CH 2.1-2.3

CHEM111, Dalton’s Atomic Theory, Laws, and Atomic Structure Overview

Matter

Matter is composed of atoms.

Atoms

Atoms are the smallest units of an element that have all the properties of that element.

Element

An element is composed entirely of one type of atom.

Compound

A compound contains atoms of two or more different elements.

Relative number of atoms

The relative number of atoms of each element in a compound is always the same.

Law of constant composition

All samples of a pure substance contain the same elements in the same proportion by mass.

Law of multiple proportions

When the same elements form more than one compound, the mass of one element that combines with a fixed mass of a second element are in a ratio of small whole numbers.

Law of Conservation of Mass

There is no detectable change in mass when a chemical reaction occurs.

Electrons

Electrons are negatively charged particles demonstrated by J.J. Thomson.

Protons

Protons are positively charged particles found in the nucleus of an atom.

Neutrons

Neutrons are neutral particles found in the nucleus of an atom.

Millikan Oil Drop Experiment

Robert A. Millikan performed experiments that determined the charge of the electron as -1.602 x 10^-19 coulombs.

Electron charge

Electron has a charge of -1.602 x 10^-19.

Electron mass

Electron has a mass of 9.109 x 10^-31 kg.

Proton charge

Proton has a charge of +1.602 x 10^-19.

Proton mass

Proton has a mass of 1.673 x 10^-27 kg.

Neutron charge

Neutron has a charge of 0.

Neutron mass

Neutron has a mass of 1.675 x 10^-27 kg.

Nuclear Model of the Atom

Atoms consist of a nucleus that is very small compared to the atom, has a high positive charge and contains most of the mass of the atom.

Proton as a fundamental particle

Rutherford proposed that the hydrogen nucleus was a fundamental particle called the proton.

Mass of proton compared to electron

The mass of the proton (1.673 x 10^-27 kg) is 1836 times that of the electron.

Atomic number (Z)

Atomic number (Z) is the number of protons in the nucleus of an atom.

Mass number (A)

Mass number (A) is the sum of the numbers of protons and neutrons in the nucleus.

Isotopes

Isotopes are atoms of an element whose nuclei contain different numbers of neutrons (same Z, different A).

Isotopes of Hydrogen

Hydrogen has no neutrons, Deuterium has one neutron, Tritium has two neutrons.

Symbol for Isotopes

A symbol to identify a specific isotope is ^{A}_{Z} \text{X} where A = mass number, Z = atomic number, and X is the one or two letter symbol of the element.

Hydrogen Isotopes Symbols

The 3 isotopes of Hydrogen are: ^{1}{1} \text{H}, ^{2}{1} \text{H}, ^{3}_{1} \text{H}.

Oxygen Isotopes Symbols

Oxygen has three stable isotopes, containing 8, 9, and 10 neutrons, respectively. The symbols are: ^{16}{8} \text{O}, ^{17}{8} \text{O}, ^{18}_{8} \text{O}.

Oxygen Isotopes Simplified Symbols

Since the value of Z and the symbol both identify the element, Z is often omitted from the symbol: ^{16} \text{O}, ^{17} \text{O}, ^{18} \text{O}.

Ions Definition

In many chemical reactions, atoms gain or lose electrons producing charged particles called ions.

Cation Definition

A cation has a positive charge and forms when an atom loses one or more electrons.

Anion Definition

An anion has a negative charge and forms when an atom gains one or more electrons.

Ion Charge Representation

The element's symbol is followed by a superscript number and a sign that shows the charge on the ion in electron charge units.

Cation Example

Ca^2+ is the symbol for the calcium atom that has lost two electrons.

Anion Example

Cl^- is the symbol for the ion formed when a neutral chlorine atom gains one electron.

Charge Representation

If the charge is 1 the number is omitted, and a charge of zero is not shown.

Dalton's Atomic Theory

Dalton's Atomic Theory states that matter is made of indivisible atoms, elements contain one type of atom, compounds form in fixed ratios, and atoms don't change identity in reactions.

Experiments by Thomson, Millikan, and Rutherford

These experiments revealed the existence and properties of electrons, protons, and neutrons.

Rutherford's Nuclear Model

Rutherford's nuclear model showed that atoms have a dense, positively charged nucleus with electrons in mostly empty space around it.

Atomic Number Definition

The atomic number identifies an element.

Mass Number Definition

The mass number counts protons plus neutrons.

Isotopes Definition

Isotopes are atoms of the same element with different neutron counts.

Subatomic Particles

Atoms are made of smaller subatomic particles: electrons (discovered by Thomson and measured by Millikan), protons (identified by Rutherford), and neutrons.