Chapter 4.1. Atomic Models. Atomic Structure.
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A. Studying the structure of atoms poses a problem due to the fact that they are extremely small. Even with a microscope scientists cannot see the structure of an atom. Since the structure of atoms cannot be seen indirect evidence must be used to develop models of atoms.
B. Ancient Greek Models of Atoms
A. Democritus believed that all matter consisted of extremely small particles.
1. He called these particles atoms.
2. He though they could not be divided.
3. He thought there were different types of atoms with specific sets of properties.
Ex. Atoms in liquids were round and smooth, but the atoms in solids were rough and prickly.
1. Believed there was no limit to the number of times matter could be divided.
2. All substances were built up from four elements: Earth, air, fire, and water.
3. These elements had four qualities: hot, cold, dry, and wet.
C. John Dalton
A. John Dalton was born in England in 1766. He became a teacher and spent his spare time doing experiments.
B. Correctly concluded that a gas consists of individual particles.
C. Gathered evidence of the existence of atoms by measuring the masses of elements that combine when compounds form.
D. The ratio of the masses of the elements in the compound is always the same.
E. Compounds have a fixed composition.
Ex. When magnesium burns it combines with oxygen. 100g of magnesium combines with 65.8g of oxygen. 10g of magnesium combines with 6.58g of oxygen.
D. Dalton’s Atomic Theory
A. All matter is made up of individual particles called atoms, which cannot be divided.
1. All elements are composed of atoms.
2. All atoms of the same element have the same mass, and atoms of different elements have different masses.
3. Compounds contain atoms of more than one element.
4. In a particular compound, atoms of different elements always combine in the same way.
B. While this theory was not perfect it was not discarded but revised to account for new discoveries.
C. What did Dalton notice that all compounds have in common?
The ratio of masses of elements in a compound is always the same.
Pic: Dalton represented elements as solid spheres. Each type of atom is represented by tiny, solid spheres with a different mass.
A. The device used to create an electric current was a glass tube with most of the air vacuumed out sealed with a metal disk on each end. When the electric current is turned on one disk becomes positively charged while the other disk becomes negatively charged. This created a glowing beam in the glass tube.
B. Thomson hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow.
C. Thomson found, in an experiment, that the beam of charged particles was attracted to a positively charged plate.
D. He hypothesized that the particles came from inside atoms.
E. Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.
F. This changed how scientists thought about atoms.
G. Thomson revised Dalton’s model to account for these subatomic particles.
F. Thomson’s Model
A. An atom is neutral, meaning it has neither a negative nor a positive charge. However, Thomson knew from his experiments that atoms contained negatively charged particles.
B. How can an atom contain negative particles and still be neutral?
C. Plum Pudding Model (Chocolate Chip Ice Cream)
1. In Thomson’s model the negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter.
2. Think of chocolate chips as negative particles and the ice cream as a positively charged mass of matter. With the chocolate chips spread evenly throughout the ice cream the charges of the chocolate chips and ice cream balance each other out and create an overall neutral charge.
D. What evidence did Thompson’s experiments provide?
Atoms are made of even smaller particles.
G. Ernest Rutherford (1871-1937)
A. Surprised by unexpected results that lead to important discoveries.
B. Discovered that uranium emits fast moving particles that have a positive charge. He named these particles alpha particles.
C. Rutherford’s experiment consisted of sending a beam of alpha particles through a thin sheet of gold foil.
E. He hypothesized, based on Thomson’s model, that the mass and charge of the alpha particles traveling through the gold foil would be too small to change the path of the particles. The particles would travel straight through the gold foil and strike a screen that would light up when hit.
F. Some of the particles that hit the gold foil were deflected. He found that this hypothesis was false.
G. This led Rutherford to believe that the positive charge of an atom is not evenly spread throughout the atom.
H. The positive charge is concentrated in a very small, central area called the nucleus.
I. The particles that were deflected came close to a nucleus, while the ones that traveled straight through it did not.
J. Rutherford’s model showed that all of the positively charged particles of an atom are concentrated in its nucleus not spread evenly out like Thomson’s model.