Atoms The Basics… or Part I CHM 1010 PGCC Barbara A. Gage.

Atoms

The Basics…or Part I

CHM 1010 PGCC Barbara A. Gage

The Atom

• ~400 BCE Democritus: small particle (“atomos” is Greek for indivisible)

• 1803 John Dalton: atomic model with several postulates

• 1897-now Modern Investigations…

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Dalton’s Atomic Dalton’s Atomic TheoryTheory

1. All matter consists of atoms which are indivisible and indestructible.

2. Atoms of one element cannot be converted into atoms of another element.

3. Atoms of an element are identical in mass and other properties and are different from

atoms of any other element.

4. Compounds result from the chemical combination of a specific ratio of atoms of different elements.

The Postulates

From Silberberg, Principles of Chemistry

And so…• Law of Conservation of Matter –

matter cannot be created or destroyed during a physical or chemical change.

CHM 1010 PGCC Barbara A. Gage

reactant 1 + reactant 2 product

total mass total mass=

calcium oxide + carbon dioxide calcium carbonate

CaO + CO2 CaCO3

56.08g + 44.00g 100.08g

And so…• Law of Definite Proportions A given

compound always contains the same proportion of elements by mass

– CaCO3 10 g Ca + 3 g C + 12 g Oor40 g Ca + 12 g C + 48 g O

– H2O 1 g H + 8 g O or7 g H + 56 g O

CHM 1010 PGCC Barbara A. Gage

And so…• Law of Multiple Proportions - When

two elements form more than one compound the ratio of the masses of the second element with 1 g of the first can always be reduced to small whole numbers

• H2O (water) H2O2 (hydrogen peroxide)

1 g H 1 g H8 g O 16 g O

16/8 = 2CHM 1010 PGCC Barbara A. Gage

Atomic Structure

• ~1870-1935 saw experimentation that showed that Dalton was right and wrong.

• 1874 – Stoney: electricity is made of individual particles with negative charge called electrons

• 1879 – Crookes: discovered “cathode rays” have distinct properties like electrons

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Figure 2.4 Silberberg, Principles of Chemistry

Experiments to determine the properties of cathode rays.

Atomic Structure

• 1896 – Becquerel discovers emissions from materials (radioactivity)

• 1898 – Rutherford uncovers properties of the some Becquerel emissions and names them alpha and beta

www.lbl.gov/abc/graphics/magnet.gif

CHM 1010 PGCC Barbara A. Gage

Atomic Structure

• 1897 – Thomson discovers “canal rays” are the same as positive hydrogen atoms

• Model of atom is “plum pudding” with + and – particles mixed together

http://commons.wikimedia.org/wiki/File:Plum_pudding_atom.svg

• 1911 – Rutherford conducts “gold foil experiment”

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

Atomic Structure

• After gold foil experiment, atomic model changes to one with + charges (protons) in dense center with – charges (electrons) surrounding the center

• 1932 – Chadwick: discovers the missing mass in the atom comes from neutral particles named neutrons

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

The coulomb (C) is the SI unit of charge.

The atomic mass unit (amu) equals 1.66054x10-24 g.

CHM 1010 PGCC Barbara A. Gage

Figure 2.8 Silberberg, Principles of Chemistry

Atomic Symbols, Isotopes, Numbers

X = Atomic symbol of the element

A = mass number; A = Z + N

Isotope = atoms of an element with the same number of protons, but a different number of neutrons

A

Z

Z = atomic number (the number of protons in the nucleus)

N = number of neutrons in the nucleus

X The Symbol of the Atom or Isotope

CHM 1010 PGCC Barbara A. Gage

CHM 1010 PGCC Barbara A. Gage

The Modern Reassessment of Dalton’s Atomic Theory

1. All matter is composed of atoms that are indivisible and indestructible. The atom is the smallest body that retains the unique identity of the element. However, it can, under unusual circumstances, be destroyed (converted to energy) and it can be divided into smaller parts.

2. Atoms of one element cannot be converted into atoms of another element in a chemical reaction. Elements can only be converted into other elements in nuclear reactions.

3. All atoms of an element have the same number of protons and electrons, which determines the chemical behavior of the element. Isotopes of an element differ in the number of neutrons, and thus in mass number. A sample of the element is treated as though its atoms have an average mass.

4. Compounds are formed by the chemical combination of two or more elements in specific ratios.