Ch. 5 Atomic Structure and the Periodic Table Ch. 5.1 Atoms Ch. 5.2 Structure of the Nuclear Atom Ch. 5.3 Distinguishing between Atoms Ch. 5.4 The Periodic Table: Organizing the Elements

Ch. 5.1 Atoms Early models of the atom Democritus (4th century BC) John Dalton (1766-1844) 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

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

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

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

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

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

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)

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.

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

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

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