What are we learning and why are we learning this? Content, procedures, or skills.
Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.
Tier III: Low frequency, domain specific terms.
Building on what we already know
Make connections to prior knowledge. This is where we build from.
Take a piece of matter. Doesn’t matter whether it is metal, a rock, or Play Doh. Cut it into two pieces. Each piece still has the same properties as the original:
Same color, elasticity, temperature at which it freezes, ability conduct electricity, etc.
Then cut each of those into smaller pieces. And then cut each of those smaller pieces into yet smaller pieces.
Each of those pieces still has the same color, elasticity, temperature at which it freezes, ability conduct electricity, etc.
We can do the same for pieces of rock or metal.
Interesting – Each of those smaller pieces still has the same color, elasticity, temperature at which it freezes, ability conduct electricity, etc.
So is matter continuous and infinitely divisible?
Or is matter divisible only until a basic, tiny particle is reached?
This was a question discussed by the ancient Greek philosophers.
Democritus (Greek: Δημόκριτος, meaning “chosen of the people”; c. 460 – c. 370 BCE)
Ancient Greek pre-Socratic philosopher remembered today for his formulation of an atomic theory of the universe.
He proposed that all matter was composed of small indivisible particles called atoms.
Aristotle (330 BCE) asserted that the elements of fire, air, earth, and water were not made of atoms, but were continuous.
One ancient idea was that there were four basic elements:
Why would some believe that there were four elements?
Think of what they observed when wood burns: Fire being released, smoke (“air”), and after the burning was done you would have ashes (“earth”).
Water being the opposite of fire would extinguish it.
One could even make a rudimentary chemical equation to justify this idea.
Wood (earth/air/fire) —> Earth (ashes) +air (smoke) + fire (flame)
Over time we realized that thinking about this subject wasn’t enough. We needed to learn more through experimentations.
Models of the Atom: The Scientific Revolution
By the 1700s scientists defined an element as:
a substance that cannot be further broken down by ordinary chemical means.
They inferred that elements combined to form compounds that have different physical and chemical properties than those of the elements that make them.
By 1800 scientists had discovered basic laws of chemistry.
For instance, the law of conservation of mass – mass is neither created nor destroyed during ordinary chemical reactions or physical changes.
Soon after they discovered the law of multiple proportions:
Compounds are made of a fixed proportion of elements.
For example, NaCl (sodium chloride, table salt) always consists of :
39.34% by mass of sodium, Na
60.66% by mass of chlorine, Cl.
John Dalton Model of Atom
In 1808, an English schoolteacher named John Dalton proposed the first scientific model of the atom:
1. All matter is composed of extremely small particles – atoms.
2. Atoms of an element are identical in size, mass, and other properties;
atoms of different elements differ in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.
- Adapted from Modern Chemistry, Raymond Davis et al, Holt, Rinehart an Winston
When Dalton proposed his model, sub-atomic particles inside atoms were unknown.
J. J. Thomson Model of atom 1897
Discover of the electron
Here’s one way to model the atom.
Ernest Rutherford experiment in 1899
At this point, here is how they imagined that they atom worked.
Neils Bohr, semi-quantum model of the atom
In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.
To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits.
Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.
Here’s a model of what happens when an electron “falls” from one orbital down to a lower energy orbital. As the e- loses energy, this energy is converted into a photon.
Here we see what happens when a photon (light particle) interacts with an atom.
Erwin Schrödinger, electron cloud model, 1920s
matrix based version of quantum mechanics
Early development of our understanding of the Atom
Historical development of atomic models