How Do We Know What Atoms Look Like? Write down the stuff in yellow.

400 B.C. 1805 1897 1904 1911 1913 1923 1930s Today Timeline write these dates down the left side of a new page – skip lines:

Make an entry next to each date Make an entry next to each date. Draw a picture of the atomic model at each stage, to the far right of the corresponding date.

People have always been interested in what all of this "stuff" is made of. What makes a tree different from water or rocks? What does all this matter have in common?

About 400 bc, Democritus thought that the universe was made of empty space and tiny bits of stuff. He thought that these bits of stuff were so tiny that they couldn't be cut any smaller, so the Greeks named them "atoms." Atom means "INDIVISIBLE" in Greek.

400 B.C. Matter is made of atoms Add this to your timeline: 400 B.C. Matter is made of atoms

Democritus' idea that matter is made up of tiny particles and empty space was a good start, but many questions still needed to be answered…

Loading…Please Wait not much happening on the atomic theory front for the next 2000 years…

In the 1700s, Scientists started conducting actual EXPERIMENTS dealing with matter, but they still wondered what atoms actually looked like.

Under your timeline, write this “Big Idea.” We have a better model of the atom today than in the past because we have better technology.

In 1805, a British chemist named John Dalton did experiments which showed that each element is made of atoms, and that all atoms are alike, except that different elements have different masses. Ooooh, that’s heavy!

Elements have different masses H C Add this to your timeline: Elements have different masses H C

Ninety years later, J.J. Thompson built on Dalton's ideas and conducted experiments which showed that atoms had negative charged particles that he named "electrons."

1897 Atoms have negative electrons Add this to your timeline: 1897 Atoms have negative electrons

Thompson's model was sort of like a blueberry muffin Thompson's model was sort of like a blueberry muffin. The atom-muffin had electron-blueberries scattered throughout the atom and also stuck to the outside.

In 1904, a Japanese physicist named nagaoka built on the work of Thompson and Dalton. His experiments showed that the electrons were orbiting around OUTSIDE of the Positively charged nucleus

1904 Electrons are on outside of nucleus Add this to your timeline: 1904 Electrons are on outside of nucleus The nucleus has a positive charge

Under your timeline, write this “Big Idea.” Scientists base their experiments on the work of others who came before. These experiments give us EVIDENCE.

Just 7 years later, another Brit named Ernest RutheRford was doing his own experiments. These trials presented evidence that the atom is mostly empty space, and the electrons are far, far away from the nucleus.

1911 Atom is mostly empty space Add this to your timeline: 1911 Atom is mostly empty space

Make an inference… Why do you think we usually draw atoms like this When we know that the electrons are not that close to the nucleus?

In 1913 in Holland, Niels Bohr conducted experiments with evidence that the electrons orbit the nucleus in layers, or Valence shells. He observed that atoms give off energy when electrons move between layers.

Bohr‘s model is also in use today when we need to picture how the electrons are arranged.

1913 Electrons are arranged in shells Add this to your timeline: 1913 Electrons are arranged in shells

in 1923 James Chadwick‘s evidence showed that there was something else in the nucleus besides protons, but they didn't have a charge – they were neutral. He named these blobs NEUTRONS. We had to re-write all of the textbooks again!

1923 Nucleus has protons and neutrons Add this to your timeline: 1923 Nucleus has protons and neutrons

Heisenberg Many scientists worked together to gather evidence that the electrons don't travel in definite ovals, they zing all over the place and they're moving so fast that they are just a blur, or a cloud. Schrodinger De Broglie

Electrons make a "cloud" around the nucleus that has several layers Electrons make a "cloud" around the nucleus that has several layers. Kind of like bees buzzing around a hive. A balloon inside a balloon inside a balloon…

1930s Electrons buzz around nucleus in the “electron cloud” Add this to your timeline: 1930s Electrons buzz around nucleus in the “electron cloud”

Scientists‘ experiments have also produced evidence that protons and neutrons have even tinier particles inside called quarks, held together by gluons. But this is another story….

Have we actually SEEN an atom? This movie was made by moving a single atom at a time inside a scanning-tunneling electron microscope.

Today Quarks, gluons, neutrinos, dark matter, anti-matter… Add this to your timeline: Today Quarks, gluons, neutrinos, dark matter, anti-matter…

A Quiz to see if you’ve been paying attention… (write these down below your timeline)

1- Throughout history, scientific knowledge has grown and progressed 1- Throughout history, scientific knowledge has grown and progressed. Why is it that in the last century we have had an explosion of knowledge about the atom? The scientific method has been improved. Scientists are smarter than in the past. The technology we have has improved. Scientists can receive government grants now.

2- Our current model of the atom is the electron cloud model 2- Our current model of the atom is the electron cloud model. How did scientists decide on this model? Scientists saw the path of electrons around the nucleus. This model is supported by the most evidence. Scientists discovered this model on the Internet.

3- Which of the following statements is true about the atomic model to the right? This model changed as natural laws changed. This model shows exactly what an atom looks like. This model has always been the correct model. This model is based on experiment results.