SUMMARY AND KEY TERMS

THE ATOMIC THEORY OF MATTER THE DISCOVERY OF ATOMIC STRUCTURE (SECTIONS 2.1 AND 2.2) Atoms are the basic building blocks of matter. They are the smallest units of an element that can combine with other elements. Atoms are composed of even smaller particles, called subatomic particles. Some of these subatomic particles are charged and foUow the usual behavior of charged particles Particles with the same charge repel one another, whereas particles with unlike charges are attracted to one another. 

We considered some of the important experiments that led to the discovery and characterization of subatomic particles. Thomson s experiments on the behavior of cathode rays in magnetic and electric fields led to the discovery of the electron and allowed its charge-to-mass ratio to be measured. Millikan s oil-drop experiment determined the charge of the electron. Becquerel s discovery of radioactivity, the spontaneous emission of radiation by atoms, gave further evidence that the atom has a substructure. Rutherford s studies of how thin metal foils scatter a particles led to the nuclear model of the atom, showing that the atom has a dense, positively charged nucleus. 

Elements can be classified by atomic number, the number of protons in the nucleus of an atom. AU atoms of a given element have the same atomic number. The mass number of an atom is the sum of the numbers of protons and neutrons. Atoms of the same element that differ in mass number are known as isotopes. 

ATOMIC WEIGHTS (SECTION 2.4) The atomic mass scale is defined by assigning a mass of exactly 12 amu to a atom. The atomic weight 

THE PERIODIC TABLE (SECTION 2.5) The periodic table is an arrangement of the elements in order of increasing atomic number. Elements with similar properties are placed in vertical columns. The elements in a column are known as a group. The elements in a horizontal row are known as a period. The metallic elements (metals), which comprise the majority of the elements, dominate the left side and the middle of the table the nonmetallic elements (nonmetals) are located on the upper right side. Many of the elements that lie along the line that separates metals from nonmetals are metalloids. 

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