Atoms and Elements

1. Atoms and Elements Mr. Olsen 7th Grade Integrated Science

2. Paper Clip Demonstration Take a pile of paper clips (all of the same size and color). Divide the pile into two equal piles. Divide each of the smaller piles into two equal piles. Repeat step 3 until you are down to a pile containing only one paper clip. That one paper clip still does the job of a paper clip (i.e., hold loose papers together). Now, take a pair of scissors and cut that one paper clip in half. Can half of the paper clip do the same job as the single paper clip?

3. The Indivisible Atom If you do the same thing with any element, you will reach an indivisible part that has the same properties of the element, like the single paper clip. This indivisible part is called an atom.

4. Atoms The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles to small to be seen. These particles were called atoms

5. Atomic Ideas • The idea of the atom was first devised by Democritus in 530 B.C. In 1808, an English school teacher and scientist named John Dalton proposed the modern atomic theory. Modern atomic theory simply states the following:

6. Atomic Theory Every element is made of atoms - piles of paper clips. All atoms of any element are the same - all the paper clips in the pile are the same size and color. Atoms of different elements are different (size, properties) – like different sizes and colors of paper clips. Atoms of different elements can combine to form compounds - you can link different sizes and colors of paper clips together to make new structures. In chemical reactions, atoms are not made, destroyed, or changed - no new paper clips appear, no paper clips get lost and no paper clips change from one size/color to another. In any compound, the numbers and kinds of atoms remain the same - the total number and types of paper clips that you start with are the same as when you finish.

7. Atomic Models Over time, the scientific model of what an atom looks like has evolved beginning with Democritus and his “uncuttable” atom.

8. John Dalton (1803) The Billiard Ball Model: John Dalton viewed the atom as a small solid sphere.  He really got the "ball" rolling for modern chemistry! • Each element was composed of the same kind of atoms. • Each element was composed of different kinds of atoms. • Compounds are composed of atoms in specific ratios. • Chemical reactions are rearrangements of atoms (mass is conserved).

9. Joseph John Thompson (1897) Plumb Pudding Model: In 1903, J. J. Thomson proposed a subatomic model of the atom. The model pictured a positively-charged atom containing negatively charged electrons. Thomson visualized electrons in homogeneous spheres of positive charge in a way that was analogous to raisins in English plum pudding Thus, the Thomson proposal became popularly know as the plum pudding model of the atom. This model was not correct, but it was consistent with the evidence at the time.

10. Ernest Rutherford Solar System Model: Ernest Rutherford discovered that the atom is mostly empty space with a dense positively charged nucleus surrounded by negative electrons.  Rutherford received the Nobel Prize in chemistry in 1908 for his contributions into the structure of the atom.  In 1913 Neils Bohr proposed that electrons traveled in circular orbits and that only certain orbits were allowed.  This model of the atom helped explain the emission spectrum of the hydrogen atom.  He received the Nobel Prize in physics in 1922 for his theory.

11. Solar System Model

12. The Bohr Model 1913, Bohr speculated that electrons orbit around the atomic nucleus just as planets circle around the Sun. He suggested that the electron orbits were at a fixed distance from the nucleus and had a definite energy. The electron was said to travel in a fixed-energy orbit that was referred to as an energy level.

13. The Bohr Model

14. Current Model of the Atom The Quantum-Mechanical Model: According to this model of the atom, the positively charged protons and the neutral neutrons are still located in the nucleus of the atom.   The electrons, no longer thought of as locked into "fixed“ orbits, are collectively located in an area called the electron cloud.  These electrons, moving at extremely high speeds, effectively occupy the entire area of the cloud.

15. Electron Cloud

16. Current Model of the Atom

17. Atoms and Matter Atoms are the basic building blocks of matter.

18. Resources http://science.howstuffworks.com/atom1.htm http://www.csmate.colostate.edu/cltw/cohorpages/viney/atomhistory.html http://wind.cc.whecn.edu/~mechalke/chapter5/Chapter5.htm http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson33.htm