Basic Atomic Structure

Chemistry Timeline #1B.C.400 B.C. Demokritos and Leucippos use the term "atomos”

1500's Georg Bauer: systematic metallurgy Paracelsus: medicinal application of minerals

1600'sRobert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements

1700s'Georg Stahl: Phlogiston TheoryJoseph Priestly: Discovery of oxygen Antoine Lavoisier: The role of oxygen in combustion, law of conservation of mass, first modern chemistry textbook

2000 years of Alchemy

Chemistry Timeline #2

1800'sJoseph Proust: The law of definite proportion (composition) John Dalton: The Atomic Theory, The law of multiple proportionsJoseph Gay-Lussac: Combining volumes of gases, existence of diatomic moleculesAmadeo Avogadro: Molar volumes of gasesJons Jakob Berzelius: Relative atomic masses, modern symbols for the elements Dmitri Mendeleyev: The periodic table J.J. Thomson: discovery of the electron Henri Becquerel: Discovery of radioactivity

1900's Robert Millikan: Charge and mass of the electron Ernest Rutherford: Existence of the nucleus, and its relative size Meitner & Fermi: Sustained nuclear fission Ernest Lawrence: The cyclotron and trans-uranium elements

Law of Definite Proportion

• Different samples of a pure compound always contain the same elements in the same proportions by mass.

• Water, for example, always has an 8:1 proportion of Oxygen to Hydrogen

16 amu Oxygen2 amu Hydrogen

32 amu Oxygen4 amu Hydrogen

48 amu Oxygen6 amu Hydrogen

Typically, this is written as a mass percent, so we say that water is 88.9% Oxygen (by mass) – what percent by number is this?

Law of Definite ProportionExamples

• What Mass of Oxygen is contained in a 50.0 g sample of water?– 44.5 g O

• What mass of Hydrogen is in a 35.5 g sample of water?– 3.94 g H

• An unknown water like substance was broken down and found to be composed of 58.8 g of Oxygen and 3.7 g Hydrogen. Could this substance be water? Show a calculation to support your answer.– No (shown to be 94.1 % Oxygen)

Law of Multiple Proportion

• If two elements combine to make different compounds, keeping one elements mass constant will always result in a whole number ratio of the second element.

• 50 grams of carbon can combine with 66.5 g of oxygen or 133 g of oxygen. What is the ratio of oxygen to oxygen in each compound?

• 14 grams of Nitrogen can combine with 8, 16, 24, 32 or 40 g of oxygen to form 5 different compounds. What is the ratio of oxygen in each?

Dalton’s Atomic Theory (1808)

Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other propertiesJohn Dalton

Modern Atomic TheorySeveral changes have been made to Dalton’s theory.

Dalton said:

Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

Modern theory states:

Atoms of an element have a characteristic average mass which is unique to that element.

Modern Atomic Theory #2

Dalton said:

Modern theory states:

Atoms cannot be subdivided, created, or destroyed

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

Discovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

The rays showed that the charge to mass ratio of the negative particles was always the same, no matter the gas used.

Thomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J.J. Thomson

Mass of the Electron

1909 – Robert Millikan determines the mass of the electron.

The oil drop apparatus

Mass of the electron is 9.109 x 10-31 kg

Conclusions from the Study of the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Rutherford’s Gold Foil Experiment

Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Rutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

Atomic Particles

Particle

RelativeChar

ge

Mass (kg) Location

Electron

-1 9.109 x 10-31 Electron cloud

Proton +1 1.673 x 10-27 Nucleus

Neutron

0 1.675 x 10-27 Nucleus

The Atomic Scale

Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space

Helium-4

Image: User Yzmo Wikimedia Commons.

1 million femtometers = 1 nanometer1 million nanometers = 1 millimeter

About Quarks…

Protons and neutrons are NOT fundamental particles.

Protons are made of two “up” quarks and one “down” quark.

Neutrons are made of one “up” quark and two “down” quarks.

Quarks are held together by “gluons”

Images: Arpad Horvath, Wikimedia Commons.

IsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Isotope Protons

Electrons

Neutrons

Nucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2

Atomic Masses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons6 neutrons

98.89%

Carbon-13

13C 6 protons7 neutrons

1.11%

Carbon-14

14C 6 protons8 neutrons

<0.01%

Atomic mass is the average of all the naturally isotopes of that element.

Carbon = 12.011

Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Element # of protons

Atomic # (Z)

Carbon 6 6

Phosphorus

15 15

Gold 79 79

Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10

- 33 42

- 31 15

8 8 1818

Arsenic 75 33 75Phosphorus 15 3116

O188As75

33

P31

Calculating Average Atomic MassCalculate the average atomic mass of Chlorine and Silicon given the following information:

Chlorine Silicon

Mass number

Exact mass

Percent abundance

Mass number

Exact mass Percent abundance

35 34.968852 75.77 28 27.976927 92.23

37 36.965903 24.23 29 28.976495 4.67

30 29.973770 3.10

45.35)2423.0)(5903.36()7577.0)(968852.34( Chlorine: 35.45

Silicon: 28.09