1. Ch. 5 Atomic Structure and the Periodic Table
Ch Atoms
Ch Structure of the Nuclear Atom
Ch Distinguishing between Atoms
Ch The Periodic Table: Organizing
the Elements

2. Ch. 5.1 Atoms Early models of the atom Democritus (4th century BC)
John Dalton ( )
Dalton’s atomic theory
All elements are composed of tiny indivisible particles
Atoms of the same element are identical, but different from any other element
Atoms of different elements can physically or chemically combine in simple whole-number ratios to form compounds
Atoms of each element are never changed in a chemical reaction

3. Ch. 5.2 Structure of the Nuclear Atom
Sub-atomic particles
Electrons
Negatively charged particles
Discovered and named by J.J. Thomson in 1897 who used a cathode ray tube to pass an electric current through a gas at low pressure
Cathode rays travel from a cathode (-) to an anode (+)
Thomson proposed that cathode rays were composed of tiny, negatively charged particles moving at high speed
Thomson and others determined that an electron has a mass of 1/2000 of an H atom
Robert Milliken determined the quantity of the negative charge carried by the electron

4. Ch. 5.2 Structure of the Nuclear Atom
Protons
E. Goldstein found that some rays in a cathode tube travel in the opposite direction of a cathode ray – he named these canal rays, and realized they were composed of positively charged particles
Named these particles “protons” (1886)
Neutrons
James Chadwick discovered a third subatomic particle in 1932
Named them neutrons – they have no charge and are equal to protons in mass

5. Ch. 5.2 Structure of the Nuclear Atom
The atomic nucleus
Until 1911, scientists were unsure how the 3 subatomic particles fit together in an atom
Ernst Rutherford devised an experiment that allowed scientists to “see” an atom
The “gold foil” or “alpha- scattering” experiment
Found that most massive alpha particles passed through gold foil, a few were deflected at large angles, and a few bounced straight back toward the source
Led Rutherford to propose a new theory of the atom- all the positive charges and most of the mass ware at the center, or nucleus, of the atom

6. Ch. 5.3 Distinguishing Between Atoms
Atomic number
The number of protons in the nucleus of an atom
The atomic number is unique for each element
All atoms are electrically neutral, so number of protons equals number of electrons
Mass number
Equal to the number of protons and neutrons in the nucleus
Mass number and atomic number are represented along with the chemical symbol of the element
16O 8

7. Ch. 5.3 Distinguishing Between Atoms
Isotopes
Atoms of the same element that have the same number of protons, but different numbers of neutrons
Isoptopes are chemically alike because they have the same number of protons (atomic number) and electrons
Contradicts part of Dalton’s atomic theory
Hydrogen has 2 isotopes, deuterium and tritium (H-1, H-2, and H-3)
In nature, most elements occur as mixture of 2 or more isotopes

8. Ch. 5.3 Distinguishing Between Atoms
Atomic mass
An isotope of carbon, carbon-12, has been designated as the standard mass for atoms
This isotope of carbon was assigned a mass of exactly 12 atomic mass units
The atomic mass unit is defined as 1/12 the mass of the carbon-12 atom
The atomic mass is the weighted average mass of the atoms in a naturally occurring sample (isotopes included)

9. Ch. 5.4 The Periodic Table: Organizing the Elements
Development of the Periodic Table
By the mid-1800s, about 70 elements were known
Dmitri Mendeleev ordered the elements in columns in order of increasing atomic mass in the very 1st periodic table
He left blank spaces when there were no known elements with the appropriate properties and masses
Later these elements were discovered and found to fit
In 1913, Henry Moseley determined the atomic number of atoms of elements, and used increasing atomic number to re-order the periodic table. It still stands today.

10. Ch. 5.4 The Periodic Table: Organizing the Elements
The Modern Periodic Table
The horizontal rows are known as periods
There are 7 periods
Properties of elements change as you move across each period
Gives rise to the periodic law
When elements are arranged in increasing atomic number, there is a periodic repetition of their physical and chemical properties
The vertical columns are called groups
There are 18 groups, or families
Elements in groups have similar physical and chemical properties

11. Ch. 5.4 The Periodic Table: Organizing the Elements
Groups are given a number and a letter
The numbers move left to right across the table
The letters, A and B, signify properties
Group A elements are called the representative elements because they exhibit a wide range of physical and chemical properties
The periodic table consists of 3 main types of elements: metals, non-metals and metalloids

12. Ch. 5.4 The Periodic Table: Organizing the Elements
Metals
High heat and electrical conductivity
High luster, ductility and malleability
Alkali metals, alkaline earth metals (Gr.A)
Transition metals and inner transition metals (Gr.B)
Non-metals
Upper right corner of the periodic table
Poor conductors, low luster, ductility, malleability
Many exist as gases at room temperature
Metalloids
Consists of elements with properties that are intermediate between those of metals and non-metals