Chemistry Basics: History & Properties (PDF)

Physical Property - a characteristic of a substance - describes how the substance is - ie. taste, texture, colour, state, change of state - Physical change: change in matter that does not change its identity/composition. - ie. Breaking apart, dissolving, change of state Chemica...

Physical Property - a characteristic of a substance - describes how the substance is - ie. taste, texture, colour, state, change of state - Physical change: change in matter that does not change its identity/composition. - ie. Breaking apart, dissolving, change of state Chemical Property - the reactive characteristics of a substance when it changes to a new substance with new properties - ie. calcium reacts with water, paper burns - Chemical change: change in matter that happens when the identity/composition of a substance changes (a NEW substance is formed). Elements are pure substances that contain only one type of atom. They are found on the periodic table. ie. Na, Cu, Li, Fe - diatomic elements: "HOBrFINCl" - these elements exist as two atoms covalently bonded together (H2, Br2, N2) Molecules have two or more atoms bonded together (H2O, C6H12O6, F2) Compounds are molecules that contain more than one type of element bonded together Some history... Dobereiner (1829) – "Triads" - found that groups of three elements had similar behaviours - did NOT work for all elements Newlands (1864) - organized elements by mass - elements with similar properties repeated every 8 elements - "Law of Octaves" - he left NO GAPS for unknown elements Mendeleev (1869) - Periodic Law - based on atomic mass (like Newlands) - repeat of 8 elements (like Newlands) - BUT left gaps for unknowns - could predict properties of elements yet to be discovered - problems with atomic masses not in order Mosley (1913) - arranged the periodic table by atomic number not atomic mass The modern theory of the atom is only about 200 years old, BUT its origin can be traced back to ancient Greece. Democritus (~460 - ~370 BCE)  Greek philosopher  first to propose atomic theory of matter  all matter could be divided into smaller pieces until a single indivisible particle is reached  he coined the term "atom" meaning indivisible Non experimental "progress" led to Aristotle criticizing Democritus' theory. Aristotle (384-322 BCE) supported four element theory of matter proposed by Empedocles 1. Earth 2. Air 3. Water 4 Fire This theory held for almost 2000 years. Then along came: John Dalton (1766-1844) scientist and schoolteacher revitalized atomic theory different from Greek philosophers as his theories were based on experimental evidence 1. Atoms cannot be subdivided. 2. Atoms cannot be created or destroyed. 3. All atoms of the same element are identical in size, mass, and chemical and physical composition (we currently know that isotopes differ in neutron number). 4. The properties of the atoms of one element differ from those of all other elements. Various atoms and molecules as depicted in 5. Atoms combine in small, whole John Dalton's A New number ratios to form compounds. System of Chemical Philosophy The development of the atomic theory was made possible by the construction of the cathode ray tube. Noble Gas (inside tube) Cathode Anode Is it a beam of light? NO!!! Why Not? 1. The beam can be bent when forced through a magnetic field. 2. The beam has mass. Thompson (1856-1940)  Looked at the mass to charge ratio for the particles passing through the cathode ray tube.  Proposed that the cathode rays were subatomic particles, and called these particles electrons.  Proposed the raisin bun model of the atom: a positively charged, mostly empty sphere, containing negative (-) charged electrons throughout. Ernest Rutherford (1871-1937) The gold foil experiment: - bombarded gold foil with positively charged alpha particles (helium nucleii) - most particles passed through suggesting the foil was mostly empty space rather than a sheet of solid atoms - some alpha particles were deflected or bounced directly back towards the emitter implying some positive solid matter  *Rutherford's work showed that atoms consist of primarily empty space surrounding a well defined core, the nucleus. He called the positive charges in the nucleus the protons.* James Chadwick (1891-1974) Demonstrated that atom nuclei contain neutral particles in addition to the positive particles (protons). He called the neutral particles neutrons.