- Chapter 2: Atoms and Elements - The Law of Conservation of Mass o Antoine Lavoisier, 1789o In a chemical reaction, matter is neither created nor destroyedo 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 1754-1826o 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 1766-1844o When two elements form two differentcompounds, the masses of B thatcombine with 1 g of A can be expressedas a ratio of small, whole numbers- Oxides of Carbon o Carbon combines with oxygen to formtwo different compounds, carbonmonoxide and dioxide o Carbon dioxide contains 2.67 g ofoxygen for every 1.00 g of carbono Carbon monoxide contains 1.33 g ofoxygen for every 1.00 g of carbono 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 ithaving ultimate, indivisible particles to explainthese lawso Each element is composed of tiny, indestructibleparticles called atomso All atoms of a given element have the same massand other properties that distinguish them fromatoms of other elementso Atoms combine in simple, whole-number ratiosto form molecules of compoundso In a chemical reaction, atoms of one elementcannot 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 electronso 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 rayso These energetic particles could penetrate mattero 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 energyo But not so small that electrons will affect ito 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 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 atomo The nucleus is positively charged The amount of positive charge balances the negative charge of the electronso The electrons are dispersed in the empty space of the atom surrounding the nucleus- Nucleus Summary 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 protonso Protons are subatomic particles found in the nucleus with a charge = +1.60 x 10^19 C and a mass = 1.67262 x 10 ^ -24o Because protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electronso 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 neutronso 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 seeno 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 Ao Number of Protons (p) + number of neutrons (n)o C – chemical Symbolo Atomic number = Z = number of protons (p) = number of electrons = number of position in Periodic Table. Atomic number is unchangeableo 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 o Soddy discovered that the same element could have atoms with two different masses, which he called Isotopes.o Dalton’s Theory There are two isotopes of chlorine found in nature, one that has a mass of about35 amu and another that weighs about 37 amu The atomic mass of chlorine is 35.45 amuo Natural abundance – Relative amount found in sample- Charged Atoms o When atoms gain or lose electrons, they acquire a chargeo Ions – Charged atoms or groups of atomso Anion – When atoms gain electrons, they become negatively charged ionso 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