Atomic Structure and the Periodic Table Ch. 5

Atoms 5-1

Early Models of the Atom Democritus – 4th century BC – world made up of empty space and tiny particles called atoms (atomos) ‘indivisible’ Hypothesized without using experiments

Dalton’s Atomic Theory All matter is made of atoms Atoms are indestructible and cannot be divided into smaller particles (later proved wrong) All atoms of one element are exactly alike (later proved wrong), but they are different from atoms of other elements Atoms combine in simple whole number ratios to form compounds In chemical reactions, atoms combine, separate, or are rearranged. Done in the 1800’s through experiments!

Structure of the Nuclear Atom 5-2

Atomic Structure 1. Electron: (e-) Discovered by Thompson’s cathode-ray tube (1897) Cathode Ray = positive and negative electrodes connected, beam travels from cathode (-) to anode (+) Found electrons have negative charge Beam in cathode ray attracted to anode (+), must be negatively charged. Relatively no mass (experiment by Millikan 1916)

Thompson’s Cathode-Ray Experiment

Proton – has positive charge (p+), greater mass than an electron - Discovered by Goldstein in 1886 Neutron – has neutral charge (no), equal in mass to proton -Discovered by Chadwick in 1932

-Discovered by Rutherford’s gold foil experiment in 1911 4. Nucleus = small, positively charged central core, made up of protons and neutrons -Discovered by Rutherford’s gold foil experiment in 1911 -Fired stream of positive particles at gold foil, most passed right through (atom mostly empty space) while a few bounced off (very small positive nucleus at center)

Play movie

Development of Modern Atomic Theory Law of conservation of matter (mass) = in a chemical reaction matter is neither created nor destroyed. All elements are recycled/rearranged!! Law of definite proportions = elements within a compound are always in certain proportions by mass Ex: NaCl (table salt) is always 39% Na and 61% Cl.

Distinguishing between Atoms 5-3

Atomic Number Atomic # = Number of protons in the nucleus of an atom atomic # = p+ Determines the identity of the element Periodic table is organized by atomic number Ex: What is the atomic number of… He ______ C ______ N ______ O ______ Also! # of protons = # of electrons p+ = e- Ex: How many electrons does Cl have? _______

Why do we not include electrons??? Mass Number Mass # = Sum of the protons and neutrons mass # = p+ + no Why do we not include electrons??? neutron = mass # - atomic # Periodic Table symbols: 12 C 6 Mass # (rounded) Element Atomic #

Let’s Practice! How many? Protons ____ Neutrons ____ Electrons ____ Write it: ? C Protons ______ Neutrons ______ Electrons ______ S

Remember!!! Mass # = p+ + n Atomic # = p+ e- = p+ n = mass # - atomic #

Atomic Mass + Isotopes Atomic Mass = weighted average mass of isotopes Isotopes = one element with same proton # but different neutron # Carbon has 3 possible isotopes: 14 13 12 C C C 6 6 6 Carbon-14, Carbon-13, Carbon-12

Try it! Ex: Isotope of Chlorine: 37 Cl 17 How many? Protons ______ Neutrons ______ Electrons ______

Atomic Mass Unit Atomic Mass Unit (amu) = 1/12 the mass of a carbon-12 atom. Used to compare element’s masses to a standard. Units for measuring atom’s masses

The Periodic Table: Organizing the Elements 5-4

Development of the Periodic Table In 1869, Mendeleev created Periodic Table of Elements Organized it by increasing atomic mass and elements with similar properties in same columns. Able to predict future elements properties In 1913, Moseley organized it by atomic number.

Modern Periodic Table Organized by atomic number Periodic Law: grouped according to physical and chemical properties Period = horizontal row of periodic table; numbered down the side, 7 periods Group = vertical column, also called a family; 18 groups

Group Designations Representative elements = Group 1,2,13-18 Group 1 – Alkali metals Group 2 – Alkaline earth metals Group 17 – Halogens Group 18 – Noble gases Transition metals = groups 3-12

Physical States of Elements Periodic Table shows the states of the elements at room temperature and normal pressure Most are solids Only two are liquids Gaseous elements are in the top right hand corner except for Hydrogen The rest are made synthetically (man-made)through nuclear reactions (43, 61, 85, 87, and > 93)

Classifying Elements Metals - majority of elements, occupy left side and center Nonmetals - occupy top right hand corner, (includes H) Metalloids - along the boundary

Metals Have luster, many colors, conduct heat and electricity, usually bend w/o breaking (malleable) Almost all are solids at room temp. and have extremely high melting points

Nonmetals Are abundant in nature Oxygen and Nitrogen: 99% atmosphere Carbon in more compounds than all others combined! Poor conductor of heat and electricity, brittle when solid Many gases at room temp, low melting point

Metalloids Have properties of both metals and nonmetals 7 Metalloids: B, Si, Ge, As, Sb, Te, Po (touch staircase, no Al + At) Some are semiconductors = element that does not conduct electricity as well as metal, but does conduct slightly better than nonmetal Ex: Silicon – used in computer industry