ATOMIC THEORY

Defining the Atom An atom is the smallest particle of an element that retains its identity in a reaction. The basic building blocks of matter that make-up everyday objects.

DemoCritus Early Greek Scholar-was the first to suggest the existence of atoms He believed that atoms were indivisible and indestructible. Never developed a theory  lacked experimental support

Dalton’s Atomic Theory Put Democritus’s ideas into a scientific theory. 1) All matter is composed of tiny indivisible particles called atoms. 2) Atoms of the same element are identical. 3) Atoms of different elements can mix together to form compounds 4) Chemical reactions occur when atoms are separated, joined, or rearranged.

Dalton’s Theory Revised Most of Dalton’s theory is still accepted today EXCEPT Atoms can be broken down into 3 subatomic particles: electrons, protons and neutrons. Atoms of the same element may have different masses. (Isotopes)

J.J. Thomson Used a cathode ray tube to prove the smallest particles present must have a negative charge. He discovered the ELECTRON! http://www.youtube.com/watch?v=RW_zfKOU9uM&feature=related

J.J. Thomson Atoms were electrically neutral, so there must be a + particle to cancel out the – charge from the electron. Developed the Plum Pudding Model (positive ball containing scattered electrons)

Ernest Rutherford Former student of Thomson, disproved the Plum Pudding Model of the atom. The Gold Foil Experiment: Sent a beam of + charges (alpha particles) through a piece of very thin gold foil. Angles of deflection were measured.

Rutherford Results: Most of the alpha particles passed straight through, most of the foil must be regions of “empty” space – not a + sphere like Thomson believed. + charges and the atoms mass must be found in the center  discovered the nucleus

The Nuclear Atom In Rutherford’s atomic model, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atoms.

ATOMIC STRUCTURE Particle Charge Mass proton + ve charge 1 neutron No charge 1 electron -ve charge nil

number of electrons = number of protons ATOMIC STRUCTURE He 2 Atomic number the number of protons in an atom 4 Atomic mass the number of protons and neutrons in an atom number of electrons = number of protons

HELIUM ATOM + - + - Shell proton neutron electron Are atoms electrically neutral? Why?

Complete the following table in your notes Atomic # Mass # # of Protons # of Neutrons # of Electrons 9 10 14 15 47 22 55 25

Isotopes Atoms that have the same number of protons but different numbers of neutrons Different mass numbers Chemically alike because they have identical numbers for the characteristic chemical behavior of each element Ex: Three known isotopes for H Hydrogen (no neutrons, mass # of 1) Deuterium (one neutron, mass # of 2) Tritium (two neutrons, mass # of 3)

Your turn… Two of the isotopes for Carbon are Carbon-12 and Carbon-13, write the chemical symbol for both 6C12 6C13 Three isotopes for Oxygen are oxygen-16, oxygen-17, and oxygen-18, write the chemical symbol for all three 8O16 8O17 8O18

Calculating Average Atomic Mass of Isotopes In nature, isotopes occur in various percentages In order to figure out the average mass of each element the percent abundance and mass of each isotope need to be considered We can calculate average atomic mass in much the same way as we calculate your grade in this class…

What are the different categories that you are graded on in this class? Classwork: 79% Practice: 7% Final: 14% What would your semester grade be if you received an 80% for classwork, 50% for practice, and 72% on your final? 0.80 x 79 = 63.2 0.50 x 7 = 3.5 0.72 x 14 = 10.1 Add all answers together to get % semester grade: 63.2 + 3.5 + 10.1 = 76.8 % (a C)

Now lets try with an element! Copper has two isotopes: copper-63 and copper-65. The relative abundances of these isotopes are 69.2% and 30.8% respectively. Calculate the average atomic mass of copper. 0.692 x 63 = 43.60 0.308 x 65 = 20.02 43.6 + 20.02 = 63.62 Take rel. abundances divided by 100 Multiple by isotope mass Add to get average atomic mass

One more example… Uranium has three naturally occurring isotopes with the following percent abundances: U-234 (0.0058%), U-235 (0.71%), and U-238 (99.23%). What do you expect the average atomic mass to be and why? What is the average atomic mass? 237.85

ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom (idea developed by Neiels Bohr) first shell a maximum of 2 electrons second shell a maximum of 8 electrons third shell a maximum of 8 electrons

SUMMARY The Atomic Number of an atom = number of protons in the nucleus. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. The number of Protons = Number of Electrons. Electrons orbit the nucleus in shells. Each shell can only carry a set number of electrons.

1. Electronic Configuration ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1. Electronic Configuration 2. Dot & Cross Diagrams

ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Nitrogen configuration = 2 , 5 7 2 in 1st shell 5 in 2nd shell N 2 + 5 = 7 14

ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; 20 11 8 Na O Ca a) b) c) 16 23 40 2,8,8,2 2,8,1 2,6 17 14 5 Cl Si B d) e) f) 11 35 28 2,8,7 2,8,4 2,3

N DOT & CROSS DIAGRAMS Nitrogen With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; X Nitrogen N 7 X X N X X 14 X X

DOT & CROSS DIAGRAMS O Cl Draw the Dot & Cross diagrams for the following elements; X 8 17 O Cl a) b) X 35 X 16 X X X X X Cl X X X X X X X O X X X X X X X X X X