4.1
TRANSCRIPT
CHAPTER 4 SECTION 1
Introduction to Atoms
History of Atom
All atoms share the same basic structureDuring past 200 years, scientists have
proposed different models.An atom is the smallest particle of an
element.Atomic theory grew as a series of models that
developed from experimental evidence. As more evidence was collected, the theory and models were revised.
Dalton’s Model
Based on experiments, Dalton developed a theory of structure of matter
4 main concepts: All matter is composed of tiny, indivisible particles
called atoms Atoms of each element are exactly alike Atoms of different elements have different masses Atoms of different elements can join to form
compounds
Dalton’s Model
Thomson’s Model
End of 1800sThomson discovered that atoms were not
simple, solid spheresAtoms contained subatomic particles
Very small, negatively charged Called them electrons
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively
charged particle: the electron
Modern Cathode Ray Tubes
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
Television Computer Monitor
Thomson’s Model
Also knew that atoms were electrically neutral Must contain enough positive charge to balance
negative charge of electronsDeveloped model where electrons were stuck
into a positively charged sphere Like chocolate chips in cookie dough
Thomson’s Model
Rutherford’s Model
By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons
A proton is a positive charged particle in the nucleus of an atom.
1911—Rutherford begins to test theoryHis experiments led him to believe that
protons are concentrated in a small area at center of atom Called this area the nucleus
Rutherford’s Model
Rutherford’s model describes an atom as mostly empty space, with a center nucleus that contains nearly all the mass Like the pit in a peach
Bohr’s Model
Modified Rutherford’s model in 1913Proposed that each electron has a certain
amount of energy Helped electron move around nucleus
Electrons move around nucleus in region called energy levels
The energy level is the specific amount of energy an electron has.
Energy levels surround nucleus in rings, like layers of onion
Bohr’s Model
Has been called planetary model Energy levels occupied by electrons are like orbits of
planets at different distances from the sun (nucleus)
Electron Cloud Model
Model accepted todayElectrons dart around in an energy levelRapid, random motion creates a “cloud” of
negative charge around nucleusElectron cloud gives atom its size and shape
Electron Cloud Model
Findings
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
Atomic Number
Atoms are composed of identical protons, neutrons, and electronsHow then are atoms of one element different
from another element?Elements are different because they
contain different numbers of PROTONSThe “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.
Element # of protons Atomic # (Z)
Carbon 6 6
Phosphorus 15 15
Gold 79 79
Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0
Nuclide p+ n0 e- Mass #
Oxygen - 10
- 33 42
- 31 15
8 8 1818
Arsenic 75 33 75
Phosphorus 15 3116
The Complete Set-UP
Contain the symbol of the element, the mass number and the atomic number.
X Massnumber
Atomicnumber
Subscript →
Superscript →
Isotopes
Dalton was wrong about all elements of the same type being identical
Atoms of the same element can have different numbers of neutrons.
Thus, different mass numbers.These are called isotopes.
Isotopes
Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.
Naming Isotopes
We can also put the mass number after the name of the element:carbon-12carbon-14uranium-235
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Isotope Protons Electrons Neutrons Nucleus
Hydrogen–1 (protium) 1 1 0
Hydrogen-2(deuterium) 1 1 1
Hydrogen-3(tritium)
1 1 2
IsotopesElements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that differ in the number of neutrons.