Chemistry– 11/11/2018 Objectives 3.1 Describe the crucial contributions of scientists and the critical experiments that led to the development of the modern atomic model 3.2 Describe the early models of the atom and construct Bohr Models of the atom.  Bell Ringer – Pretest Name 3 scientist who contributed to our understanding of the atom Homework: Read chapter 4 and complete study guide

Early Greek Theories This led to the idea of atoms in a void. fire air Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void. fire air water earth Aristotle 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years.

Alchemists (about 1000-1650) Alchemists – Scientist who used chemistry and magic. Made many discoveries without knowing why.. Attempted to : change lead and other base metals to gold discover a universal solvent discover a life-prolonging elixir

Problems with combustion Hundreds of years went by with scientists debating why things burn. Stahl, Georg (1660-1734) - Suggested that a substance he called “phlogiston” flowed out of burning material. Priestley, Joseph (1733-1804) - Discovered oxygen (“dephlogiston air”), carbon monoxide and nitrous oxide. Optional Demonstration: Elephant Toothpaste

Lavoisier, A.L. (1743-1794) Generally regarded as the father of chemistry. Discovered nitrogen; studied acids and described composition of many organic compounds.

Mendeléev, D.I. (1834-1907) Discovered periodicity of the elements and compiled the first Periodic Table.

John Dalton All matter is made of atoms. 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

Changes to Dalton’s Theory Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element. Dalton said: Atoms cannot be subdivided, created, or destroyed Atoms cannot be subdivided, created, or destroyed. Modern theory states: However, these changes CAN occur in nuclear reactions!

Early Theories of Matter Experiments around the turn of the twentieth century provided evidence that the atom consisted of smaller particles.

Early Theories of Matter Day 1 - History of the Atomic Theory Early Theories of Matter When J.J. Thomson applied a high voltage to two electrodes sealed inside an evacuated tube, an invisible beam, or ray, was found to emanate from the negative electrode (cathode) of the tube and travel toward the positive electrode (anode). The path of the beam could be made visible by the glow it caused when it passed across a plate coated with fluorescent material. The ray, called a cathode ray, could be deflected by electric and magnetic fields. Thomson concluded that the ray consisted of a stream of negatively charged particles that had been dislodged from atoms. These particles became known as electrons. How did he discover an invisible beam?

Thomson, Sir J.J. (1856-1940) Research on cathode rays resulted in proof of existence of electrons (1896).

Thomson’s Plum Pudding Model of the Atom

Millikan, Robert (1868-1953) In 1909 Robert Millikan accurately determined the mass of an electron to be 1/1840 the mass of a hydrogen atom. He also determined the electron’s charge to be 1-. This value was accepted as a fundamental unit of electrical charge.

Early Theories of Matter Day 1 - History of the Atomic Theory Early Theories of Matter In 1911, Ernest Rutherford conducted experiments in which he aimed a beam of alpha particles (positively charged helium nuclei) at a thin sheet of gold. The fact that a few of these heavy particles were deflected almost directly backwards revealed that the positive charge of an atom, along with nearly all of the atom’s mass is concentrated in a very small region in the center of the atom. This positively charged mass is called the nucleus of the atom. How did he discover an invisible beam?

Rutherford, Sir Ernest (1871-1937) Proved the existence of the nucleus. Explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers Target #1 Target #2

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged

Expected and Actual Results of Rutherford’s Experiment

Early Theories of Matter Day 1 - History of the Atomic Theory Early Theories of Matter The particles that make up the positive charge of the nucleus are called protons, and each has a charge of 1+ (equal to but opposite the charge of an electron). How did he discover an invisible beam?

Early Theories of Matter Day 1 - History of the Atomic Theory Early Theories of Matter In 1932, Chadwick demonstrated the existence of the neutron, a nuclear particle having no charge but with nearly the same mass as a proton. How did he discover an invisible beam?

Bohr, Niels (1885-1962) Niels Bohr – Suggested that electrons in an atom move in set paths around the nucleus in energy levels much like the planets orbit the sun in our solar system. Developed a quantum model for the hydrogen atom.

NEXT DAY LESSONS IF EXTRA TIME

Atomic Structure Nucleus – the center of an atom; made up of protons and neutrons Proton – A positively charged subatomic particle in the nucleus of an atom Neutron – A neutral subatomic particle in the nucleus of an atom Electron – a tiny negatively charged subatomic particle moving around outside the nucleus of an atom  

Atomic Structure Atomic number – the number of protons in a nucleus of an atom of that element Atomic (average) mass number – the number of protons plus the number of neutrons

Properties of Subatomic Particles Symbol Electrical Charge Relative Mass Location Electron e- 1- 1/1840 Orbiting the nucleus Proton p+ 1+ 1 In nucleus Neutron n0 Quiz on this information in 2 days