Studying the structure of atoms is a little like studying wind. Because you cannot see air, you must use indirect evidence to tell the direction of the wind. Atoms pose a similar problem because they are extremely small. Even with a microscope, scientists cannot see the structure of an atom.
Ancient Greek Models of Atoms If you cut a piece of aluminum foil in half, you have two smaller pieces of the same shiny, flexible substance. You could cut the pieces again and again. Can you keep dividing the aluminum into smaller pieces? Greek philosophers debated a similar question about 2500 years ago.
Ancient Greek Models of Atoms Democritus all matter consisted of extremely small particles that could not be divided called these particles atoms Greek word atomos, which means uncut or indivisible
Ancient Greek Models of Atoms Aristotle substances made of four elements Earth Air Fire Water Thought matter could be divided indefinitely By the 1800s, scientists had enough experimental data to support an atomic model
Which philosopher came up with the term atom? 1. Aristotle 2. Democritus
Dalton s Atomic Theory all matter is made up of individual particles called atoms, that cannot be divided Atoms of the same element are identical Atoms of different elements can join to form molecules In a particular compound, atoms of different elements always combine in the same way
Dalton s Atomic Theory Evidence for Atoms John Dalton Determined that because gases exert pressure, they consists of individual particles Determined that masses of elements that make up a compound was equal to the mass of the compound compounds have a fixed composition
Dalton s Atomic Theory When magnesium burns, it combines with oxygen. In magnesium oxide, the ratio of the mass of magnesium to the mass of oxygen is always about 3 : 2. Magnesium dioxide has a fixed composition.
True or False? A compound has a greater mass than the total mass of all the atoms that make up that compound. 1. True 2. False
Dalton s Atomic Theory Dalton s Theory All elements are composed of atoms. All atoms of the same element have the same mass, and atoms of different elements have different masses. Compounds contain atoms of more than one element. In a particular compound, atoms of different elements always combine in the same way.
Dalton s Atomic Theory Dalton made these wooden spheres as a model to represent the atoms of different elements. A tiny, solid sphere with a different mass represents each type of atom.
Which of the following is not part of Dalton s atomic theory? 1. Elements are made of atoms 2. Atoms of the same element have the same mass 3. Compounds contain more than one element 4. Atoms in a particular compound differ from one sample to the next
Dalton s Atomic Theory A theory must explain the data from many experiments Dalton s theory was supported by experiments and became widely accepted. In time, some of Dalton s ideas were found to be incorrect theory was revised due to new discoveries.
Thomson s Model of the Atom Thomson s experiments provided the first evidence that atoms are made of even smaller particles.
Cathode Ray Tube http://www.youtube.com/watch?v=o9goys cbazk
Thomson s Model of the Atom Some materials are said to have either a positive or a negative electric charge. Objects with like charges repel, or push apart. Objects with opposite charges attract, or pull together.
Thomson s Model of the Atom Amber is the hardened form of a sticky, viscous liquid that protects trees from insects and disease. If amber is rubbed with wool, it becomes charged and can attract a feather.
Thomson s Model of the Atom Thomson s Experiments In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas. Sealed tube filled with gas at low pressure Glowing beam Metal disk Metal disk Source of electric current Metal disk Source of electric current
Thomson s Model of the Atom Thomson s Experiments In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas. Sealed tube filled with gas at low pressure Glowing beam Positive plate Metal disk Negative plate Metal disk Source of electric current Metal disk Source of electric current
Thomson s Model of the Atom When the current was turned on, the disks became charged, and a glowing beam appeared in the tube. hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow beam was repelled by the negatively charged plate beam was attracted by the positively charged plate
Thomson s Model of the Atom Evidence for Subatomic Particles He concluded that the particles in the beam had a negative charge they were attracted to the positive plate hypothesized that the particles came from inside atoms because no matter what metal Thomson used for the disk, the particles produced were identical the particles had about 1/2000 the mass of a hydrogen atom, the lightest atom
Thomson s Model of the Atom Thomson s Model He revised Dalton s model to account for these subatomic particles atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it.
In Thompson s experiment, the beam was attracted to the plate. 1. Positive 2. Negative
Thomson s Model of the Atom Thomson s model is called the plum pudding model. Today, it might be called the chocolate chip ice cream model. The chips represent negatively charged particles, which are spread evenly through a mass of positively charged matter the vanilla ice cream.
Rutherford s Atomic Theory According to Rutherford s model, all of an atom s positive charge is concentrated in its nucleus.
Gold Foil Experiment http://www.youtube.com/watch?v=xbqhkr af8ie
Rutherford s Atomic Theory Rutherford s Hypothesis designed an experiment to find out what happens to alpha particles when they pass through a thin sheet of gold. Alpha particles are fast-moving, positively charged particles. He hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle. He predicted that most particles would travel in a straight path from their source to a screen that lit up when struck.
Rutherford s Atomic Theory The Gold Foil Experiment Deflected particle Undeflected particle Alpha particles Slit Gold atoms Beam of alpha particles Screen Alpha particles Source of alpha particles Nucleus
Rutherford s Atomic Theory Discovery of the Nucleus alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection. many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object.
Rutherford s Atomic Theory Thomson s model did not explain all of the evidence from Rutherford's experiment. Rutherford proposed a new model. The positive charge of an atom is not evenly spread throughout the atom. Positive charge is concentrated in a very small, central area. The nucleus of the atom is a dense, positively charged mass located in the center of the atom.
A fast-moving, positively charged particle is called a(n). 1. Nucleus 2. Atom 3. Alpha particle 4. Beam
Rutherford s Atomic Theory The Houston Astrodome occupies more than nine acres and seats 60,000 people. If the stadium were a model for an atom, a marble could represent its nucleus. The total volume of an atom is about a trillion (10 12 ) times the volume of its nucleus.
Assessment Questions 1. Dalton s theory did not include which of the following points? a. All elements are composed of atoms. b. Most of an atom s mass is in its nucleus. c. Compounds contain atoms of more than one element. d. In a specific compound, atoms of different elements always combine in the same way.
Assessment Questions 2. J. J. Thomson s experiments provided the first evidence of a. atoms. b. a nucleus. c. subatomic particles. d. elements.
Assessment Questions 1. The concept of an atom as a small particle of matter that cannot be divided was proposed by the ancient Greek philosopher, Democritus. True False