Atoms, Molecules, and Ions Defining the Atom The Greek philosopher Democritus (460 B.C. – 370...

Atoms,Molecules,and Ions

Defining the Atom

• The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)– He believed that atoms were indivisible and

indestructible

– His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

Atomic Theory of Matter

The theory that atoms are the fundamental building blocks of matter reemerged in the early nineteenth century, championed by John Dalton.

Dalton's Postulates

All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

Dalton's Postulates

Each element is composed of extremely small particles called atoms.

Dalton's Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

Dalton's Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Law of Conservation of Mass

The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

Discovery of the Electron

In 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.http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#

Chem Talk

The Electron

• Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence.

• J. J. Thomson is credited with their discovery (1897).

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.

The Atom, circa 1900

• The prevailing theory was that of the “plum pudding” model, put forward by Thomson.

• It featured a positive sphere of matter with negative electrons imbedded in it.

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

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

Millikan Oil-Drop Experiment

Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

Millikan Oil-Drop Experiment

Robert Millikan (University of Chicago) determined the charge on the electron in 1909.

Other Subatomic Particles

• Protons were discovered by Rutherford in 1919.

• Neutrons were discovered by James Chadwick in 1932.

Radioactivity

• Radioactivity is the spontaneous emission of radiation by an atom.

• It was first observed by Henri Becquerel.• Marie and Pierre Curie also studied it.

Radioactivity

• Three types of radiation were discovered by Ernest Rutherford: particles particles rays

Discovery of the Nucleus

Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

The Nuclear Atom

Since some particles were deflected at large angles, Thomson’s model could not be correct.

The Nuclear Atom

• Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.

• Most of the volume of the atom is empty space.

Law vs. TheoryLaw vs. Theory

A A lawlaw summarizes what summarizes what happens happens

A A theorytheory (model) is an attempt (model) is an attempt to explain to explain whywhy it happens it happens

Chem Talk

Subatomic Particles

• Protons and electrons are the only particles that have a charge.

• Protons and neutrons have essentially the same mass.

• The mass of an electron is so small we ignore it.

Symbols of Elements

Elements are symbolized by one or two letters.

Atomic Number• Atoms are composed of identical

protons, neutrons, and electrons– How then are atoms of one element

different from another element?

• Elements are different because they contain different numbers of PROTONS

• The “atomic number” of an element is the number of protons in the nucleus

• # protons in an atom = # electrons

Atomic Number

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

ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)

CarbonCarbon 66 66

PhosphorusPhosphorus 1515 1515

GoldGold 7979 7979

Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0

NuclideNuclide pp++ nn00 ee-- MassMass ##

Oxygen Oxygen - - 1010

- - 3333 4242

- - 3131 1515

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

Symbols of Elements

All atoms of the same element have the same number of protons, which is called the atomic number, Z.

Isotopes

• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons.

Atomic Mass

Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.

Average Mass

• Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.

• Average mass is calculated from the isotopes of an element weighted by their relative abundances.

Measuring Atomic Mass

• Instead of grams, the unit we use is the Atomic Mass Unit (amu)

• It is defined as one-twelfth the mass of a carbon-12 atom.– Carbon-12 chosen because of its isotope purity.

• Each isotope has its own atomic mass, thus we determine the average from percent abundance.

Atomic Masses

IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus

% in nature% in nature

Carbon-12Carbon-12 1212CC 6 protons6 protons

6 neutrons6 neutrons

98.89%98.89%

1.11%1.11%

<0.01%<0.01%

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

Carbon = 12.011

Average Atomic mass

Av. Atomic Mass = (mass of 1st isostope x % abundance)+ (mass of 2nd isotope x % abundance ) + (mass of 3rd isotope x % abundance) + ….

(depending on how many isotopes )

% abundance to be used as decimal percent.

Periodic Table

• The periodic table is a systematic catalog of the elements.

• Elements are arranged in order of atomic number.

Periodic Table

• The rows on the periodic chart are periods.

• Columns are groups.

• Elements in the same group have similar chemical properties.

Periodicity

When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.

Groups

These five groups are known by their names.

Periodic Table

Nonmetals are on the right side of the periodic table (with the exception of H).

Periodic Table

Metalloids border the stair-step line (with the exception of Al, Po, and At).

Periodic Table

Metals are on the left side of the chart.

Chemical Formulas

The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.

Chemical Formulas

Molecular compounds are composed of molecules and almost always contain only nonmetals.

Diatomic Molecules

• These seven elements occur naturally as molecules containing two atoms:– Hydrogen– Nitrogen– Oxygen– Fluorine– Chlorine– Bromine– Iodine

Atoms, Molecules, and Ions Defining the Atom The Greek philosopher Democritus (460 B.C. – 370...

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