Atomic Structure Ch. 3

(don’t write this page) Early Philosophers Democritus – 400 BC Thought matter was made of individual, indivisible particles – “atomus” Aristotle Disagreed with Democritus Thought matter was continuous His idea was accepted until late 1700s

1790’s – Better analysis begins with improved equipment.

Dalton’s Theory Matter is made of atoms. Atoms of the same element are identical. Atoms cannot be divided. Atoms combine in whole number ratios to form compounds. Atoms are rearranged, separated, or combined in chemical reactions. Wrong – Isotopes!! Wrong – Nuclear Reactions!!

Modern Atomic Theory Atoms of the same element can be different; called isotopes. Atoms can be split.

Discovery of the Electron JJ Thomson used cathode ray tubes to study electricity and matter

Observations Shadow was detected. The paddle wheel moved. Cathode rays were attracted and deflected by a magnet. Video on Cathode Ray Tube

Thomson’s ‘Plum Pudding’ model: Conclusions Electrons have mass, are negative, and are very small. Thomson’s ‘Plum Pudding’ model:

Discovery of the Nucleus Rutherford used the Gold Foil Experiment to study the atom Alpha particles are fired at the thin gold sheet.

Observations Some particles were deflected (1 in 8000). Most particles passed straight through.

Conclusions The nucleus is very small, dense, positive. The atom is mostly empty space. Rutherford’s model:

Neils Bohr Studied light – Hydrogen spectra – made up of particles (atoms) Conclusion: Electrons have specific amounts of energy and exist in specific energy levels (Quantum jumps) Bohr’s model:

Subatomic Particles Protons (p+) Neutrons (n0) Electrons (e-) Positively charged, found in nucleus Neutrons (n0) Neutral, found in nucleus Electrons (e-) Negative, found in electron cloud surrounding nucleus equals the number of protons in a neutral atom Negligible mass

Atomic Number Whole number on periodic table Indicates the number of protons Also indicates number of electrons in the neutral atom. EX: How many protons does Carbon have? Calcium? Uranium?

Mass Number Equals the number of protons plus neutrons (whole number) NOT the mass found on the Periodic Table Has to be provided to you by either Hyphen Notation Symbol (Nuclear) Notation EX: What is the mass number of Carbon-14? Uranium-235?

Summary Protons = Atomic number Electrons (in neutral atom) = Atomic number Neutrons = Mass number minus # of protons Mass number = Protons plus neutrons

Nuclear Symbol Notation Mass Number (p+ + n0) Atomic Number (number of protons) Protons? Neutrons? Electrons?

Hyphen Notation Chlorine – 35 protons? neutrons? electrons? Mass Number

Isotopes Elements have the same number of protons but different number of neutrons. Different mass numbers. Ex: Carbon-12 and Carbon-14 How many protons, neutrons and electrons? Which one is more common?

Average Atomic Mass Weighted average of all isotopes found in nature based on % abundance (amu) Found on PT – Helps identify most common isotope. (remember – mass number is not a fraction) Example: 99.985% of hydrogen found in nature is hydrogen-1 and 0.015% is hydrogen-2. This creates an average mass of 1.00794 amu.)

calculating Avg. Atomic Mass (mass # x %)____+____(mass # x %)… = 100 In nature: Carbon-12 is 98.89% Carbon -13 is 1.10 % Carbon-14 is 0.01 %