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Elements • Over 112 known: 88 found in nature, others are man made. Chapter 4 Atoms and Elenments Elements (cont.) • Each element has a unique one- or two-letter symbol Elements (cont.) 1 Dalton’s Atomic Theory • Elements are composed of tiny particles called atoms. • All atoms of a given element are identical. Dalton’s Atomic Theory • Atoms of different elements combine to form compounds. d – All carbon atoms have the same chemical and physical properties. • Atoms of a given element are different from those of any other element. – C Carbon b atoms have h different diff chemical h i l andd physical h i l properties than sulfur atoms. Dalton’s Atomic Theory • Law of Constant Composition: all samples of a compoundd contain t i the th same proportions ti (by (b mass)) off the elements that form the compound. • Atoms are indivisible by chemical processes. – All atoms present at beginning are present at the end. – Atoms are not created or destroyed, just rearranged in chemical reactions reactions. – Atoms of one element cannot change into atoms of another element. Formulas Describe Compounds • A compound is a distinct substance that is composed d off atoms t off two t or more elements. l t • Compounds are identified by the number and type of each atom in the simplest unit of the compound. – Molecules or ions • Cannot turn lead into gold by a chemical reaction 2 Molecular and Ionic Compounds Writing Formulas of Compounds • Each element is represented by its symbol. • The number n mber of each type t pe of atom is indicated by b a subscript written to the right of the element symbol. • If only one atom is present, do not include a subscript. • If p polyatomic y ggroups p are present p in the molecule,, they are written inside parentheses if more than one group is present. • Examples: H2O, NaCl, KNO3, Mg(NO3)2 What Are Atoms Made Of? What Are Atoms Made Of? • J.J. Thomson investigated a beam called a cathode ray. ray • He determined that the ray was made of tiny negatively charged particles we now call electrons. 3 The Electron • Tiny, negatively charged particle • Very light compared to mass of atom – 1/1836th the mass of a H atom • Move very rapidly within the atom • Atoms of different elements all produce electrons; therefore electrons are a fundamental unit of atoms. Rutherford’s Gold Foil Expt The Proton • If negative electrons exist, must also have positive particles to balance the charge and make the atom electrically neutral • Assume “Plum Pudding Model” electrons suspended in a positively charged electric field Rutherford’s Nuclear Model • The atom contains a tiny dense center called the nucleus. nucleus – The volume is about 1/10 trillionth the volume of the atom. • The nucleus is essentially the entire mass of the atom. • The nucleus is positively charged. – The amount of positive charge of the nucleus balances the negative charge of the electrons. • The electrons move around in the empty space of the atom surrounding the nucleus. 4 Components of an Atom The Modern Atom • Atoms are composed of three main pieces: protons, neutrons, and electrons. • The nucleus contains protons and neutrons. The Modern Atom (cont.) Isotopes • All atoms of an element have the same number of protons. • The number of protons in an atom of a given element is the same as the atomic number. – Found on the pperiodic table • Atoms of an element with different numbers of neutrons are called isotopes. 5 Isotopes (cont.) • All isotopes of an element are chemically id ti l identical. Isotopes of Carbon • 12C • 13C has 6 protons and 7 neutrons (atomic mass number = 6 + 7 = 13) • 14C has 6 protons (atomic number ) and 6 neutrons t (atomic ( t i mass number b = 6 + 6 = 12) – Undergo the exact same chemical reactions • Isotopes of an element have different masses. • Isotopes are identified by their mass numbers. – Mass number = # protons + # neutrons has 6 protons and 8 neutrons (atomic mass number = 6 + 8 = 14) Elements • Arranged in a pattern called the periodic table • Position on the table allows us to predict properties of the element Elements (cont.) • Metals – Ab About 75% % off all ll the h elements l – Lustrous, malleable, ductile, conduct heat and electricity • Nonmetals – Dull, brittle, insulators • Metalloids – Also know as semi-metals – Some properties of both metals & nonmetals 6 The Modern Periodic Table The Modern Periodic Table (cont.) • Elements with similar chemical and physical properties are in the same column. • Groups (Families) and Periods The Modern Periodic Table (cont.) • Main group = representative elements – “A” columns • Transition elements – All metals • Bottom rows = inner transition elements = rare earth elements – Metals – Lanthanides and actinides Natural States of Elements • Gases: monatomic gases such as noble gases and diatomic gases such as O2 • Liquids such as Hg or Br2 • Solids such as metals or allotropes p of carbon 7 Natural States of Elements (cont.) Ions • Cations: ions that have a positive charge – Form when an atom loses electrons • Anions: ions that have a negative charge – Form when an atom gains electrons • Ions with opposite charges attract – Therefore cations and anions attract each other Ions (cont.) • Moving ions conduct electricity. • Compound must have no total charge, therefore we must balance the numbers of cations and anions in a compound to get 0 g total charge. Atomic Structures of Ions • Metals form cations. • For each positive charge the ion has 1 less electron than the neutral atom. – Na = 11 e-, Na+ = 10 e– Ca = 20 e-, Ca+2 = 18 e- • Cations are named the same as the metal sodium Na → Na+ + 1e- sodium ion calcium Ca → calcium ion Ca+2 + 2e- 8 Atomic Structures of Ions (cont.) • Nonmetals form anions. • For each negative charge the ion has 1 more electron than the neutral atom. – F = 9 e-, F- = 10 e– P = 15 e-, P3- = 18 e- Ionic Charges and Compounds • Charges on the ions of some elements can be predicted from the periodic table. • Cations and anions usually form simultaneously to yield an ionic compound. • Anions are named by changing the ending of the name to -ide – Fluorine – Oxygen F + 1e- → FO + 2e- → O2- fluoride ion oxide ion Ionic Charges and Compounds (cont.) Formulas of Ionic Compounds • The formula of an ionic compound can be determined by balancing the positive charge of the cation(s) with the negative charge of the anion(s) to yield a net charge of zero. 9 Formation of Ionic Compounds 10