1. Atomic Structure and the Periodic Table
Ch. 5

2. Atoms
5-1

3. Early Models of the Atom
Democritus – 4th century BC – world made up of empty space and tiny particles called atoms (atomos) ‘indivisible’
Hypothesized without using experiments

4. Dalton’s Atomic Theory
All matter is made of atoms
Atoms are indestructible and cannot be divided into smaller particles (later proved wrong)
All atoms of one element are exactly alike (later proved wrong), but they are different from atoms of other elements
Atoms combine in simple whole number ratios to form compounds
In chemical reactions, atoms combine, separate, or are rearranged.
Done in the 1800’s through experiments!

5. Structure of the Nuclear Atom
5-2

6. Atomic Structure 1. Electron: (e-)
Discovered by Thompson’s cathode-ray tube (1897)
Cathode Ray = positive and negative electrodes connected, beam travels from cathode (-) to anode (+)
Found electrons have negative charge
Beam in cathode ray attracted to anode (+), must be negatively charged.
Relatively no mass (experiment by Millikan 1916)

7. Thompson’s Cathode-Ray Experiment

8. Proton – has positive charge (p+), greater mass than an electron
- Discovered by Goldstein in 1886
Neutron – has neutral charge (no), equal in mass to proton
-Discovered by Chadwick in 1932

9. -Discovered by Rutherford’s gold foil experiment in 1911
4. Nucleus = small, positively charged central core, made up of protons and neutrons
-Discovered by Rutherford’s gold foil experiment in 1911
-Fired stream of positive particles at gold foil, most passed right through (atom mostly empty space) while a few bounced off (very small positive nucleus at center)

10. Play movie

11. Development of Modern Atomic Theory
Law of conservation of matter (mass) = in a chemical reaction matter is neither created nor destroyed.
All elements are recycled/rearranged!!
Law of definite proportions = elements within a compound are always in certain proportions by mass
Ex: NaCl (table salt) is always
39% Na and 61% Cl.

12. Distinguishing between Atoms
5-3

13. Atomic Number Atomic # = Number of protons in the nucleus of an atom
atomic # = p+
Determines the identity of the element
Periodic table is organized by atomic number
Ex: What is the atomic number of…
He ______ C ______
N ______ O ______
Also! # of protons = # of electrons
p+ = e-
Ex: How many electrons does Cl have? _______

14. Why do we not include electrons???
Mass Number
Mass # = Sum of the protons and neutrons
mass # = p+ + no
Why do we not include electrons???
neutron = mass # - atomic #
Periodic Table symbols: 12 C 6
Mass # (rounded)
Element
Atomic #

15. Let’s Practice! How many? Protons ____ Neutrons ____ Electrons ____
Write it: ? C
Protons ______
Neutrons ______
Electrons ______ S

16. Remember!!! Mass # = p+ + n Atomic # = p+ e- = p+
n = mass # - atomic #

17. Atomic Mass + Isotopes Atomic Mass = weighted average mass of isotopes
Isotopes = one element with same proton # but different neutron #
Carbon has 3 possible isotopes:
C C C
Carbon-14, Carbon-13, Carbon-12

18. Try it! Ex: Isotope of Chlorine: 37 Cl 17 How many? Protons ______
Neutrons ______
Electrons ______

19. Atomic Mass Unit
Atomic Mass Unit (amu) = 1/12 the mass of a carbon-12 atom.
Used to compare element’s masses to a standard.
Units for measuring atom’s masses

20. The Periodic Table: Organizing the Elements
5-4

21. Development of the Periodic Table
In 1869, Mendeleev created Periodic Table of Elements
Organized it by increasing atomic mass and elements with similar properties in same columns.
Able to predict future elements properties
In 1913, Moseley organized it by atomic number.

22. Modern Periodic Table Organized by atomic number
Periodic Law: grouped according to physical and chemical properties
Period = horizontal row of periodic table; numbered down the side, 7 periods
Group = vertical column, also called a family; 18 groups

24. Group Designations Representative elements = Group 1,2,13-18
Group 1 – Alkali metals
Group 2 – Alkaline earth metals
Group 17 – Halogens
Group 18 – Noble gases
Transition metals = groups 3-12

25. Physical States of Elements
Periodic Table shows the states of the elements at room temperature and normal pressure
Most are solids
Only two are liquids
Gaseous elements are in the top right hand corner except for Hydrogen
The rest are made synthetically (man-made)through nuclear reactions (43, 61, 85, 87, and > 93)

26. Classifying Elements
Metals - majority of elements, occupy left side and center
Nonmetals - occupy top right hand corner, (includes H)
Metalloids - along the boundary

27. Metals
Have luster, many colors, conduct heat and electricity, usually bend w/o breaking (malleable)
Almost all are solids at room temp. and have extremely high melting points

28. Nonmetals Are abundant in nature Oxygen and Nitrogen: 99% atmosphere
Carbon in more compounds than all others combined!
Poor conductor of heat and electricity, brittle when solid
Many gases at room temp, low melting point

29. Metalloids Have properties of both metals and nonmetals
7 Metalloids: B, Si, Ge, As, Sb, Te, Po (touch staircase, no Al + At)
Some are semiconductors = element that does not conduct electricity as well as metal, but does conduct slightly better than nonmetal
Ex: Silicon – used in computer industry