Chapter 2: The Components of Matter- Democritus – father of atomism. “If you cut an element into smaller and smaller pieces, eventually you will reach a particle of the element so small that it can no longer be cut.” (460-370 BC)- Aristotle – famous Western philosopher. Said it was impossible for “nothing” to exist andthe concept of atoms was therefore suppressed for 2,000 years.- Robert Boyle – 17th century English scientist, argued that by definition, an element is composed of “simple Bodies, not made up of any other Bodies, of which al are mixed Bodies are compounded, and into which they are ultimately resolved.”2.1 Elements, Compounds, and Mixtures: An Atomic Overview- Matter can be classified into three types based on its composition: elements, compounds, and mixtures.- Elements and compounds are kinds of substances – a matter whose composition is fixed.- Elements – simplest type of matter with unique physical and chemical properties.- Elements consist of only one type of atom. o Each element is unique because the properties of its atoms are unique.o Several elements ocurr in molecular form – a molecule is an independent structureof 2+ atoms bound together.- Compound – consist of 2+ different elements that are bonded chemically.o No matter what the compound, one defining feature is that the elements are present in fixed parts by mass (fixed mass ratio.)o This is so because each unit of the compound consists of a fixed number of atoms of each element.- Another defining feature of a compound is that its properties are different from the properties of its component elements. - A compound can be broken down into simpler substances – its component elements. Thisis a chemical change (the breakdown is), not a physical one.- Mixtures – consist of 2+ substances that are physically intermingled.o The components of a mixture can vary in their parts by mass.o A mixture retains many of the properties of its components.o A mixture can be separated into their components by physical changes.2.2 The Observations That Lead to An Atomic View of Matter- An atomic theory developed by John Dalton in the early 19th century explained mass laws(mass conservation, definite composition, and multiple proportions.)Mass Conservation- Law of mass conservation – total mass of substances doesn’t change during a chemical reaction.o Total amount of matter remains constant.o Matter cannot be created nor destroyed.  However, Albert Einstein proposed that the mass before and after a reaction is not exactly the same; some mass is converted to energy and vice versa, but the different is too small to measure on a balance.Definite Composition- Law of definite (or constant) composition – no matter what its source, a particular compound is composed of the same elements in the same parts (fractions by mass)- Fraction by mass – part of the compound’s mass that each element contributes.- Percent by mass – fraction by mass expressed as a percentageo In a compound, each element has a fixed mass fraction/percent.o Mass fraction is fixed no matter what the size of the sample.- We can use info to find mass of element in any sample of the compound.- Mass of element in sample = mass of compound in sample x (mass of element in compound/mass of compound)Multiple Proportions- Law of multiple proportions – if elements A and B react to form two compounds, the different masses of B that combine to form with a mixed mass of A can be expressed as aratio of small whole numbers.- The mass fraction of one element relative to the other element changes in increments based on the ratio of small whole numbers.2.3 Dalton’s Atomic Theory- John Dalton (1766 – 1844) presented his atomic theory of matter in “A New System of Chemical Philosophy.”Postulates of the Atomic Theory- 1. All matter consists of atoms – tiny indivisible particles of an element that cannot be created nor destroyed.- 2. Atoms of one element cannot be converted into atoms of another element.- 3. Atoms of an element are identical in mass and other properties and are different from atoms of any other element.- 4. Compounds result from the chemical combination of a specific ratio of atoms of different elements.2.4 The Observations That Led to the Nuclear Atom Model- Dalton’s model established that masses of reacting elements could be explained in terms of atoms but not why atoms bond as they do.Discovery of the Electron and Its Properties- To discover the nature of an electric current, investigators tried passing current through nearly evacuated glass tubes fitted with metal electrodes. A ray could be seen, striking theend of the tube. - The rays were called cathode rays, because they originated at the negative electrode (cathode) and moved to the positive electrode (anode)- Cathode rays consist of negatively charged particles found in all matter. - Cathode ray particles were later named electrons.Mass and Charge of the Electron- 2 classic experiments revealed the mass and charge of the electron.- 1.) Mass-charge ratio: JJ Thompson (1856-1940) measured the ratio of the mass of a cathode ray to its charge. Estimated the electron particle weighed less than 1/1000 as much as the lightest atom. Suggested that atoms contain even smaller particles but was ridiculed over this statement by the modern scientists of the time.- 2.) Robert Millikan (1868 – 1953) measured the charge of the electron.Discovery of the Atomic Nucleus- JJ Thompson proposed a plum pudding model.o A spherical atom composed of diffused, positively charged matter with electrons embedded like “raisins in plum pudding.”- Rutherford tested this model and concluded that o 1.) an atom is mostly space occupied by electronso 2.) in the center is what Rutherford called the nucleus, that contains all the positive charge and essentially all the mass of the atom.o 2.) proposed that positively charged particles lay within the nucleus, called them protons.- 1932, James Chadwick discovered the neutron, an uncharged dense particle that also resides in the nucleus. 2.5 Atomic Theory TodayStructure of the Atom- An atom is an electrically neutral, spherical entity composed of positively charged centralnucleus surrounded by one or more negatively charged electrons.- The nucleus contributes 99.97% of the atom’s mass.- An atom is neutral because the number of protons in the nucleus equals the number of electrons surrounding the nucleus.Atomic Number, Mass