Atomic Structure & Past Atomic Models Chapters 5 & 13

Atoms Atoms are the basic unit of matter Atoms are so small, that billions can fit into a period at the end of a sentence Atoms aren’t solid, they are made of various subatomic particles

Atomic Structure Nucleus: massive center of the atom, holds the protons and neutrons Proton: positively charged particle, mass = 1amu

Atomic Structure Neutron: has NO charge (neutral), mass = 1amu Electron: orbits the nucleus, negatively charged, very small mass

Atomic Structure Energy Level: where an electron can be found (e- orbital, e- shell) Ion: an atom with a charge, more or less e- than a neutral atom

Atomic Structure Atomic Number: # of protons Mass Number: # of protons plus number of neutrons (always whole number) Isotope: atom with more or less neutrons but the same # of protons, mass # is different, atomic # will stay the same

Atomic Structure Atomic Mass: the weighted average of all the isotopes of an element (usually a decimal) The mass number of the most abundant isotope is usually the atomic mass rounded to a whole number

Atomic Structure To find the number of neutrons without a mass number, round the atomic mass to the nearest whole number, then subtract the # of protons

Atomic Structure A neutral atom will have the same number of electrons as it does protons

Atomic Structure Fill in the Chart: (assume neutral) Name Symbol Mass # # P # e #N carbon 6 8 He 4 2 14 7

Atomic Structure Name Symbol Atomic # Mass # # P # e #N carbon C 6 14 8 helium He 2 4 oxygen O 16 nitrogen N 7

“Formulas” to Remember Atomic number = # protons Mass number = # protons + # neutrons # neutrons = mass # - # protons Charge = #protons - # electrons # electrons = # protons - charge

Average Atomic Mass Average atomic mass is the weighted average of all the isotopes of that element To calculate, you need to do a weighted average calculation Multiply the mass numbers by the percent abundances and add them all together – do NOT divide by the total number!!

Average Atomic Mass Avg. Atomic Mass calculation: Suppose we have element X, with two isotopes, X-200 (85%) and X-202 (15%). What is the atomic mass of X? (200)(.85) + (202)(.15) = 170 + 30.3 = 200.3 amu

Average Atomic Mass amu is the unit of atomic mass 1 amu is equal to 1/12 of a carbon-12 atom We cannot use grams as our unit when speaking of a single atom, but we will use grams when speaking of a much larger quantity of atoms (mole) later on

Early Models & Atomic Theory Democritus – thought atoms were indivisible & indestructible Lacked experimental support 4th century B.C.

Early Models & Atomic Theory John Dalton – (1766-1844) Dalton’s Atomic Theory All elements composed of tiny, indivisible atoms Atoms of same element are identical. Atoms of 1 element are different from another element

Early Models & Atomic Theory Dalton’s Atomic Theory (cont) Atoms of different elements can either physically mix or chemically combine in simple, whole number ratios to form compounds

Early Models & Atomic Theory Dalton’s Atomic Theory (cont) Chemical Rxn’s occur when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical rxn.

Early Models & Atomic Theory Dalton’s Theory……… Was tested experimentally Dalton formulated hypotheses to explain his observations

Early Models & Atomic Theory Dalton’s Theory is mostly accepted today Except: we now know atoms CAN be divided - into subatomic particles AND – not all atoms of the same element are exactly identical (ions & isotopes)

A Limerick of 5 Hypotheses The Atom: A Limerick of 5 Hypotheses They make up all the matter you’ll find And each element has its own kind They wont mix in a fraction; (For a chemical reaction) For reactions, they’re just realigned! But Dalton, that Brit, Thought they could not be split. Now we know they have 3 bits combined!

Early Models & Atomic Theory JJ Thompson (1856-1940) discovered electrons using a cathode ray tube passed electric current through gases at low pressure

Early Models & Atomic Theory A glowing beam formed between the 2 electrodes Called the cathode ray

Early Models & Atomic Theory The cathode ray is attracted to metal plates that have a positive charge Negatively charged plates repelled the ray

He called these particles electrons Atomic Structure The ray must be negatively charged particles moving at a high speed He called these particles electrons

Atomic Structure JJ Thomson’s model of the atom is called the plum-pudding model – the electrons were randomly placed throughout the atom

Rutherford Ernest Rutherford discovered the nucleus He had a thin piece of gold foil & bombarded it with alpha particles (helium nucleus) Most particles passed through the foil like he expected Some were deflected slightly

Other particles bounced back at him! He was as surprised as if he had sent a bowling ball at a tissue and it came back to hit him He figured that there must be a dense positive part of the atom, but most of the atom is empty space

Bohr’s Model Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus

Bohr’s Model The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of NRG to move from one to another

Review Dalton – thought atoms were solid and indivisible JJ Thomson – discovered the electron, & made the plum-pudding model

Review Rutherford – discovered the nucleus Bohr – proposed electrons in orbitals around nucleus

Evolution of Atomic Models: Summary First were Dalton’s atomic 5 traits. Then came Thompson’s e- Pudding update Nucleus found by Rutherford. But Bohr’s orbits, in other words, Won him free beer forever, how great!