Download Atoms, Molecules, and Ions Chapter 2 Handout 1 The Atom Dalton`s

Atoms, Molecules, and Ions Chapter 2 Handout 1 The Atom Dalton’s Atomic Theory Dalton’s Atomic Theory of matter involved the following assumptions: 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all the other elements. 3. Atoms are neither created nor destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine in a simple numerical ratio. Dalton’s Atomic Theory explained the chemical laws: Law of Conservation of Mass Matter is not created or destroyed only changed (Lavoisier) Using balances, he found that the total mass does not change after chemical reactions. Law of Definite Proportions In a given chemical compound, the elements are always combined in the same proportions by mass. (Proust) Each compound has a definite percent of each element in that compound. The Structure of the Atom Atom­ Word is from Greek, meaning “unable to be cut” or indivisible. Proposed by Greek philosophers: Democritus, Empedocles (about 450 BC) A philosophical idea, rather than scientific theory. Basically speculative, not based on observation or measurement. Some of the Greek philosophers ascribed shapes to the atoms. Atoms of water: spherical (smooth, flow) Atoms of earth: cubical (rigid, solid) Atoms of fire: jagged (hurts to touch) Modern chemistry and modern atomic theory, relating to scientific observation and measurement, dates back approximately 200 years, to the late 18th and early 19th centuries. For Full Atomic Theory Read: Section 2.2 in Textbook Modern Knowledge of Atomic Structure Subatomic Particles Particle Symbol Mass (g) Charge (Coulomb) Relative Charge Location in Atom Proton p+ 1.67262x10­24 +1.6022x10­19 +1 Inside Nucleus Neutron n 1.67262x10­24 0 0 Inside Nucleus Electron e­ 9.10939x10­28 ­1.6022x10­19 ­1 Outside Nucleus Atomic Number, Mass Number, and Nucleus atomic number (Z) = the number of protons in the nucleus of an atom mass number (A) = the sum of the number of protons and neutrons in an atom element symbol (X) = the symbol of the element Note: mass number= number of protons + number of neutrons Therefore …. mass number = atomic number + number of neutrons ……. A= Z + number of neutrons ….. Number of neutrons = A­Z Note: For any given element on the periodic table: Number of protons = Number of electrons In order to symbolically represent elements and isotopes chemists use the following notation: Mass Number X Atomic number Ex: The nucleus, containing protons and neutrons, accounts for almost all the mass of the atom. The electron cloud accounts for almost all the space (volume) of the atom, but almost none of the mass. Isotopes are atoms with identical atomic numbers but different mass numbers (that is, same number of protons but different number of neutrons) A given element, with a fixed atomic number, will have several isotopes with different mass number. Such isotopes differ only in the number of neutrons in the nucleus. Thus, unlike Dalton’s original assumption, the same element can have atoms with different masses.