Section 4.2 Pages 106 -114

Early Ideas about Matter - Greek Philosophers: Democritus (460 – 370 BC) He thought matter was not infinitely divisible, and that matter was made up of tiny particles called atomos which move through empty space. He believed that atomos could not be created, destroyed, or divided. He did not know what holds the atoms together

Early Ideas about Matter - Greek Philosophers: Aristotle (384 – 322 BC) Extremely well respected and influential philosopher. He believed that empty space cannot exist, and therefore he denied the existence of atoms.

Early Ideas about Matter John Dalton (1766 – 1844) Revised Democritus’ ideas. Created Dalton’s Atomic Theory Matter is made up of extremely small particles called atoms. Atoms are indivisible and indestructible. Atoms of an element are all the same in regards to size, mass and chemical properties

Early Ideas about Matter - Dalton John Dalton (1766 – 1844) Created Daltons’s Atomic Theory (CONTINUED) Different elements have different atoms. The Law of Multiple Proportions In a chemical reaction, atoms are separated, combined or rearranged.

Mass is conserved in any process Conservation of Mass Mass is conserved in any process The same number of atoms used as reactants, will be present in the product(s). X + Y XY

Conservation of Mass

Comparison of Dalton and Democritus Take some time to compare and contrast Dalton and Democritus using a double bubble map.

Atoms could not be created, divided or destroyed. Both believed Matter is composed of extremely small particles called Atoms. All atoms of a given element are identical , but differ from the atoms of other elements. Atoms could not be created, divided or destroyed. Democritus Believed that matter is composed of empty space through which atoms move Different kinds of atoms come in different sizes and shapes. Differing properties of atoms are due to the size, shape and movement of the atoms. Dalton Specified that different atoms combine in simple whole number ratios to form compounds.

The Atom Atoms are the smallest particles of an element that retain the properties of this element. Atoms are made up of three subatomic particles: Protons Electrons Neutrons

Atoms We can view atoms using a scanning tunneling microscope

The Electron Sir William Crookes Noticed a flash of light in the cathode ray. Lead to further research – determined that cathode rays were a stream of negatively charged particles found in all forms of matter. These negatively charged particles are called electrons

J.J. Thompson (1856-1940) The Electron Did a series of experiments to determine the charge to mass ratio of the charged particles He concluded that the mass of the charged particle was much less than that of a Hydrogen atom which is the lightest known atom.

The Electron Robert Millikan (1868 – 1953) Performed an oil drop experiment Determined the charge of an electron Calculated the mass of an electron: 1840 times lighter than the mass of Hydrogen.

Plum Pudding Model Electrons are negatively charged, but matter is neutral overall J.J. Thompson proposed a model that had uniformly distributed positive matter with negative charges residing in it.

Rutherford’s Experiment The Nucleus Rutherford’s Experiment A narrow beam of positively charged alpha particles was aimed at a thin sheet of gold foil.

Expected: alpha particles to only move a little Actual: particles collided and went all over! Few particles were deflected at large angles, and several were deflected straight back to the source.

Rutherford’s Atomic Model Concluded: An atom consisted mostly of empty space through which the electrons move. Almost all of the atom’s positive charge and almost all of its mass were contained in a tiny dense region in the center of the atom which he called nucleus. The negative charges (electrons) are held in place by their attraction to the positive charges

James Chadwick Rutherford’s coworker, English Physicist Showed that the nucleus also contained another subatomic neutral particle, called neutron Neutron is a subatomic particle that has a mass nearly equal to that of a proton Neutrons carry “no electric charge” Chadwick received the Nobel Prize in Physics for proving the existence of neutrons

The volume of space through which the electrons move is huge compared to the volume of the nucleus. If an atom had a diameter of 2 football fields , the nucleus would be the size of a nickel

The Nucleus It contains: Proton: sub-atomic particle with a charge of +1 Neutron: sub-atomic particle with no charge. Mass is nearly equal to that of a proton

Summary of Atom Proton Electron Neutron Symbol p e- n Charge Positive It is made up of 3 sub-atomic particles with varying properties Proton Electron Neutron Symbol p e- n Charge Positive Negative Neutral (none) Location Nucleus Outside nucleus Mass 1 1/1840

Section 4.2: Assessment Questions page 114 Question 7: Describe the structure of a typical atom. Identify where each subatomic particle is located A typical atom consists of a central, small, dense nucleus containing protons and neutrons. The nucleus is surrounded by a cloud of negatively charged electrons.

Question 8: Compare and contrast Thompson’s plum pudding model with Rutherford’s nuclear model Tompson’s plum pudding model describes atoms as spherical particles with uniformly distributed positive charge in which individual, negatively charged electrons are located in fixed positions. In contrast, Rutherford’s model states that an atom is mostly empty space, with a small, dense, central nucleus containing all of an atom’s positive charge and most of its mass. The negatively charged electrons move through the empty space and are held in the atom by their attraction to the positively charged nucleus.

Question 9: Evaluate the experiments that led to the conclusion that electrons are negatively charged particles found in all matter The deflection toward positively charged plates demonstrated the negatively charged nature of electrons; the fact that changing the type of electrode or the type of gas used in the cathode-ray tube did not affect the ray produced led to the conclusion that electrons are present in all matter.

Question 10: Compare the relative charge and mass of each subatomic particle Relative Mass Electron -1 1/1840 Proton +1 1 Neutron ~1