Models of the Atom A Historical Perspective CHEMISTRY J.P. HANCOCK, B.S., M.Ed

EVOLUTION OF ATOM

Early Greek Theories fire air water earth Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely. Derived from the greek word atomos=uncuttable fire air water earth Aristotle 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.

DALTON’S ATOMIC THEORY 1800 –John Dalton proposed a modern atomic model based on experimentation not on pure reason. 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. 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).

Size of an Atom A pure copper coin the size of a penny has 2.4x1022 atoms Earth’s population is 6x109 people 4x1012 times as many atoms in the coin as people on Earth Radii of most atoms 5x10-11m to 2x10-10m

History of Current Atomic Model John Dalton’s Model(1803) English chemist and school teacher (1776-1844) Studied ratios of atoms in chemical reactions Atom was solid and indivisible “Billiard Ball” model Eugen Goldstein 1886 Discovered protons J.J.Thomson’s Model (1897) New Zealand physicist Atom is positive sphere with negative particles embedded throughout. Plum Pudding Model-like a plum pudding or chocolate chip cookie

JJ Thomson’s Experiment Passed an electric current through low pressure gases sealed in glass tubes with electrodes at both ends Anode + Cathode - Caused a glowing beam(cathode ray) Electrons are discovered Opposites attract!!

Atom has central nucleus Hantaro Nagaoka(1904) Japanese physicist Atom has central nucleus Electrons move around in orbits like rings around Saturn Rutherford Model (1911) called the Nuclear Atom Most of the atom’s mass was in a positive central sphere surrounded by negative electrons Protons and neutrons are located in nucleus Electrons are around the nucleus and occupy almost all volume of atom

Ernest Rutherford (movie: 10 min.) Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles 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.

Bohr’s Model-1913 Niels Bohr Danish Physicist & student of Rutherford Electrons orbit the nucleus in “shells” Electrons can be bumped up to a higher shell if hit by an electron or a photon of light called a Quantum of Energy Robert A. Millikan-1916 U.S. Physicist Calculated mass of electron Louis de Broglie-1923 French Physicist Electrons have some properties of waves as well as particles

Quantum Mechanical Model 1926 Erwin SchrÖdinger Austrian physicist(1887-1961) Used mathematical equations to determine where electrons are in the atom Electron Cloud Model- “probability cloud” Determined the energies of electrons and how likely to find their location around nucleus James Chadwick-1932 English physicist Confirmed existence neutrons No charge with a mass near the proton’s

THE ATOM

THE ATOM Basic building block of all matter Elements on periodic table are represented as one neutral atom Chemical symbols are abbreviated ways of representing elements C, H, N, O Na, Cl, Ne, Cu NaCl, H2O, CO2

Parts of the Atom Protons Neutrons Electrons Quarks= smallest particles arranged in groups of threes to make p+, e-, or n0

6 Types of Quarks

Properties of Subatomic Particles Particle Symbol Charge Relative Actual Mass Mass Electron e- 1- 1/1840 9.11x10-26 g Proton p+ 1+ 1 1.67x10-24 g Neutron n0 0 1 1.67x10-24 g

Location of Subatomic Particles 10-13 cm electrons protons neutrons 10-8 cm nucleus LecturePLUS Timberlake

1st Periodic Table Dmitri Mendeleev- a Russian chemist who arranged elements in order of their increasing atomic masses

Henry G.J.Moseley determined the atomic number for each known element Modern Periodic Table Father of Modern Periodic Table 1913 British Physicists Henry G.J.Moseley determined the atomic number for each known element Arranged the periodic table according to increasing atomic numbers

The Periodic Table

Periodic Table Arrangement of atoms in increasing order of atomic numbers Represents physical and chemical behavior of elements Collects atoms with similar properties in vertical columns LecturePLUS Timberlake

Properties of elements in any vertical column do differ Properties of elements in any vertical column do differ. The properties are similar but not identical in chemical behavior. Adjacent elements in each horizontal row(period) differ in both chemical and physical properties but the properties change in a regular way across each period.

Vertical columns = families or groups 1-18 Have similar properties due to similar electron arrangements Horizontal rows= periods or series Elements to the left of the “stair step” line are metals except hydrogen Elements to the right of the “stair step” line are non metals

Why do elements in a group have similar properties? Similar electron arrangements While atoms of different elements have diff. # of e-. The atoms of diff. e- may have the same # of e- in their outer energy levels.

The number of e- in the outer energy level determines the chemical properties of the element i.e. different elements w/ the same # of e- in the outer energy levels will have similar chemical properties!

Electron Cloud Structure The goal of every atom is stability. OCTET RULE An atom is stable when it has 8 valence electrons(a full outer energy level) Hydrogen and Helium are the exceptions to the rule The valence electrons will determine the reactivity of the element Valence electrons are electrons in the outermost energy level

PERIODIC TABLE

Atomic Symbols Atomic Number Symbol Element Name Atomic Mass Number

Atomic Number The number of protons in the nucleus of an atom All atoms of an element have the same number of protons LecturePLUS Timberlake

Number of Electrons An atom is neutral The net charge is zero Number of protons = Number of electrons Atomic number can be used to infer the Number of electrons LecturePLUS Timberlake

Mass Number (protons + neutrons) Approx. equal to the mass of the nucleus in atomic mass units(amu). Whole number of the amu

Number of Neutrons (mass number – atomic number) In a neutral atom the number of e- and number of p+ are equal Every atom of the same element has the same number of protons MASS NUMBER =ATOMIC MASS

Atomic Mass Units Do we now know how much atoms really weigh, in pounds or grams? Yes, but we don't normally use those units for measuring the mass of an atom. It's much more convenient to use something called the atomic mass unit, or amu.

Using AMU is simplier! One gram is about 600,000,000,000,000,000,000,000 amu (that's 600 sextillion, or a 6 followed by 23 zeros) A pound is just shy of 300 septillion amu that’s, 300,000,000,000,000,000,000,000,000 (3 followed by 26 zeros)

Na 22.99 Atomic Mass Unit Listed on the periodic table Gives the mass of “average” atom of each element compared to 12C Average atom based on all the isotopes and their abundance % Atomic mass is not a whole number 1amu= 1.6606 x 10-24 grams LecturePLUS Timberlake

RECAP

Isotopes Atoms with the same number of protons, but different numbers of neutrons. atomic number is the same mass number is different Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37 LecturePLUS Timberlake

Protium Deuterium Tritium ISOTOPES Protium Deuterium Tritium

Calculating # of Neutrons Protons + Neutrons = Mass Number   6p+ + 6no = 12, the mass number of the first isotope of carbon 6p+ + 7no = 13, the mass number of the second isotope of carbon 6p+ + 8no = 14, the mass number of the third isotope of carbon

Chapter 4 Vocabulary page 121-(15 words plus the 11 below) Quantum Quantum Mechanical Model Bohr Model Electron Cloud Model Thomson Model Dalton Model Rutherford Model Stability Octet Rule Orbital Energy Levels