Defining Atoms & Electrons in Atoms

Democritus ( BC) Originated idea of the atom

John Dalton ( ) 1803 Dalton’s Atomic Theory

Several changes have been made to Dalton’s theory. Modern Atomic Theory Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element.

Modern Atomic Theory Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

J.J. Thomson ( ) 1897 Discovered the electron (“plum pudding” model)

Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. The oil drop apparatus Mass of the electron is x kg

Conclusions from the Study of Electrons Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons. Electrons have so little mass that atoms must contain other particles that account for most of the mass

Rutherford ( ) 1911 Discovered the nucleus (gold foil experiment)

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected  The nucleus is small  The nucleus is dense  The nucleus is positively charged “Like howitzer shells bouncing off of tissue paper!” Conclusions:

Since opposite charges attract each other, why don’t the electrons fall into the nucleus?

Niels Bohr ( ) 1913 proposed Planetary model

Neils Bohr I pictured electrons orbiting the nucleus much like planets orbiting the sun. But I was wrong! They’re more like bees around a hive. WRONG!!! The Bohr Model of the Atom

Based upon the work of several men, a new mathematical model was developed to describe the structure of the atom The Quantum-Mechanical Model Louis de Broglie ( ) Werner Heisenberg ( ) Erwin Schrodinger ( )

Atomic Number Atomic number of an element is the number of protons in the nucleus of each atom of that element. Element Atomic ## of protons # of electrons Carbon6 Phosphorus15 Gold79 In a neutral atom: # electrons = # protons

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 of the volume of the atom is the electron cloud.

ParticleChargeMass (g)Location Electron e x (1/1840 amu) Electron cloud Proton p x (1 amu) Nucleus Neutron n x (1 amu) Nucleus Atomic Particles

Reading the Periodic Table Atomic Number 3 Li Lithium Element Symbol Element Name Atomic Mass # p + # e - (in a neutral atom) # p + + # n 0 # n 0 = Atomic Mass - #p +

The Quark… Oops… wrong Quark!

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 of the volume of the atom is empty space “q” is a particle called a “quark”

About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark. Neutrons are made of one “up” quark and two “down” quarks. Quarks are held together by “gluons”

Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

Mass Number Mass # = p + + n 0 Mass number is the number of protons and neutrons in the nucleus of an isotope.

Symbols of Isotopes Carbon-12 Atomic Mass C 12 6 Atomic Number 14 6 Carbon-14 Atomic Mass C Atomic Number Atomic Number Chlorine-35 Atomic Mass Cl

Mass Number Mass # = protons + + neutons 0 Element p+p+ n0n0 e-e- Mass # Oxygen Neon Arsenic Phosphorus Copper

Mass Number Mass # = p + + n 0 Isotope p+p+ n0n0 e-e- Mass # Oxygen Arsenic Phosphorus O188 As7533 P3115

IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) 110 Hydrogen-2 (deuterium) 111 Hydrogen-3 (tritium) 112 Isotopes…Again (must be on the test) Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

IsotopeSymbol Composition of the nucleus % in nature Carbon C 6 protons 6 neutrons 98.89% Carbon C 6 protons 7 neutrons 1.11% Carbon C 6 protons 8 neutrons <0.01% Atomic mass is the weighted average of all the naturally occuring isotopes of that element. Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Carbon = Atomic Masses

Isotope nameIsotope mass (amu)percentage Silver Silver remainder Silver has two naturally occurring isotopes: Find the missing percentage. Find the average atomic mass of an atom of silver.

Silicon has three naturally occurring isotopes: Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28, 29, or 30? Find the average atomic mass of silicon. Isotope nameIsotope mass (amu)Relative Abundance Silicon Silicon Silicon

Iron has four naturally occurring isotopes: Estimate the average mass. Find the average atomic mass of iron. Isotope nameIsotope abundanceIsotope mass (amu) Iron %53.94 Iron %55.93 Iron %56.94 Iron %57.93

The Periodic Table Period: horizontal rows of the periodic table. Group or Family: vertical columns of the periodic table. Elements within a group have similar chemical and physical properties.

Period (rows) Group or Family (columns)