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 massHydrogen 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

Tuesday12/3/13

Bell Ringer

Pick up one of each of the three papers off the front demo table.

You also need your composition notebook for notes today.

How many protons, neutrons, and electrons are in a neutral atom of Carbon?

What is Carbon’s mass number? Atomic number? Atomic mass?

Schedule

1.Discuss Midterm

2.Atom Notes

HOMEWORK: Practice Problems

Chem

The train of failure usually runs on the track of laziness.

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 NotationA 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.