The Atomic Theory

Dalton Relied on the premise that their existed a different kind of atom for each element Key points for Dalton’s Model of the Atom: Elements are composed of indivisible particles called atoms Atoms of same element are alike in size and mass Atoms of different elements have different sizes and masses Compounds form when two or more atoms of different elements combine Atoms combine in simple whole number ratios; 1:1, 1:2, 2:3, ect… Atoms of two elements may combine in different ratios to form different compounds: CO, CO2

Exceptions to Dalton Atoms consist of subatomic particles Atoms of the same element may have different masses Under specific circumstances, atoms are able to be broken down

Composition of Compounds Compounds contain the same elements in the same proportion, by mass Example: H2O Water is a 2:1 ratio of hydrogen atoms to oxygen atoms, always It is 11.2 % hydrogen to 88.8% oxygen by mass Hydrogen and oxygen may combine in another ratio however it would not form water: H2O2 is 5.9 % H to 94.1% O by mass

Law of Definite Composition A compound always contains two or more elements combined in a definite proportions by mass

Law of Multiple Proportions Atoms of two or more elements may combine in different ratios to produce more than one compound

Electric Charge Positive and negative charge Charge may transfer Contact Induction Unlike charges attract Force of charges increases as distance between the charges decreases

The Ion Farday and Arrhenius Conduct electricity when dissolved in water Ions are atoms with a positive or negative charge Cation: positively charged atom Anion: negatively charged atom How does this happen? Stoney and Thompson show existence of the electron: negatively charged subatomic particle in the atom

Subatomic particles Electron e- Negatively charged subatomic particle Mass of 9.110 x 10-28 g 1/1837 the mass of the hydrogen atom Electrical charge is -1

Subatomic particles Proton Goldstein and Thompson Mass is ~1837 x that of an electron 1.673 x 10-24 g p+ Equal in magnitude to the electron but opposite in charge

Modifications to Dalton’s Model Thompson offers new theory to Atomic model Although it seemed as if the atom was indivisible, it is now clear that it is composed of subatomic particles This clearly negates parts of Dalton’s Model of the Atom Modifications were suggested by Thompson

Thompson’s Model of the Atom Electrons are embedded in the atomic sphere Atoms are neutral therefore they must contain an equal number of positive protons Ions are formed by gaining or losing electrons Cations form by losing electrons Group I forms 1+ ions by losing 1 electron Group 2 forms 2+ ions by losing 2 electrons Group 3 forms 3+ ions by losing 3 electrons Anions form by gaining electrons Nitrogen group forms 3- ions by gaining 3 electrons Oxygen group forms 2- ions by gaining 2 electrons Halogens form 1- ions by gaining 1 electron

Subatomic Particles Neutron Chadwick Neutral n◦ 1.675 x 10-24 g

Dimensional Analysis and the mass of an atom Be familiar with the example problems on page 89, at the bottom…

Rutherford and the Nucleus of the Atom Rutherford used the radioactive alpha particles discovered by Becquerel to establish the nature of the nucleus of the atom He directed the positively charged He atoms (alpha particles) at a piece of gold foil Most passed through Some were deflected A few bounced back Rutherford was able to reason that their must be a positive portion in the center of the atom because like repels like This portion was referred to as the nucleus Most of the atom is empty space

Mass of an atom? Mass of the atom is usually referred to as the nucleus of the atom The nucleus contains protons and neutrons 99.9% of the total mass of the atom Mass of electrons is mostly negligible

The Neutral Atom Atoms consist of a dense portion called the nucleus The nucleus contains the positively charges protons and the neutral neutrons Electrons are found outside the nucleus The number of protons in a neutral atom equal the number of electrons

Elements and their Numbers Atomic number is the whole number on the peridic table that increases from left to right The atomic number = the number of protons in the nucleus of an atom Atomic number is unique for each atom as it determines the identity for each element Atomic # 1 = Hydrogen , 2 = Helium, 3 = Lithium

In a neutral atom, the number of protons will = the number of electrons The mass number = the number of protons + the number of neutrons The number of neutrons = mass number – the number of protons

Isotopes Atoms of the same element have the same atomic number If the same atoms have the same atomic number but different mass numbers, they are isotopes Atoms of elements with the same number of protons and electrons but different neutrons are called isotopes

Proper Notation A number is mass number ( protons + neutrons or nucleons) Z number is number of protons (atomic number)

A = Z + N N= A – Z Remember: A = number or nucleons or protons plus neutrons…A is the mass number Z is the number and N is the number of neutrons

Example Protons = 79 Electrons = 79 Neutrons = 197 – 79 = 118 Nucleons= 197

Atomic Mass Read section in Chapter and walk through example problems on pages 96 and 97.