1. 4.1 Defining the Atom Atoms are too small to see. Because of this, we must perform experiments and analyze the results to study them. An atom is the smallest particle of an element that retains its identity in a chemical reaction.

2. 4.1 Defining the Atom Democritus’s Atomic Philosophy ------------ Greek philosopher who lived over 2400 years ago. Among the first to suggest that matter is composed of atoms. Believed that atoms were indivisible and indestructible. His ideas lacked scientific support. His beliefs agreed with later scientific theory, however they were not accepted by the people of his time. The views of Aristotle, however, were widely excepted for over a thousand years.

3. 4.1 Defining the Atom 1. 2. 3. 4. All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical in mass and properties. Compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms. John Dalton (1766-1844)

4. 4.1 Defining the Atom 1) All elements are composed of tiny indivisible particles called atoms.

5. 4.1 Defining the Atom 2) Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

6. 4.1 Defining the Atom 3) Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

7. 4.1 Defining the Atom Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into atoms of another element in a chemical reaction.

8. 4.1 Defining the Atom What’s Wrong with Dalton’s Theory? Problem with Dalton’s atomic theory- atoms can be broken down into subatomic particles There are 3 subatomic particles: electrons, protons, and neutrons Atoms of the same element aren’t necessarily identical. Atoms of the same element can have different masses (different isotopes) due to different numbers of neutrons.

9. 4.1 Defining the Atom Imagine grinding this penny into very small pieces. - Each tiny piece has the exact same composition. How small can a substance be, and still be considered that substance? As small as 1 atom of that substance. A copper penny contains about 2.4 x 10 22 atoms of copper. You could line up 100,000,000 copper atoms and they would only be 1 cm long.

10. 4.1 Defining the Atom Individual atoms- can be seen through scanning tunneling electron microscope Generally used for metal samples Electron cloud extends above the atoms Needle is brought close and current is sent through Current changes as the needle gets closer to the electron cloud Current changes are mapped to show the surface Red ridge is a series of Cesium atoms Ring of Iron Atoms