1. The Atomic Theory

2. 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

3. 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

4. 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

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

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

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

8. 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

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

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

11. 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

12. 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

13. Subatomic Particles
Neutron
Chadwick
Neutral n◦
1.675 x g

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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

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

22. 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

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

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