Chapter #3 Atoms: The Building Blocks of Matter Chapter 3A ATOMS: West Valley High School General Chemistry Mr. Mata Chapter #3 Atoms: The Building Blocks of Matter

Standard 2a To compare and contrast the atomic number and atomic mass.

Essential Question Describe how atomic theory developed to give us our modern view of the atom.

3-1 Early Atomic Theory Atoms – smallest particle of matter. Democritus (400 B.C) stated world was made of atoms. (atomos = “indivisible”). Antoine Lavoisier (1800’s) discovered mass didn’t change after chemical rxn. Proposed “matter can be changed, but it cannot be created or destroyed “ (Law of Conservation of Mass).

Dalton’s Atomic Theory All matter composed of atoms. Atoms of same element identical in size, mass, properties; atoms of diff elements diff. in size, mass, properties. Atoms can’t be subdivided, created, destroyed. Atoms of different elements combine in whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged.

Modern Atomic Theory All matter is composed of atoms. Atoms of any one element differ in properties from atoms of another element. Element’s average mass unique to element. Atoms cannot be subdivided, created, or destroyed in ordinary chemical rxns. Changes CAN occur in nuclear rxns!

Section 3-2 Atom- smallest particle of element that retains chemical properties of element. Nucleus- positively charged, dense central portion of the atom; contains nearly all mass (~ 99.7%).

Subatomic Particles Electrons e- Negative charged particles. Found in electron clouds. Thomson (1897) Protons p+ Positively charged particles. Found in the nucleus. Rutherford (1918) Neutrons N No charge. Found in the nucleus. Chadwick (1932)

The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud). Most volume of atom=empty space.

Famous Scientist Scientist Experiment Name Conclusion JJ Thomson Cathode Ray Electron (-) Robert Millikan Oil Drop Mass of a single electron. Ernest Rutherford ______________ James Chadwick Gold foil _______________ Beryllium Detection Proton (+) _____________ Neutron (0)

Discovery of the Electron J.J. Thomson (1897) used a cathode ray tube to deduce the presence of a negatively charged particle. Discovered the – electron particle.

(1906)Thomson awarded the Nobel Prize in chemistry for discovery of the electron. The atom could be broken down into smaller particles.

Thomson’s Atomic Model Thomson believed electrons were like plums in a + charged “pudding”. He called it the “plum pudding” model.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei. Particles fired at a thin sheet of gold foil. Particle hits on screen (film) are detected.

Rutherford’s Findings Most particles passed right through screen. Few particles deflected. VERY FEW were greatly deflected. Conclusions: The nucleus is small. The nucleus is dense. The nucleus is + charged.

Section 3-3 Atomic number: number of protons in the nucleus of atom. # p(+) = # e(-) 6 C Carbon 12.011

Atomic Mass number of protons & neutrons in the nucleus. Atomic Mass=protons + neutrons Atomic Mass C = 12.011 Mass number = rounded atomic mass Mass Number C = 12

235 92 Nuclear Symbols Mass number (p+ + n) Atomic Mass (p+ + n) Element symbol Atomic number (# of p+) U 235 92

Hyphen Notation Sodium-23 (23 is the atomic mass) Sooo… 23- 11 (atomic #) = 12 for the # of neutrons. Atomic number of 11 is the # of protons (11) and electrons(11).