Kevin Lockwood Chemistry 1211K Dr Stroeva 220 Urban Life TR 4 00 5 15 PM January 27 2014 Chapter 2 Atoms and Elements The Law of Conservation of Mass o Antoine Lavoisier 1789 o In a chemical reaction matter is neither created nor destroyed o Because only whole atoms combine and atoms are not changed or destroyed in the process the mass of the compound made must equal the total mass of the atoms that combine together Law of Definite Proportions o Joseph Proust 1754 1826 o All samples of a given compound regardless of their source or how they were prepared have the same proportions of their constituent elements Law of Multiple Proportions o John Dalton 1766 1844 o When two elements form two different compounds the masses of B that combine with 1 g of A can be expressed as a ratio of small whole numbers Oxides of Carbon o Carbon combines with oxygen to form two different compounds carbon monoxide and dioxide o Carbon dioxide contains 2 67 g of oxygen for every 1 00 g of carbon o Carbon monoxide contains 1 33 g of oxygen for every 1 00 g of carbon o Because there are twice as many oxygen atoms per carbon atom in carbon dioxide of in carbon monoxide the oxygen mass ratio should be 2 Dalton s Atomic Theory o Salton proposed a theory of matter based on it having ultimate indivisible particles to explain these laws o Each element is composed of tiny indestructible particles called atoms o All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements o Atoms combine in simple whole number ratios to form molecules of compounds o In a chemical reaction atoms of one element cannot change into atoms of another element They simply rearrange the way they are attached Charge o Two kinds of Charges Positive Negative o Opposite charges attract attracted to o Like charged repel repels repels o To be neutral something must have no charge or equal amounts of opposite charges Major Steps of Development Modern Atomic Theory o The discovery of the electron in the late 1800s J J Thomson had discovered the electron a negatively charged low mass particle present within all atoms It also measured the charge to mass ratio of the particles within electron beam which is 1 76 x 10 8 coulombs C g o In 1909 Robert Melikan measured charge of electron oil drop experiment and based on Thomson s result calculate mass of electron as Thomson s Plum Pudding Atom o The structure of the atoms contains many negatively charged electrons o These elections are held in the atom by their attraction for a positively charged electric field within the atom There had to be a source of positive charge because the atom is neutral Thomson assumed there were no positively charged pieces Electrons are the only particles in the Plum Pudding atoms therefore the only source of mass Radioactivity o In the late 1800s Henri Becquerel and Marie Curie discovered that certain elements would constantly emit small energetic particles and rays o These energetic particles could penetrate matter o Ernest Rutherford Discovered that there were three different kinds of emissions Rutherford s Experiment o How can you prove something is empty space Put something through it Use large target atoms Au atoms o Use very thin sheets of target so it will no absorb bullet Use very small particle as bullet with very high energy o But not so small that electrons will affect it o Bullet alpha particles target atoms gold foil Rutherford s Interpretation The Nuclear Model o The atom of the Nucleus contains a tiny dense center called the nucleus Nucleus Summary The amount of space taken by the nucleus is only about 1 10 trillionth the volume of the atom o The nucleus has essentially the entire mass of the atom The electrons weigh so little they give practically no mass to the atom o The nucleus is positively charged The amount of positive charge balances the negative charge of the electrons o The electrons are dispersed in the empty space of the atom surrounding the nucleus o Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign these particles are called protons o Protons are subatomic particles found in the nucleus with a charge 1 60 x 10 19 C and a mass 1 67262 x 10 24 o Because protons and electrons have the same amount of charge for the atom to be neutral there must be equal numbers of protons and electrons o Neutrons are subatomic particles with a mass 1 67493 x 10 24 g and no charge found in the nucleus Atomic Mass Unit amu o Protons and neutrons have nearly identical masses A more common unit to express these masses is the atomic mass unit amu defined as 1 12 the mass of a carbon atom containing six proton and six neutrons o Expressed in this unit the mass of proton mass of neutron 1 amu Why Does Matter Appear Continuous if the Atom is Mostly Empty Space o The emptiness of the atom is on such a small scale that the variation in density cannot be seen o Most of our macroscopic observations involve particles colliding that are so much larger than this scale that the particles appear solid instead of mostly empty Atomic Representation in Periodic Table o Mass number is whole number A o Number of Protons p number of o C chemical Symbol o Atomic number Z number of protons p number of electrons number of position neutrons n in Periodic Table Atomic number is unchangeable o Each element is defined by a unique atomic number Z the number of protons in the nucleus of every atom of that element Structure of the Nucleus which he called Isotopes o Dalton s Theory o Soddy discovered that the same element could have atoms with two different masses There are two isotopes of chlorine found in nature one that has a mass of about 35 amu and another that weighs about 37 amu The atomic mass of chlorine is 35 45 amu o Natural abundance Relative amount found in sample Charged Atoms o When atoms gain or lose electrons they acquire a charge o Ions Charged atoms or groups of atoms o Anion When atoms gain electrons they become negatively charged ions o Cation When atoms lose electrons they become positively charged ions Ions and Compounds o Ions behave much differently than the neutral atoms The metal sodium made of neutral Na atoms is highly reactive and quite unstable however the sodium cations Na found in table salt are very non reactive and stable o Because materials such as table salt are neutral there must be equal amounts of charge from cations and anions