Download Topic 3 Review

Topic 3 Review
• Atomic Theory
• Atomic structure
• Valence electrons
Topic 3
The development of the Atomic theory
 Democritus concluded that matter could
not be divided into smaller and smaller
pieces forever.
 Democritus named this small, indivisible
piece, an atom; atom comes from the Greek
word atomos, which means “not to be cut”
or “indivisible.”
Topic 3
The development of the Atomic theory
• Law of conservation of mass: mass is neither created
nor destroyed during ordinary chemical reactions or
physical changes.
• Law of definite proportions: a chemical compound contains
the same elements in exactly the same proportions by
mass regardless of the size of the sample or source of the
compound.
• Law of multiple proportions: if two or more different
compounds are composed of the same two elements, then
the ratio of the masses of the second element combined with
a certain mass of the first element is always a ratio of small
whole numbers.
Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory
1. All matter is composed of extremely small
particles called atoms.
2. Atoms of a given element are identical in size,
mass, and other properties; atoms of different
elements differ in size, mass, and other
properties.
3. Atoms cannot
destroyed.
be
subdivided,
created,
or
Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory, continued
4. Atoms of different elements combine in simple
whole-number ratios to form chemical
compounds.
5. In chemical reaction, atoms are combined,
separated, or rearranged.
Topic 3
The development of the Atomic theory
Modern Atomic Theory
• Aspects of Dalton’s atomic theory that have proven
to be incorrect:
• Atoms are divisible into even smaller particles (by a
nuclear change)
• A given element can have atoms with different masses
(isotopes)
Topic 3
The development of the Atomic theory
Modern Atomic Theory
Concepts that remain unchanged.
• All matter is composed of atoms.
• Atoms of any one element differ in properties
from atoms of another element.
Discovery of the electron
• Thomson concluded: All cathode rays are
composed of electrons.
• He also found the charge to mass ratio of the
electrons.
• Millikan measured the charge of the electrons.
• The mass of an electron was found to be about
1/2000! the mass of a hydrogen atom.
Thomson’s plum pudding model
• Thomson proposed a model for the atom that is called
the plum pudding model,
• The negative electrons were spread evenly throughout
the positive charge of the rest of the atom.
Discovery of the atomic nucleus
Ernest Rutherford discovery:
• The Atom is mostly empty space
• The nucleus is positively charged
• The nucleus carries most of the atom mass
Rutherford’s atomic Model
• Dense, positive nucleus
at the center of the
atom.
• Most of atom is empty
space.
• But, can electrons just
float around in empty
space?
The Bohr Model
• Niels Bohr proposed a model in which he placed each
electron in a specific energy level.
• Electrons move in definite orbits around the nucleus,
like planets circle the sun. These orbits, or energy
levels, are located at certain distances from the
nucleus.
Electron shells and valence electrons
a) Electron shells determine how an atom
behaves when it encounters other atoms.
b) The 1st shell can hold up to two electrons, and
each shell thereafter can hold up to 8 electrons.
Valence electrons are the electrons in the
highest occupied energy level of an atom.
To find the number of valence electrons in an atom
of a representative element, simply look at its
group number.