2. Early models of the atom
The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms.
He believed that atoms were indivisible and indestructible.

3. Early Models of the atom
John Dalton ( )
Using experimental methods, Dalton transformed the ideas of Democritus into a scientific theory.

4. Early Models of the atom
Dalton’s Atomic Theory
1.) All elements are composed of tiny indivisible particles called atoms.
2.) Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

5. Early Models of the atom
Dalton’s Atomic Theory
3.) Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
4.) Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

6. Modern Atomic Theory The essence of Dalton’s theory remains valid
Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions
There are also different kinds of atoms within an element known as isotopes

7. ATOMS
Atoms. An atom is the smallest particle of an element that retains its identity in a chemical reaction.

8. ATOMS Atoms are very small An atom has 3 kinds of particles:
Protons (positive charge), neutrons (neutral), electrons (negative charge)
Protons and neutrons are located in the atomic nucleus (center of atom)
Electrons are located around the nucleus in an electron cloud and occupies almost all of the volume of the atom.

9. What forces keep an atom together?
4 Fundamental Forces
Gravitational force of nature
Electromagnetic force of nature
Strong nuclear force of nature
Weak nuclear force of nature
Stable nucleus stays together

10. Electromagnetic Forces
Oppositely charged particles attract each other, while like particles repel one another.
Electrons are kept in the orbit around the nucleus by the electromagnetic force, because the nucleus in the center of the atom is positively charged and attracts the negatively charged electrons.

11. Strong Force
The strong forces oppose the electromagnetic force of repulsion between protons. Like ”glue” the strong force keeps the protons together to form the nucleus.
The strong forces and electromagnetic forces both hold the atom together.

12. Weak forces
     Weak forces are important because they are responsible for stabilizing particles through the process of radioactive decay.

13. Gravitational Force
Gravity is the force of attraction exerted between all objects in nature.
 Gravity is most easily observed in the behavior of large objects.  Inside the tiny nucleus of an atom, the effect of gravity is small compared to the effects of the other three forces.

14. Subatomic Particles Properties of Subatomic Particles Particle Symbol
Relative Charge
Relative Mass
(mass of proton = 1)
Actual Mass
Electron e- 1-
1/1840
9.11 x 10-28
Proton p+ 1+ 1
1.67x 10-24
Neutron n0

15. Elements
Elements are the simplest form of matter that has a unique set of properties.
Cannot be broken down into simpler substances by chemical means.

16. How many known elements are there?

17. Distinguishing Among Atoms
Elements are different because they contain different numbers of protons.
The atomic number of an element is the number of protons in the nucleus of that element.
Atomic number
Identifies an element.

18. Atomic number Element Atomic Number Number of Protons Hydrogen (H) 1
Beryllium (Be) 4
Carbon (C) 6
Nitrogen (N) 7
Neon (Ne) 10

19. Atomic number
Atoms are electrically neutral therefore number of electrons= number of protons.
Ions- have unequal number of protons and electrons (+/-)
Element
Atomic Number
Number of Protons
Number of Electrons
Hydrogen (H) 1
Beryllium (Be) 4
Carbon (C) 6
Nitrogen (N) 7
Neon (Ne) 10

20. Mass number
Remember that most of the mass of an atom is found in the nucleus (protons + neutrons).
Mass number= # of protons + # of neutrons
How can we determine the number of neutrons?

21. Protons= atomic number
Electrons= atomic number (if atom has no charge)
Neutrons= atomic mass – atomic number

22. Element
Atomic Number
Mass number
Number of
Protons/ Electrons
Number of Neutrons
Hydrogen (H) 1
Beryllium (Be) 4 9
Carbon (C) 6 12
Nitrogen (N) 7 14
Neon (Ne) 10 20

23. Ions and Isotopes

24. Ions An atom or molecule that has a charge.
Loss of electrons  Positive charge (cation)
Gain of electrons  Negative charge (anion)

25. Ions Ion Atomic # Symbol Cation or anion? Number of Protons
Number of Electrons
Hydrogen ion 1
Fluoride ion
F-1
Oxide
ion 10
Calcium ion
Ca+2
Aluminum ion

26. What is different between the three isotopes of hydrogen?

27. Isotopes
Isotopes= atoms of the same element that have the same atomic number but different atomic masses due to a different number of neutrons.

28. Atomic Mass Units
Today it is possible to determine the mass of an element using a mass spectrometer.
However, numbers are small and impractical to work with. (i.e. Mass of F= x g)
It is easier to use relative masses of atoms using a reference isotope as a standard.

29. Atomic Mass Units
The reference isotope= Carbon 12. This isotope is assigned a mass of exactly 12 amu.
Atomic mass units= 1/12 of the mass of a Carbon-12 atom.

30. Atomic Mass
In nature, most elements occur as a mixture of two or more isotopes.
Atomic mass= the weighted average mass of the atoms in a naturally occurring samples of the element.
Takes into account the relative abundance

32. Atomic Mass
To determine atomic mass, you must know the number of isotopes, the mass of each isotope and the percent abundance of each isotope.
Atomic mass= multiply the mass of each isotopes by natural abundance and add the products.

33. Atomic Mass
Atomic mass= multiply the mass of each isotopes by natural abundance and add the products.

34. Atomic Mass of Carbon
Example: Carbon has 2 isotopes: Carbon-12 which has a natural abundance of 98.89% (0.9889) and Carbon-13 which has a natural abundance of 1.11% (0.0111).
Atomic Mass of carbon=

35. Atomic Mass of Magnesium
Magnesium has three naturally occurring isotopes % of Magnesium atoms exist as Magnesium-24 ( g/mol), 10.03% exist as Magnesium-25 ( g/mol) and 11.17% exist as Magnesium-26 ( g/mol). What is the average atomic mass of Magnesium?
Atomic Mass of Magnesium=

36. Atomic Mass of Lithium
What is average atomic mass of Lithium if 7.42% exists as Li-6 (6.015 g/mol) and 92.58% exists as Li-7 (7.016 g/mol)?
Atomic Mass of Lithium=