Chemistry 101 : Chap. 2 Atoms, Molecules and Ions (1) Atomic Theory of Matter (2) The discovery of Atomic Structure (3) The Modern View of Atomic Structure (4) Atomic Weight (5) Periodic Table (6) Molecules and Molecular Compounds (7) Ions and Ionic Compounds (8) Naming Inorganic Compounds
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Chemistry 101 : Chap. 2 Atoms, Molecules and Ions (1) Atomic Theory of Matter (2) The discovery of Atomic Structure (3) The Modern View of Atomic Structure.
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Chemistry 101 : Chap. 2Chemistry 101 : Chap. 2
Atoms, Molecules and Ions
(1) Atomic Theory of Matter
(2) The discovery of Atomic Structure
(3) The Modern View of Atomic Structure
(4) Atomic Weight
(5) Periodic Table
(6) Molecules and Molecular Compounds
(7) Ions and Ionic Compounds
(8) Naming Inorganic Compounds
The Atomic Theory of MatterThe Atomic Theory of Matter
The history of development of atomic theory of matter begins in ancient Greece. However, modern atomic theory has it’s origin in a burst of scientific discovery between 1870 and 1930.
Democritus (460 ~ 370 BC)
Democritus proposed atomic theory of matter.
He and other Greek philosophers believed that material world must be made up of hard and tiny indivisible particles that they called atomos, which are in constant motion.
The Atomic Theory of MatterThe Atomic Theory of Matter
Aristotle (384 ~ 322 BC)
Aristotle proposed 4 element theory of matter.
Earth
Water
Fire
Air
hot dry
wet cold
The school of thought laid out by Socrates, Plato and Aristotle dominated the western philosophy for 2000 years and the atomic theory of matter was completely buried.
The Atomic Theory of MatterThe Atomic Theory of Matter
John Dalton (1766 ~ 1844)
Dalton proposed that all matter is made up of atoms and stated that elements are the simplest form of matter.
Dalton’s Atomic Theory(1)Each element is composed of atoms
(2)All atoms of a given element are identical,
but they are different from the atoms of
all other elements
(3) Atoms are neither created nor destroyed
in chemical reactions.
(4) Compounds are formed from chemical
combination of two or more atoms.
The Atomic Theory of MatterThe Atomic Theory of Matter
What can Dalton’s theory explain? (1) Law of constant composition
In a given compound, the relative numbers and kinds of atoms
are constant. [postulate 4]
(2) Law of conservation of mass
The total masses of material present before and after a chemical
reaction are identical [postulate 3]
(3) Law of multiple proportions
If elements A & B combine to form more than one compound, the
masses of B which can combine with a given mass of A are in
the ratios of small whole numbers
12g C + 16g O CO or 12g C + 32g O CO2 16g : 32g = 1:2
The Discovery of Atomic StructureThe Discovery of Atomic Structure
After Dalton’s atomic theory, not much of progress had been made andno one had direct evidence for the existence of atom. Then, things startedto change in late 1800s…
William Crooks (1832 ~ 1919): Cathode-ray tube (CRT) [1879]
A high voltage between two electrodes in a partiallyevacuated tube generates electrical discharge(cathode ray)
The Discovery of Atomic StructureThe Discovery of Atomic Structure
J. J. Thomson (1856 ~ 1940) : Discovery of electron [1897]
(1)Rays are the same regardless of the
identity of the cathode material
(2) Conduct quantitative analysis of the
effect of electric and magnetic field
determine the charge to mass ratio
charge/mass = 1.76 108 C/g
He discovered that cathode raysare negatively charged particles, whichhe originally called ``corpuscles’’ . He won a Nobel prize in physics [1906].
The Discovery of Atomic StructureThe Discovery of Atomic Structure
Robert Millikan (1868 ~ 1953) : Determine the charge of electron [1907]
Millikan’s oil-drop experiment
Measured charge = 1.60 10-19 C
Electron mass = charge/[charge/mass]
= 9.10 10-28 g
The machine on the right hand side isthe original apparatus Millikan usedto perform his oil-drop experiment.He won a Nobel prize in physics [1923].
The Discovery of Atomic StructureThe Discovery of Atomic Structure
Ernest Rutherford (1871~1937): Discovery of nucleus [1911]
He directed his graduate student Hans Geiger and undergraduate studentErnest Marsden to carry out -paticleexperiment. He won a Nobel prize in chemistry [1908].
The Discovery of Atomic StructureThe Discovery of Atomic Structure
Radioactivity: Generation of - particles
- ray: particles with +2 charge - ray: particles with 1 charge
- ray: high energy radiation with no charge
The Discovery of Atomic StructureThe Discovery of Atomic Structure
From the scattering experiment….
(1) Most -particles simply pass through the gold foil.
(2) Small amount of scattering was observed at large
angles.
Rutherford postulated that..
(1) Most of the total volume of an atom is empty space. (2) Most of the mass of an atom and all of its positive charge reside in a very small region, called nucleus.
Rutherford also found the existence of protons inside of nucleus [1919].Another particle in nucleus, neutron, was found by James Chadwick in 1932.
Early Models of an Atom Early Models of an Atom
J. J. Thomson’s model “plum-pudding model”
Rutherford’s model
+
Rutherford's Model:
Electrons are negatively charged, but atoms as a whole are neutral.
Modern View of Atomic Structure Modern View of Atomic Structure
The list of subatomic particles has grown considerably since the discovery of electrons, but only the electron, proton and neutron have a bearing on chemical behavior.
A convenient unit (non-SI) to describe the dimensions of atoms and molecules is Angstrom (Å). 1 Å = 1 10-10 m = 100 pm
Modern View of Atomic Structure Modern View of Atomic Structure
Properties of subatomic particles
Particle Charge (C) Mass (g) Mass (amu)
Proton +1.60 10-19 (+1) 1.6727 10-24 1.0073
Neutron 0 ( 0) 1.6750 10-24 1.0087
Electron 1.60 10-19 (1) 9.1097 10-28 5.486 10-4
Every atom has an equal number of protons and electrons so that it hasno electrical charge
Atomic Mass Unit (amu)
1 amu = 1/12 of the mass of carbon (12C) atom
= 1.66054 10-24 (g)
Modern View of Atomic StructureModern View of Atomic Structure
The characteristics of each atom are determined by the numbers ofproton, neutron and electrons.
Atomic Number: The number of protons in the nucleus of an atom. Mass Number: The total number of protons plus neutrons in the atom Isotopes : Atoms with identical atomic numbers but different mass
numbers such as C-14 and C-12.
Hydrogen:1 proton
Helium:2 protons
2 neutrons
Lithium:3 protons
4 neutrons
Beryllium:4 protons
5 neutrons
Modern View of Atomic StructureModern View of Atomic Structure
Same information : An element is defined by the number of protons