Atomic Theory and Structure: Key Concepts for Chemistry

Who first proposed the concept of atoms?

Greek philosophers in the fifth century BC

atomos

a term derived from the Greek word for "indivisible"

What was Aristotle's belief about matter?

Matter consisted of various combinations of the four elements: fire, earth, air, and water.

Who proposed atomic theory in 1807?

John Dalton, an English schoolteacher.

What are the five postulates of Dalton's atomic theory?

1. Elements consist of indivisible atoms.

2. All atoms of the same element are identical.

3. Different elements have different types of atoms.

4. Atoms combine in whole-number ratios to form compounds.

5. Atoms cannot be created or destroyed.

What's not true about Dalton's first postulate in his atomic theory?

Atoms ARE divisible into subatomic particles

What's not true about Dalton's second postulate in his atomic theory?

Atoms of the same element can have DIFFERENT MASSES due to ISOTOPES

Dalton's Atomic Theory provides...

a microscopic explanation of the many macroscopic properties of matter that you've learned about.

What does the Law of Conservation of Matter state?

The total mass of matter remains constant during a chemical change, as atoms are neither created nor destroyed.

What does the Law of Definite Proportions state?

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

Who illustrated the Law of Definite Proportions?

French chemist Joseph Proust.

table

table

What is the Law of Multiple Proportions?

When 2 elements react to form more than one compound, a fixed mass of 1 element will react with masses of the other element in a ratio of small, whole numbers.

Example: compounds containing chlorine and copper

-A green solid contains 0.558 g Cl to 1 g Cu.

-A brown solid contains 1.116 g Cl to 1 g Cu.

What experiment did J.J. Thomson conduct?

He experimented with cathode ray tubes.

Cathode Ray Tubes

-A sealed glass tube from which almost all the air had been removed

-Contained two metal electrodes

Thompson calculated...

the charge-to-mass ratio of the cathode ray particles.

What did Thomson discover about cathode rays?

They are negatively charged particles much lighter than atoms, now known as electrons.

Electron

a negatively charged, subatomic particle with a mass more than one thousand times less than that of an atom. (very very small)

Who discovered Electron Charges via oil drops?

Robert A Millikan

What did Millikan do?

-created microscopic oil droplets, which were electrically charged

-was able to determine the charge on individual drops

What was the charge of a single electron determined by Robert A. Millikan?

1.6 × 10-19 C.

What was Thomson's early model of the atom?

The plum pudding model.

What was Nagaoka's early model of the atom?

Atoms resembled the planet Saturn, with a ring of electrons surrounding a positive "planet."

Who discovered the Nucleus via Gold Foil scattering?

Ernest Rutherford

What did Rutherford do?

-Aimed a beam of alpha particles (α particles) at a very thin piece of gold foil.

-The scattering of these α particles was examined using a luminescent screen that would glow briefly when hit.

What did Ernest Rutherford's Gold Foil Experiment reveal?

Atoms consist of a large amount of empty space and contain a small, dense, positively charged nucleus. The nucleus contains most of the atom's mass. Negatively charged electrons surround the nucleus.

Nucleus

A small, relatively heavy, positively charged body; at the center of each atom

Proton

a positively charged, subatomic particle located in the nucleus.

Isotope

Atoms of the same element that differ in mass. Isotopes of an element have the same number of protons but a different number of neutrons.

Who discovered the isotope?

Frederick Soddy of England. Noble Prize in 1921.

Neutrons

Uncharged, subatomic particles with a mass approximately the same as that of protons; located in nucleus

Who discovered the Neutron?

James Chadwick in 1932

The nucleus contains

the majority of an atom's mass (Protons and neutrons are much heavier than electrons)

Electrons occupy

almost all of an atom's volume

A carbon atoms weighs

less than 2 × 10-23 g.

Atomic mass unit (amu)

a unit of mass equal to 1/12 the mass of a carbon-12 atom

-1 amu =

1.6605 x 10-24 g (1/12 mass of one carbon - 12 atom)

Proton Mass & Charge

-Mass = 1.0073 amu

-Charge = +1

Neutron Mass & Charge

-Mass = 1.0087 amu (slightly heavier than a proton)

-Charge = 0

Electron Mass & Charge

-Mass = 0.00055 amu

-Charge = -1

Atomic Number (Z)

the number of protons in the nucleus of an atom

Neutral Atoms

atom that contains the same number of positive and negative charges. (# of protons = # of electrons)

Mass Number (A)

The total number of protons and neutrons in an atom.

Mass Number (A) - Atomic Number (Z) =

Number of neutrons

Ions

When the number of protons and electrons are NOT equal, the atom is electrically charged and called an ion.

Ions are

Charged atoms

Charge of an atom =

number of protons - number of electrons

Atoms (and molecules) acquire charge by

losing or gaining electrons.

Anion

An atom that gains one or more electrons will exhibit a negative charge and is called an anion.

Anion Example

A neutral oxygen atom (Z = 8) has eight electrons, and if it gains two electrons it will become an anion with a 2− charge (8 − 10 = 2−).

Cation

An atom that loses one or more electrons will exhibit a positive charge and is called an cation.

Cation Example

A neutral sodium atom (Z = 11) has 11 electrons. If this atom loses one electron, it will become a cation with a 1+ charge (11 − 10 = 1+).

Chemical Symbols are

derived from the common name of the element; others are abbreviations of the name in another language.

3 Letters in Chemical Symbols

have been used to describe some elements that have atomic numbers greater than 112.

Isotope Symbol

The symbol for a specific isotope of any element is written by placing the mass number as a superscript to the left of the element symbol.

Hydrogen...

exists as a mixture of three isotopes

Protium (hydrogen-1)

no neutrons and a mass of 1.0078, 99.989% natural abundance

Deuterium (hydrogen-2)

one neutron and a mass of 2.0141, 0.0115% natural abundance

Tritium (hydrogen-3)

two neutrons and a mass of 3.01605,

trace natural abundance

All Hydrogen isotopes

have atomic number of one and one proton

Mass Sizes

Each proton and each neutron has a mass of ~ 1 amu. Each electron weighs far less.

Atomic Mass

equal to a single atoms mass number.

Mass Spectrometry (MS)

The occurrence and natural abundances of isotopes can be experimentally determined using an instrument called a mass spectrometer (separated by their mass and charge)