Chemistry Week 2

Atoms and the Periodic Table

Dalton’s Atomic Theory

• Law of Conservation of Mass

• Law of Definite Proportions

• Law of Multiple Proportions

• An element only consists of one type of atom that is unique to the element and found in all other exampled of that element

Law of Conservation of Mass

• The total mass present before a chemical reaction is equal to the amount of mass present after the reaction has finished

• This is sometimes changed by mass being converted into large amounts of energy instead, Ex. Uranium Decomposing

Law of Definite Proportions

• Elements always combine in the same proportions by mass

• Percent of atoms from each element also combine in set proportions

Law of Multiple Proportions

• Great majority of molecules combine into small number ratios that are fixed

• “Small” is relative to what is possible in the universe

Cathode Ray Experiment

• Discovered the electron in 1897

• Conducted by shooting a cathode ray between a positive and negative magnet

• The ray bent slightly towards the positive magnet, proved the ray had something negative in it

• Through trigonometry it was found that the mass of the negatively charged thing had a mass that was 1/12 that of an atom

Thompson’s Plum-Pudding Model

• Early / false model of an atom

• Atom is a cloud of positive electricity with negative electrons inside of it

• The positive electricity is the “pudding”

• The negative electrons are the “plums”

• Logic follows below

1. The Cathode Ray Experiment proves that an atom has negative particles (modern-day electrons) in it

2. The atom makes up all matter

3. Mater with the same charge repel each other

4. Everything in the universe doesn’t repel each other

5. There must be a positive component to the atom

6. The positive component was theorized to be a positive electrical field “pudding”

Alpha Particels

• He²+

• Positively charged particle

Beta Particels

• High energy electrons

Gama Rays

• High energy electromagnetic radiation

Gold Foil Experiment

• Discovered a positively charged nucleus at center of atom made of protons and neutrons

Proton

• Subatomic particle

• Part of all nuclei

• Postive, +1 charge

• 1.007 amu*, replace with 1 for the purposes of Chem 101

Neutron

• Subatomic particle

• Part of nearly all nuclei, not present in most H nuclei

• Neutral charge

• 1.009 amu, replace with 1 for the purpose of Chem 101

Electron

• Subatomic particle

• Found outside the nucleus

• Negative, -1 charge

• 5.486 × 10^-4 amu, replace with 0 for purpose of Chem 101

• Amount present in atom equal to that of protons

• Subdivisions: Core electrons and valence electrons

Atomic Structure

• 99% empty space

• Early on in periodic table neutrons and protons equal each other, causing the mass number to be double the atomic number

• Later on in the periodic table neutrons start to greatly outnumber the amount of protons present

• Atomic number (Z)

• Mass number (A)

Atomic Number

• Part of atomic structure

• Decides the identity of the element

• Count of number of protons in the atom

Mass Number

• Number of protons plus the number of neutrons

Isotope

• Different versions of the same element

• Contain different number of neutrons

Ion

• Different version of the same element

• Contains different number of electrons

• Subdivided into cation and anion

Cation

• A positively charged ion

• Has lost an electron

Anion

• A negatively charged ion

• Has gained an electron

Atomic Mass

• Actual mass of an atom

Atomic Mass Unit (AMU)

• 1/12 of the atomic mass

Chemical formulas

• Represent molecules

• Molecular formula and structural formula

• For the most part the element that appears first on the periodic table leads

Ex: CO

Molecular formula

• Used the chemical symbol to indicate type of atom followed by a subscript that shows the amount of that atom present in each molecule

Structural formula

• Gives same info as molecular formula but also shows how atoms are connected

Coefficient

• Comes before a molecular formula

• Indicates the number of molecules present

Subscript

• Comes after a molecular formula

• Indicates the ratio elements with a molecule

Empirical formula

• Indicates the simplest whole-number ratio of the number of atoms (or ions) in the compound

Ex: C6H6 turns into CH

Isomers

• Compounds with the same chemical formula but different molecular structures

• One isomer has great differences in properties compared to the other

Metal classifications

• Shiny solids, malleable, bendable, conductors, high melting and boiling point

Nonmetal classifications

• Brittle, insulators, low melting and boiling point

Semimetals

• “Metalloid”

• Can act as metal or nonmetal, semiconductors

Core electrons

• Electrons in every orbital that isn’t the outermost

• More tightly bound to the nucleus

• Rarely involved in chemical reactions

Valence electrons

• Electrons in the outermost orbital

• Responsible for chesty of atom

• Amount an atom has equals it’s row number on the periodic table

Orbital

• Pathway that electrons orbit the nucleus on

• Very similar to the way planets orbit the sun

• Amount an atom has equals it’s row number on the periodic table

Law of Mendeleev - Chemical Periodicity

• When elements are arranged in the periodic table by atomic mass properties repeated every 8 blocks

• Ex. Lithium (Li) Sodium (Na) and Potassium (K) are all very soft metals and located 8 blocks away from one another

Magic Numbers

• Ratio of neutrons to protons in a nucleus that makes the atom stable

• Unknown to why there numbers exist