1. The Atom

2. What is an atom? An atom is the smallest part of an element The atom remained mostly a mystery because it is unable to be seen with even a microscope Over time there have been many models of what an atom looks like

3. Ancient Greek Model Greek philosophers asked “What would happen if you kept cutting a piece of paper in half until there is nothing left?” They believed that at some point a single, smallest particle, that could not be divided would remain

4. Ancient Greek Model The Greek model was not highly accepted at the time Aristotle refuted these claims and most people agreed with his argument

5. Dalton’s Atomic Theory In 1766, Dalton concluded that gas is made up of billions of particles He gathered his evidence by measuring the masses of elements in compound form

6. Dalton’s Atomic Theory He concluded that no matter how large or small the sample of a compound is, the ratio of the masses of the elements is always the same Compounds have fixed compositions

7. Dalton’s Atomic Theory Dalton developed a theory – “All matter is made up of individual particles called atoms, which cannot be divided.”

8. Dalton’s theory cont. All elements are composed of atoms All atoms of the same element have the same mass, and atoms of different elements have masses Compounds contain atoms of more than one element In particular compounds, atoms of different elements always combine in the same way

9. Dalton’s Atomic Model

10. Thomson’s Atomic Model In 1856, J.J Thomson used an electrical current to learn more about the atom He used an electrical current, metal discs, and a beam of light to study his theory

11. Thomson’s Atomic Model He found that particles in a beam of light have a negative charge He found that these particles of light were even smaller than an atom

12. Thomson’s Atomic Model Thomson’s experiments provided the first evidence that atoms are made up of even smaller particles We call these “Subatomic Particles”

13. Thomson’s Atomic Model Thomson concluded that an atom has a neutral charge (Neither positive, nor negative) He designed an atom that has negative charges scattered throughout a positively charged mass

14. Thomson’s Atomic Model

15. Rutherford’s Atomic Theory In 1899 Rutherford discovered that uranium emits fast moving particles that have a positive charge He named theses particles “Alpha particles” He studied what would happen to alpha particles as they pass through a thin sheet of gold.

16. Rutherford’s Atomic Theory 1/20,000 alpha particles were deflected 90 degrees off of the gold The rest bounced straight off of the gold foil

17. Rutherford’s Atomic Theory Rutherford concluded that the deflected alpha particles must have come close to a charged object After studying this theory he concluded that atoms contain a nucleus

19. The Nucleus A dense, positively charged mass located at the center of an atom According to Rutherford, all of an atom’s positive charge is concentrated in its nucleus