Science 9 Unit 2 Topic 3: What are Elements?
Ancient Greek philosophers thought that all matter was made out of fire, water, earth and air • Were referred to as “elements” • Everything were a combination of 4 elements and had varying properties of dryness, hotness, coldness, and wetness
The current view of the elements began to develop several; hundred years ago • Sir Francis bacon (1561-1626) • Published book • Argued that science should be built on experimental evidence rather than on thought • Robert Boyle (1627-1691) • Recognized that elements could be combined to form compounds
Taking Matter Apart • Scientists learned to take matter part by ordinary chemical means until it would not break down any further • They were able to determine if a substance was a pure substance or a mixture
Taking Matter Apart • Antoine Lavoisier (1743- 1794) • Defined elements as pure substances that cannot be decomposed into simpler substances by means of a chemical change • Identified 23 pure substances as elements • Successful techniques as an experimenter was his careful measurements of mass • He measured the mass of all the substances involved in a chemical change • Created the law of conservation of mass • States “in a chemical change, the total mass of the new substances is always the same as the total mass of the original substances”
The Law of Definite Composition • Compounds are pure substances that contain 2 or more elements combined together in fixed (or definite) portions • Ex. water
New Discoveries • 19th Century • Voltaic pile • A device we now call a battery • Scientist began using voltaic pikes to pass electricity through water • Discovered hydrogen and oxygen gasses were produced, and that the water had been decomposed into hydrogen and oxygen • Electrolysis • The process of decomposing a chemical compound by passing an electric current through it • Used electrolysis to isolate • Potassium, sodium, magnesium, calcium, strontium, barium
Dalton’s Atomic Theory • The particle model of matter cannot account for particles of different substances combining or decomposing • John Dalton (1766-1844) • Introduced a new way of explaining chemical facts and laws in his atomic theory • All matter is made up of small particles called atoms • Atoms cannot be created, destroyed, or divided into smaller particles • All atoms of the same elements are identical in mass and size • Compounds are created when atoms of different elements link together in definite proportions.
Element • A pure substance made of one type of particle or atom • Compounds • Pure substances that are made up of 2 or more elements chemically combined together
Laws, Theories, Models, and Observations • Laws • Describe and summarize what happens • Theories • Imaginative ways to explain why something happens • Thousands of observations and hundreds of experiments are often made before a theory is accepted by scientists
Scientific ideas may change over time as more evidence is gathered • Scientific models help to picture structures or processes that cannot directly be seen
A brief history of atomic Models • J.J Thomsen • Developed the “plum pudding” or “raisin bun” model • Thought that negatively charged electrons were stuck in a positively charged mass
A brief history of atomic Models • Ernest Rutherford • Hypothesized the atomic nucleus • Almost all the mass of an atom is at the centre • Was made from protons (positively charged particles) and another particle (later named neutron- uncharged particles) • The rest of the area was mostly empty except for orbiting electrons (negatively charged particles)
A brief history of atomic Models • Niels Bohr • Both Bohr and Rutherford pictured the atom like a solar system • Rutherford’s system did not explain many of the atom’s properties • Bohr suggested that electrons move around the nucleus in fixed pathways called electron shells
A brief history of atomic Models • Louis de Broglie and Erwin Schrodinger • Concluded that atoms due in fact have electron levels • Suggested that there was an area around the nucleus where electrons were most likely found • This region was called the electron cloud
(-) negative (+) positive Neutral- no charge Very light Heavy Slightly greater than protons In Nucleus In nucleus Around nucleus, in electron cloud