chem161sg_chapter2 - Chapter 2 ATOMS MOLECULES AND IONS...

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Chapter 2 ATOMS, MOLECULES AND IONS Laws of Chemical Combinations Lavoisier : The Law of conservation of mass For an ordinary chemical reaction, the total mass (reactants plus products) remains constant.( He established chemistry as a quantitative science ) Proust : The Law of definite proportions ( the law of constant composition ) Different samples of the same compound have the same composition irrespective of the source. Conclusion: compounds contain elements in certain definite proportions. Dalton : The Law of Multiple Proportions When two elements form more than one compound ( CO, CO 2 ) , then the ratio of the differering masses of one element that combines with a fixed mass of the other element, is in a small whole number ratio. The Structure of Atoms: Dalton's Atomic Theory 1. Matter is composed of tiny particles called atoms . The atoms of a given element are identical: having the same size , mass , and chemical properties . 2. Compounds are composed of atoms of more than one element. 3. A chemical reaction involves only the separation, combination, or rearrangement of atoms. It DOES NOT create or destroy the atom. Modern Atomic Theory differs in some aspects from Dalton's Atomic Theory: - atoms consist of even smaller particles. .. subatomic particles - although atoms of an individual element have the same chemical properties they are not necessarily identical in size or mass (isotopes - more later) 1
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SUBATOMIC PARTICLES Atoms cannot be broken into smaller particles in ordinary chemical reactions ( as compared to nuclear reactions ) but they do contain subatomic particles located both in the nucleus of the atom and outside the nucleus. ( More in Chapter 7 ). Of the many subatomic particles known, three are needed to account for the properties of the atoms. Name Symbol Mass (g) Mass (u)* Relative Charge proton p 1.6725 x 10 -24 1.0073 +1 neutron n 1.6748 x 10 -24 1.0087 0 electron e - 9.109 x 10 -28 0.0005486 -1 * u = atomic mass unit 1u = 1.66054*10 -24 gram Notes: 1. p and n are much more massive than the e - 2. The diameter of the nucleus is about 10 -13 cm The diameter of the atom is about 10 -8 cm 3. Atomic Number (Z) denotes to the number of protons in the nucleus. d DETERMINES THE IDENTITY OF THE ELEMENT Mass Number (A) denotes to the number of protons and neutrons (jointly known as neucleons) Therefore: A = Z + N where N = number of neutrons X Na 11 23 mass # A 4. Notation of an atom: atomic # Z 5. Isotopes: atoms with the same atomic number ( number of protons, Z ) b u t different mass number ( A ).
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chem161sg_chapter2 - Chapter 2 ATOMS MOLECULES AND IONS...

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