Democritus (460-370 B.C.E.) and his mentor Leucippus
first person on record to have postulated that matter is composed of atoms.
Dalton's atomic theory has three parts
Each element is composed of tiny, indestructible particles called atoms.
All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.
Atoms combine in simple, whole-number ratios to form comp
scanning tunneling microscope
to move xenon atoms to form the letters I, B, and M.
J. J. Thomson (1856-1940)
discovered a smaller and more fundamental particle called the electron.
Thomson discovered the following:
Electrons are negatively charged.
Electrons are much smaller and lighter than atoms.
Electrons are uniformly present in many different kinds of substances.
He proposed that atoms must contain positive charge that balances the negative charge of electrons
Rutherford's gold foil experiment (1909
Tiny particles called alpha-particles were directed at a thin sheet of gold foil. Most of the particles passed directly through the foil. A few, however, were deflected�some of them at sharp angles.
Discovery of the atomic nucleus
from Rutherfords foil experiement, some paricles reflected back, showing a nuclues
Statement by Rutherford of the Nuclear Theory of the Atom.
3 points
Most of the atom's mass and all of its positive charge are contained in a small core called the nucleus.
Most of the volume of the atom is empty space through which the tiny, negatively charged electrons are dispersed.
The number of negatively charged ele
Distribution of Mass in the Atom is Not Uniform
The dense nucleus makes up more than 99.9% of the mass of the atom but occupies only a small fraction of its volume.
The electrons are distributed through a much larger region but don't have much mass.
Matter at its core is less uniform than it appears.
I
The proton is nearly 2000 times as massive as an electron.
...
Electrical Charge Is a Property of Protons and Electrons
Electrical charge is a fundamental property of protons and electrons.
� Positive and negative electrical charges attract each other.
� Positive-positive and negative-negative charges repel each other.
� Positive and negative charges cancel each other so t
The Properties of Protons, Neutrons, and Electrons
Protons and neutrons have very similar masses.
Electrons have almost negligible mass.
Electrical Storm Provides Evidence of Charge in Matter
Matter is normally charge-neutral, having equal numbers of positive and negative charges that exactly cancel.
In an electrical storm, the charge balance of matter is disturbed.
Elements
are defined by their numbers of protons.
It is the number of protons in the nucleus of an atom that identifies the atom as a particular element.
If an atom had a different number of protons, it would be a different element.
The number of protons in the nu
Origins of the Names and Symbols of the Elements
Most chemical symbols are based on the English name of the element.
Some symbols are based on Latin names.
lead Pb plumbum
mercury Hg hydrargyrum
iron Fe ferrum
silver Ag argentum
tin Sn stannum
copper Cu cuprum
Early scientists gave newly discovered elements names that reflected their properties
Argon, from the Greek argos, means "inactive."
Bromine originates from the Greek word bromos, meaning "stench.
Other elements were named after countries:
Polonium after Poland
Polonium after Poland
Francium after France
Americium after the United States of America.
Other elements were named after scientists.
.Curium is named after Marie Curie
Dmitri Mendeleev
a Russian chemistry professor, proposed from observation that when the elements are arranged in order of increasing relative mass, certain sets of properties recur periodically.
Locate Metals, Nonmetals, Metalloids on the Periodic Table..
Metals
good conductors of heat and electricity
can be pounded into flat sheets (malleability).
Metals can be drawn into wires (ductility).
Metals are often shiny (lustrous).
Metals tend to lose electrons when they undergo chemical changes.
Good examples of metal
Nonmetals
Nonmetals have more varied properties; some are solids at room temperature, while others are gases.
As a whole, nonmetals tend to be poor conductors of heat and electricity.
Nonmetals tend to gain electrons when they undergo chemical changes.
Good example
Metalloids
Metalloids are also called semiconductors because of their intermediate electrical conductivity, which can be changed and controlled
Main Group Elements and the Transition Series of Elements
Main group elements are first two and last 6 columns, transitional elements are in between the two
element groups
alkali metals
include lithium, potassium, rubidium, and cesium
alkaline earth metals
beryllium, magnesium , calcium, strontium, and barium.
Halogens
fluorine, chlorine, bromine, iodine ,astatine
Noble Gases
helium,neon,argon, krypton, and xenon
ions
In chemical reactions, atoms often lose or gain electrons to form charged particles
cations
Positive ions
anions
Negative ions
Ion charges are usually written with the magnitude of the charge first, followed by the sign of the charge.
Examples: Mg2+, O2-
Ion Charge Formula
#p (positive) - #e (negative)
isotopes
Atoms with the same number of protons but different numbers of neutrons
mass number (top number (A)
the sum of the number of protons and the number of neutrons.
Radioactive isotopes are not always harmful
Many have beneficial uses.
For example, technetium-99 (Tc-99) is often given to patients to diagnose disease.
The radiation emitted by Tc-99 helps doctors image internal organs or detect infection.
The Atomic Theory
Ancient Greeks: Matter is composed of small, indestructible particles.
Dalton: Matter is composed of atoms.
Atoms of a given element have unique properties that distinguish them from atoms of other elements.
Atoms combine in simple, whole-number ratios to
The Nuclear Model of the Atom
The atom is composed of protons and neutrons, which compose most of the atom's mass and are grouped together in a dense nucleus.
Electrons comprise most of the atom's volume.
Protons and neutrons have similar masses (1 amu), while electrons have a much sm
Charge
Protons and electrons both have electrical charge.
The charge of the proton is 1+ and the charge of the electron is 1-.
The neutron has no charge.
When protons and electrons combine in atoms, their charges cancel.
Many of the machines and computers we dep
The Periodic Table
It tabulates all known elements in order of increasing atomic number.
Columns of elements have similar properties and are called groups or families.
Elements on the left side are metals. They tend to lose electrons in their chemical changes.
Elements on t
Atomic Number
The characteristic that defines an element is the number of protons in the nuclei of its atoms.
This number is called the atomic number (Z).
Ions
When an atom gains or loses electrons, it becomes an ion.
Positively charged ions are called cations.
Negatively charged ions are called anions.
Cations and anions occur together so that matter is charge-neutral.
Isotopes
Atoms of the same element with different numbers of neutrons are called isotopes.
Isotopes are characterized by their mass number (A), the sum of the protons and the neutrons in the nucleus.
Each naturally occurring sample of most elements has the same pe
Learning Objectives
LO: Recognize that all matter is composed of atoms.
LO: Explain how the experiments of Thomson and Rutherford led to the development of the nuclear theory of the atom.
LO: Describe the respective properties and charges of electrons, neutrons, and protons.