“What are ATOMS?”

Leucippus and Democritus (400? BC)

• Greek philosopher and his “student”

• New idea: - there is a limit to how far matter can be divided

- atomos: “uncuttable” particles,

different shape/size

Robert Boyle(1627-1691)

English scientist and philosopher

Hypothesis: everything made of corpuscles (tiny particles of various sizes/shapes).

Thinking about Atoms…

Some Definitions: Matter Classification

• Element: 1) A pure substance that cannot be broken down into simpler substances by a chemical reaction.

2) A substance made of unique, (nearly) identical atoms.

• Compound:

1) A pure substance that can be broken down into simpler substances by a chemical reaction.

2) A substance made in a chemical reaction by combining two or more different elements.

Start of Modern Era of Atoms:Dalton’s Atomic Theory

John Dalton

(1766-1844)

British chemist,

lecturer, and

meteorologist

Dalton’s Atomic Theory (1803) - 1

1) All matter is made up of indivisible and indestructible basic particles called atoms.

2) All atoms of a given element are identical, both in mass and in properties. Atoms of different elements have different masses and properties.

3) Compounds are formed when atoms of different elements combine in the ratio of small whole numbers.

Dalton’s Atomic Theory (1803) - 2

4) Elements and compounds are composed of definite arrangements of atoms.

Chemical change occurs when the atomic arrays are rearranged.

Significance of Dalton’s Atomic Theory

• Broke down 18th-century view of “elements”

• Bridged gap between lab data and hypothetical atom.

- way of calculating relative atomic weights.

• Explained Law of Conservation of Mass

- “Initial Mass = Final Mass”

- Only reorganizing of unchangeable atoms occurs in chemical reaction.

Dalton: inconsistencies uncovered…

1) The basic state of an element is one atom?

Perhaps… basic natural state of an element may be a molecule made of 2 or more atoms.

2) Dalton: “Thou knows…no man can split the atom.” Not so: radioactivity.

3) Atoms of given element have same mass and properties? Not so: isotopes exist…

More Definitions:• Molecule:

- any combination of 2 or more atoms. - atoms can be same or different elements.

• Allotropes: - different molecular combinations of a

single element.

Ex] oxygen as O2 or ozone O3

• Isotopes: (more later…)- atoms of same element that differ in mass only

Identification of Elements

• Physical properties

• Chemical properties

• Flame test for solids/solutions

• Interaction with light:line-absorption spectrumline-emission spectrum

Flame Test for Element Identification

(From left) Sodium, potassium, lithium; strontium, barium, potassium.

Elements: Ages of Discovery

Classification of the Elements:Development of the Periodic Table

• Dobereiner: “Triads”

• Newlands: “Octaves”

• Mendeleev: first-published “Period” definition

• Meyer: second-published “Period” definition

Dobereiner’s “Law of Triads” (1817)

• Organized a few elements by similar chemical properties

• Hypotheses:

- Groups of three elements (triads) existed.

- In order of increasing atomic weight, middle element’s properties were the

average of other 2 elements’ properties.

Newlands’ “Law of Octaves” (1863?)

• Every eighth element repeats properties of first “Octave” element

[like middle-C to high-C on piano]

DmitriMendeleev(1834-1907)

“Creator of the Periodic Table”

(probably formulated periodic idea at same

time as Meyer)

Mendeleev’s early notes

for the Periodic Table

(1869)

Characteristics of Mendeleev’s Table

• Organized 60+ known elements…

- in families (groups) with similar properties

- by valence = “combining number” (split out elements with multiple valence)

- roughly by atomic weight (moved 17 elements based on properties rather than weight)

• Could use to predict existence of new elements (of 10, found 7; other 3 do not exist)

• NOTE: at first, no “rare gases” were classified

Comparison of eka-silicon’s predicted properties

and known Group 4 properties

Eka: “one beyond”

Discovery of Atomic Structure;Sub-atomic Particles

• Thomson: electron mass-to-charge ratio

• Millikan: electron charge

• Rutherford: mass and charge of nucleus

• Chadwick: neutron (1932). (Nobel prize in 1935)

• Bohr: electron energy levels (Topic 7)

Joseph John Thomson(1856-1940)

British physicist and mathematician

Nobel Prize in 1906 (existence of electrons)

1897: calc’d electron’s charge-to-mass ratio in cathode-ray tube expt.

Robert Millikan (1868-1953)

U.S. physicist

Nobel Prize in 1923 (charge of electron:

1909 oildrop expt.)

With Thomson’sresult, this allowed

calculation of electron mass. Millikan’s experimental apparatus.

Ernest Rutherford(1871-1937)

nuclear physicist,New Zealander teaching

in Great Britain

Nobel Prize in 1908(radioactive decay)

Gold foil experiments

Rutherford’s Experiments (1910-11) (done by undergrad Ernest Marsden/physicist Hans Geiger)

• Fired beam of alpha particles [He+2] at gold foil.

• Most particles went straight through, some were deflected, BUT a few were reflected straight back to source!

Rutherford’s Experiments (cont.)

• Rutherford’s description:“It was about as incredible as if you had fired a 15-inch shell at a piece of paper and it came back and hit you.”

• Interpretation: gold atom has small, dense, positively-charged nucleus surrounded by “mostly empty” space in which the electrons must exist.

• Calculated nuclear mass as mass of positively-charged protons. Protons about half of actual mass: suggests neutral particles of same mass as proton?

Known Properties of Subatomic Particles

Property

Particle

Mass (amu),

Mass (g)

Relative

Charge

Electron 0.00055

9.1093897 x 10-28

- 1

Proton 1.00728

1.6726231 x 10-24

+ 1

Neutron 1.00866

1.6749286 x 10-24

0

Proton/Electron Count in Atoms

• Every neutral atom has an equal number of electrons and protons.

• The number of protons, or “atomic number”, is unique for each element.

• If an atom has unequal numbers of protons and electrons, it is called an ion (which is a charged atom).

Example] Chlorine has atomic number 17. How many protons & electrons are in one Cl atom? How many protons & electrons are in one Cl- (chloride ion)?

Neutron Count in Atoms: Isotopes

• For many elements, the atoms of that specific element are only “nearly identical” to each other: the number of neutrons may vary.

• The “nearly identical” forms are called isotopes.

Example: Chlorine atoms exist in nature as a mixture of two

isotopes: Cl-35 and Cl-37. 3 in four Cl atoms are Cl-35; 1 in four is Cl-37.

The “atomic mass” of Cl in Table is an average.

Modern “Periodic Table” Organization

• Elements are NOW placed in order of increasing atomic number (# of protons).

- Why? Refer to “isotopes”: natural weight irregularities exist

• This relationship between nuclear charge and arrangement of elements in Table was discovered by Henry Moseley in 1914.

• In 1860s, Mendeleev / Meyer could NOT have predicted a relationship to subatomic particles!