COMPOSITION OF MATTER

1. COMPOSITION OF MATTER

2. COMPOSITION OF MATTER 1. Atoms- building blocks of matter (smallest particles) - “ atomos” (uncut, indivisible/ indestructible, can’t be divided) 2. Molecules – “little quantity” - composed of 2 or more kinds of atoms

3. Models of the Atom a Historical Perspective

4. Discoveries about Atoms 1. Democritus “Matter is composed of small particles which are indivisible called atoms”. 2. Aristotle “ Matter is divided into smaller particles without limit”.

6. fire Democritus earth air water Aristotle Early Greek Theories • This led to the idea of atoms in a void. • 400 B.C. - Democritus thought matter could not be divided indefinitely. • 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. • Aristotle was wrong. However, his theory persisted for 2000 years.

7. John Dalton • All matter is made of atoms. • Atoms of an element are identical. • Each element has different atoms. • Atoms of different elements combine in constant ratios to form compounds. • Atoms are rearranged in reactions. • 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. • His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

8. Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos). These rays have a small mass and are negative. Thomson noted that these negative subatomic particles were a fundamental part of all atoms. Adding Electrons to the Model • John Dalton’s “Billiard ball” model (1800-1900) Atoms are solid and indivisible. • Joseph John Thomson “Plum pudding” model (1900) • Negative electrons in a positive framework. • The Ernest Rutherford model “Nuclear Atomic Model” (around 1910) • Atoms are mostly empty space. • Negative electrons orbit a positive nucleus.

10. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Ernest Rutherford (movie: 10 min.) • Rutherford shot alpha () particles at gold foil. Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

13. Electrons orbit the nucleus in “shells” 4.Neils Bohr’s model (Solar System Model) • Electrons can be bumped up to a higher shell if hit by an electron or a photon of light. There are 2 types of spectra: continuous spectra & line spectra. It’s when electrons fall back down that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).

14. 5. Electron Cloud Atomic Model • Electron cloud-a space (n-energy level) in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • They are not moving around in random patterns. • Location of electrons depends upon how much energy the electron has.

15. Electron Cloud: • Depending on their energy they are locked into a certain area in the cloud. • Electrons with the lowest energy are found in the energy level closest to the nucleus • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. 1st energy level (n-1) = 2 e–s 2nd energy level (n-1) = 8 e–s 3rd energy level (n-3) = 18 e–s 4th energy level (n-4) = 32 e–s nth energy level (n-5 ----) = 32 e–s

20. 8 E.g. Oxygen: O Atomic numbers, Mass numbers • There are 3 types of subatomic particles. We alreadyknowaboutelectrons(e–)&protons(p+). Neutrons (n0) were also shown to exist (1930s). • They have: no charge, a mass similar to protons • Elements are often symbolized with their mass number and atomic number 16 • These values are given on the periodic table. • For now, round the mass # to a whole number. • These numbers tell you a lot about atoms. • # of protons = # of electrons = atomic number • # of neutrons = mass number – atomic number • Calculate # of e–,n0, p+ for Ca, Ar, and Br.

21. Atomic Mass p+ n0 e– Ca 20 40 20 20 20 Ar 18 40 18 22 18 Br 35 80 35 45 35

22. 3 p+ 4 n0 2e– 1e– Bohr - Rutherford diagrams • Putting all this together, we get B-R diagrams • To draw them you must know the # of protons, neutrons, and electrons • Draw protons (p+), (n0) in circle (i.e. “nucleus”) • Draw electrons around in shells He Li Li shorthand 3 p+ 4 n0 2 p+ 2 n0 Draw Be, B, Al and shorthand diagrams for O, Na

23. Be B 4 p+ 5 n° 5 p+ 6 n° O Na 8 p+ 8 n° 11 p+ 12 n° 2e– 6e– 2e– 8e– 1e– Al 13 p+ 14 n°

27. Molecular Theory of Matter 1. All matter is made up of molecules. 2. Molecules are constantly in motion. (Diffusion, Brownian Movement)

28. a. Diffusion- spreading of molecules in a container or any available space. Solid- fast Liquid – faster Gas – fastest b. Brownian Movement– zigzag/random movement of molecules * *

29. 3. There are spaces between molecules. (Intermolecular Spaces) • Intermolecular Spaces – spaces in between molecules Solid- wide Liquid – wider Gas- widest

30. 4. Molecules attract one another. (Surface tension, capillarity –cohesion & adhesion) Intermolecular Attraction – attraction of molecules

31. a. Capillarity – rising of liquid into a narrow tube • Cohesion- attraction between same kind of molecules • Adhesion- attraction between different kinds of molecules b. Surface Tension- thin, elastic sheet on the surface the surface of the water