1. Atoms & the Small Piece of Life
4.1

2. The Atom
The smallest particle that we know of (except what it is made of)
Cannot see even with microscopes

3. History
Ancient Greeks debated if you could break something down so small you couldn’t see it

4. Democritus
Believed all matter was made up of particles that could not be divided further
Called these particles atomos
Greek for indivisible

5. Aristotle Thought all particles made of only 4 elements
Supported until 1800’s

6. John Dalton John Dalton Studied behavior of gases
Based on gas pressure, Dalton concluded gas must be made of many individual particles
Measured masses of elements when compounds form
Number was ALWAYS the same
Meaning the ratio was ALWAYS the same
FIXED COMPOSITION

7. 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.

8. Dalton’s 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.

9. Dalton’s 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.

10. Dalton’s Theory
Great ideas and a good foundation, but not all ideas correct
Allowed us to build off of

11. Thomson’s Model
Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

12. Thomson’s Model
When some materials are rubbed, they gain the ability to attract or repel other materials. Such 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.

13. Thomson’s Model 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
Source of electric current

14. Thomson’s Model
When the current was turned on, the disks became charged, and a glowing beam appeared in the tube.
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.
Thomson observed that the beam was repelled by the negatively charged plate and attracted by the positively charged plate.

15. Thomson’s Model Evidence for Subatomic Particles
Thomson concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. He 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.

16. Thomson’s Revision Thomson’s Model
Thomson revised Dalton’s model to account for these subatomic particles.
The atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom.
The atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it.

17. Thomson’s Model
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.

18. Rutherford’s Model
According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

19. Rutherford’s Model Rutherford’s Hypothesis
Ernest Rutherford 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.
Based on Thomson’s model, Rutherford 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.

20. Rutherford’s Model The Gold Foil Experiment Alpha particles
Gold atoms
Alpha particles
Nucleus
The Gold Foil Experiment
Undeflected particle
Beam of alpha particles
Source of alpha particles
Slit
Deflected particle
Screen

21. Rutherford’s Model
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.

22. Rutherford’s 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 (1012) times the volume of its nucleus.

23. Parts of an Atom Neutron (o) Proton (+) Electron (-)
Neutral subatomic particle found in the nucleus
Proton (+)
Positively Charged subatomic particle in the nucleus
Electron (-)
Negatively charged subatomic particle outside the nucleus
These can be distinguished by their mass, charge and location in an atom

24. Reading the Periodic Table
Atomic Number = Number of protons
Since an element is neutral (no charge), the electrons = the number of protons
Mass number = sum of the protons and neutrons
What about electrons?
Mass number – Atomic number
Isotopes
Atoms of the same element that have different numbers of neutrons and different mass numbers

25. Practice