The Atom

1. The Atom

2. What does an atom look like? • Draw a picture of what you think an atom looks like up close • Open book to page 164 and compare your drawing to the picture of the Aluminum atoms.

3. The beginning of Atomic Theory • Matter is made of particles that we call atoms. • An atom is the smallest particle into which an element can be divided and still have the properties of that element.

4. The History of the Atom 440 BCE a Greek philosopher named Democritus hypothesized that if you cut an object into pieces, you would eventually end up with a particle that could not be cut further He called this atomos – which means ‘not able to be divided’

5. Dalton’s Atomic Theory • By the late 1700s, scientists had learned that elements combine in certain proportions based on mass to form compounds. • John Dalton’s atomic theory, published in 1803, suggested that elements combine in certain proportions because they are made of atoms.

6. Dalton’s Theory also stated: • All substances are made of atoms and atoms are the smallest particles that cannot be created, divided or destroyed • Atoms of the same element are exactly alike • Atoms join with other atoms to make new substances

7. Thomson’s Discovery of Electrons • In 1897, Thomson’s experiments led him to conclude that there must be negatively charged particles inside the atom. These particles are now called electrons. • An electron is a subatomic particle that has a negative charge.

9. Thomson’s Plum Pudding Model • Thomson discovered that electrons were a part of the atom, but it did not show where they were located. • So he guessed that they were everywhere – ‘Like plums in a pudding’ • His model shows electrons everywhere in the atom and is called the Plum Pudding Model

10. Rutherford’s Atomic ‘Shooting Gallery’ • In 1909, Ernest Rutherford designed an experiment to study the parts of an atom. • The results of Rutherford’s gold-foil experiment suggested that atoms are not all the same throughout. He concluded that atoms have different parts.

12. Rutherford and the Nucleus • Rutherford’s results led him to conclude that most matter in an atom is found in one very small area. • In 1911, Rutherford proposed that in the center of the atom is a tiny, extremely dense, positively charged area called the nucleus. • the nucleus is an atom’s central region which is made of protons (pro) and neutrons (neutral).

13. Bohr’s Findings • In 1913, Niels Bohr’s results led him to propose that electrons move around the nucleus in definite paths. • Today: Modern Atomic Theory states electrons are likely to be found moving in certain regions around the nucleus of an atom called electron clouds, or orbitals.

14. Electron Clouds • Electron clouds are regions around the nucleus of an atom where electrons are likely to be found. Each electron cloud has a definite energy level.

15. Size of an Atom • Atoms are so small that light waves are too large to be used to see them. Scientists use scanning tunneling electron microscopes to provide images of atoms.

16. The average size of an atom = .00000002 cm or 2 hundred millionths of a cm • If an atom were the size of a period at the end of a sentence, then the person reading the sentence would be 1000 miles tall!!! • Atom Song!

17. The smallest particle into which an element can be divided and still have the properties of that element is called: • Nucleus c. Atom • Electron d. Neutron 2. What particle did Thomson discover? • Neutron c. atom • Electron d. proton 3. What did Rutherford believe was in the center of the atom?

18. 4. Dalton believed that • Atoms of the same element are exactly alike. • Most, but not all, substances are made of atoms • Atoms of different elements are the same • Atoms can be divided