1-) 0