Atoms and Elements

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Atoms and ElementsAtoms and Elements Chapter 2Chapter 2

John Dalton1766-1844.Conceivedatomic weights.

Ernest Rutherford1871-1937.* Discoverer ofatomic nucleus.

D. I. Mendeleev1834-1907.Periodic Table.Predicted elements.

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• John Dalton: Elements are composed of atoms.– All atoms of an element are identical

(chemically). (Dalton stressed “identical in weight” but he didn’t know about isotopes)

– In chemical reactions, the atoms are not changed.

– Compounds are formed when atoms of more than one element combine.

The Atomic Theory of MatterThe Atomic Theory of Matter

(e.g., H2O, C6H6, C12H22O11 but not H2, Cl2)

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For example, water (H2O) is always a ratio of two hydrogen atoms to one oxygen atom.

Law of Constant CompositionIn a compound, relative amounts and kinds of atoms are fixed.

Specifically, water is always 11.1% hydrogen and 88.9% oxygen (by weight).

For water, O/H = 88.9/11.1 = 8/1 (by weight)

Dalton perceived that this constant ratio of mass of different elementsin a compound reflected a specific ratio of atoms, with each element having its specific, but unique weight. But he was confused about the exact number of atoms in a compound. For example, he thought water had a formula of HO (and not H2O).

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The ancient Greeks were the first to postulate that matter consists of indivisible constituents.Later scientists realized that the atom consisted of charged (+ or -) entities.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

Cathode Rays and ElectronsCathode Rays and ElectronsA cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.

A charged particle will have its path bend in either an electric or magnetic field.

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Cathode Rays and ElectronsCathode Rays and Electrons

The Discovery of Atomic StructureThe Discovery of Atomic Structure

(electrons are charged (-) particles)

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The Discovery of Atomic StructureThe Discovery of Atomic Structure

•a spot which is not affected by the electric field,

Three spots are noted on the detector:

•a spot in the direction of the positive (+) plate,

•a spot in the direction of the negative (-) plate.

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-radiation: Large deflection toward the positive plate corresponding to radiation which is negatively charged and of low mass. These particles are light and of low mass. particles are electrons.

-radiation: No deflection; neutral (zero charge) radiation.

-radiation: Small deflection toward the negative plate corresponding to high mass, positively charged radiation.

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The Nuclear AtomThe Nuclear AtomFrom the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities.J. J. Thomson assumed all these charged species were found in a sphere.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

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The Nuclear AtomThe Nuclear AtomRutherford’s -particle experiment:

The Discovery of Atomic StructureThe Discovery of Atomic Structure

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The Nuclear AtomThe Nuclear AtomIn order to get the majority of -particles through a piece of foil to be undeflected, the majority of the atom must consist of a low mass, diffuse negative charge the electron.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

To account for the small number of high deflections of the -particles, the center or nucleus of the atom must consist of a dense positive charge.

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The Nuclear AtomThe Nuclear AtomRutherford modified Thomson’s model as follows:assume the atom is spherical but a massive positive charge must be located at the center, with a diffuse light negative charge surrounding it.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

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The atom consists of positive, negative, and neutral entities (protons, electrons, and neutrons).

The Modern View of Atomic StructureThe Modern View of Atomic Structure

Protons and neutrons are located in the nucleusof the atom, which is small. Most of the mass of the atom is due to the nucleus.

Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons.

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(1Å = 10-8cm =10-10 m)

The Nucleus

Ångstrom unit:

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Atomic number (Z) = number of protons in the nucleus.

The Modern View of Atomic StructureThe Modern View of Atomic Structure

XAZ

Isotopes, Atomic Numbers, and Mass NumbersIsotopes, Atomic Numbers, and Mass Numbers

By convention, for element X, we write

Isotopes have the same Z but different A.

Isotopes of carbon: 116C, 12

6C, 136C, 14

6C

Mass number (A) = total number of nucleons in the nucleus (i.e., protons and neutrons).

Note the common “6” for all isotopes of carbon

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John Dalton1766-1844.

Henry Moseley 1887-1915.

Frederick Soddy1877-1956.

Dalton conceived atomic weights 200 years ago.Moseley defined atomic number, using X-ray diffraction,100 years later. Soddy defined isotopes shortly after.

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Comparison of Proton, Neutron and ElectronRelative

Particle Charge Mass (amu)

Proton 1+ 1.0073Neutron neutral 1.0087Electron 1- 5.486 x 10-4

1 amu = 1.66 x 10-24 g

about the samein thenucleus

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Isotopes, Atomic Numbers, and Mass NumbersIsotopes, Atomic Numbers, and Mass Numbers

The Modern View of Atomic StructureThe Modern View of Atomic Structure

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Exercise

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Exercise

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The Atomic Mass ScaleThe Atomic Mass Scale

Atomic and Molecular WeightsAtomic and Molecular Weights

Assume H has a relative mass of 1 “unit”H2O is 88.9 % O and 11.1% H (by mass)

Mass ratio of O to H (in water) is: 88.9/11.1 = 8/1.

Since there are two H for each O, mass ratio of O to H must be 16/1

or: mass O = 16 mass H 1

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Another example:

Then, mass C = 12 (Show this!) mass H 1

CO is 42.9% C and 57.1% O (by mass)Mass ratio of O to C= 57.1/42.9 = 4/3

or: mass O = 4 mass C 3

Since: mass O = 16 (previous slide) mass H 1

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Atomic and Molecular WeightsAtomic and Molecular WeightsCan now build up “relative” atomic mass scale.

Roughly, if H=1, then C=12 and O=16.

Can add other elements (e.g., N=14, F=19, etc).

Current scale is based on isotope 12C having mass of 12 exactly (by definition). This unit of mass is called the atomic mass unit, or amu.

Mass of 12C = 12 amu exactly

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Because of isotopes, atoms have Because of isotopes, atoms have averageaverage atomic weights atomic weights

Relative atomic mass: average masses of isotopes:Naturally occurring C: 98.892 % 12C + 1.108 % 13C.

Average mass of C: (0.98892)(12 amu) + (0.0108)(13.00335) = 12.011 amu.

Atomic weight (AW) is also known as average atomic mass (atomic mass).These average atomic weights, as found in the earth’s crust, are listed on the periodic table.

Atomic and Molecular WeightsAtomic and Molecular WeightsAverage Atomic MassAverage Atomic Mass

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Formula weights (FW): sum of AW for atoms in formula.

FW (NaCl) = AW(Na) + AW(Cl) = 23.0 amu+ 35.5 amu= 58.5 amu

(Remember, NaCl is not a “molecule” It exists as a 3-D array of ions)Molecular weight (MW) is the weight of the molecular formula.

MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu) = 180.0 amu

Formula and Molecular WeightsFormula and Molecular Weights

We will use FW and MW interchangeably

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The Periodic TableThe Periodic Table

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The Periodic Table is used to organize the 114 elements in a meaningful way

The Periodic TableThe Periodic Table

As a consequence of this organization, there are periodic properties associated with the periodic table.

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Columns in the periodic table are called groups (numbered from 1A to 8A or 1 to 18).

The Periodic TableThe Periodic Table

Metals are located on the left hand side of the periodic table (most of the elements are metals).Non-metals are located in the top right hand side of the periodic table. Elements with properties similar to both metals and non-metals are called metalloids and are located at the interface between the metals and non-metals.

Rows in the periodic table are called periods.

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The Periodic TableThe Periodic Table

Metals

Non-Metals

Metalloids“semiconductors”

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Some elements occur naturally as diatomic molecules

(Most elements can be viewed as uniatomic; but thereare unusual elemental molecules, e.g., P4, S8, C60.)

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Some of the groups in the periodic table are given special names.

The Periodic TableThe Periodic Table

These names indicate the similarities between group members:

Group 1A: Alkali metals - “al kali” = “the ashes” (of a fire)Group 2A: Alkaline earth metals (“earths” historically were oxides that were difficult to reduce to the metal).Group 6A: Chalcogens - “ore formers”Group 7A: Halogens - “salt formers”Group 8A: Noble gases - “unreactive” gasesAt the bottom are the lanthanides (“rare earths”) and the actinides.

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AlkaliMetals

AlkalineEarths

Noble or Inert Gases

Halogens

Chalcogens

Lanthanides (rare earths)

Actinides

Transition Metals

Navigating the Periodic TableNavigating the Periodic Table

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Different Kinds of CompoundsDifferent Kinds of Compounds

A salt, formed by ionic bonding,is formed between a metal anda nonmetal, (e.g., NaCl, Ag2O).

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A molecule, formed by covalentbonding, is formed between a nonmetal and a nonmetal, (e.g., CO2, PBr3, H2O).

Different Kinds of CompoundsDifferent Kinds of Compounds

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Different Kinds of CompoundsDifferent Kinds of Compounds

An alloy, formed by metallicbonding, is formed between a metal and a metal, (e.g., brass or nickel-steel)

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The MoleThe MoleThe mole connects the visible with the invisible.A fluorine molecule (F2) weighs 38.000 amu.A mole of fluorine molecules weighs 38.000 grams.The number of fluorine molecules in a mole isan incredibly large number, called Avogadro’s Number, N, which is 6.022 x 1023.We will be using the mole concept very often.

Amedeo Avogadro1776-1856

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The MoleThe MoleExamples:A mole of H is 1.008 grams.A mole of H2 is 2.016 grams.A mole of CO2 is 44.011 grams.A mole of CO is 28.01 grams.A mole of octane (C8H18) is 114.22 grams.A mole of copper (Cu) is 63.54 grams.A mole of table salt (NaCl) is 58.44 grams.A mole of sodium bicarbonate (NaHCO3) is 84.01 grams.A mole of Ag2O is 231.74 grams.A mole of glucose (C6H12O6) is 180.16 grams.A mole of chlorophyll (C55H72MgN4O5) is 893.51 grams.