Midterm Review
Topic 1
The Atom
Vocabulary
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Atom
Atomic mass
Atomic mass unit
Atomic number
Compound
Electron
Element
Excited state
Ground state
Heterogeneous
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Homogeneous
Isotope
Mass number
Mixture
Neutron
Orbital
Proton
Pure substance
Wave-mechanical model
Dalton’s Atomic Theory
• All elements are composed of identical
atom.
• All atoms of a given element are identical.
• Atoms of different elements are different;
that is, they have different masses.
• Compounds are formed by the combination
of atoms of different elements. (Law of
multiple proportions)
Parts of the Atom
• Electron
– J.J. Thomson
• Smallest unit of the atom
• Negativity charged
• “Plum Pudding” model
Parts of the Atom
• Nucleus
– Ernest Rutherford
• Gold Foil Experiment
– Alpha Particles
– If the “Plum Pudding” model was correct then the α particles
would pass through the foil with just a few being slightly
deflected.
– Some of the α particles were deflected, and some even
bounced back.
» Rutherford concluded that atoms have a small dense
positively charged central core, and the rest is mostly
empty space
Modern Atomic Theory
• The Bohr Model
– Proposed a model of the atom that showed
that atom with a dense nucleus with
electrons found in surrounding orbits
• He proposed that electrons requires a specific
amount of energy to stay in each orbit
• He suggested that the first orbital could hold only
2 electrons and that the 2nd orbital could hold 8 e-
Modern Atomic Theory
• The Wave-Mechanical Model
– Electrons have properties for both waves and
particles
– The major difference between the Bohr model
and the Wave-Mechanical Model is the way that
the electrons are pictured.
• Instead for moving in fixed orbits, the electrons move in
areas called orbitals.
– Orbital is described as a region in which an electron of a
particular amount of energy is most likely to be found.
Modern Atomic Theory
• The Modern model of the atom has been
developed over a long period of time
Wave-mechanical
Bohr
Rutherford
Thompson
Dalton
Subatomic Particles
• Because the mass of a proton is so small, it is
more convenient to use a different scale
whose units are called atomic mass units.
Subatomic Particles
• Each atom of a specific element must contain
the same number of protons as each other
atom of the same element
– The number of protons in the nucleus of the
atoms is the atomic number of that element.
• For example : The atomic number of Chlorine is 17 so
therefore each atom of Chlorine contains 17 Protons
Subatomic Particles
• Mass Number
– The nucleus of the carbon atoms,
containing 6 protons and 6
neutrons is taken as the standard
mass.
– The sum of the number of protons
and the number of neutrons is
called the mass number
Isotopes
• All atoms of a given elements contain
the same number of protons, but the number of
neutrons can vary.
– When the number of neutrons vary, then the mass
of then atoms will be different.
• These different forms of an atom are
called isotopes.
– Isotopes are atoms of the same element that have
different numbers of neutrons
Isotope Symbols
Nuclear symbol
Atomic Masses
• This is the number that is found on the
periodic table.
Avg.
Atomic
Mass
(mass)(% )  (mass )(% )
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100
Location of Electrons
• Energy levels
– The orbitals in an atom form a series of energy
levels in which electrons may be found.
– Each electron in an atom has its own distinct
amount of energy that corresponds to the energy
level that it occupies.
• Electrons can gain or lose energy and move to different
energy levels
• Electrons can absorb only distinct, or fixed about of
energy that allows it to move to a higher energy level.
Excited and Ground State
• When the electrons occupy the lowest
available orbitals, the atom is said to be in the
ground state.
• When electrons are subjected to stimuli such
as heat, light, or electricity, an electron may
absorb energy and temporally move to a
higher energy level
• This unstable condition is called an excited
state.
Excited State
• The electron quickly returns to a
lower available energy level,
emitting the same amount of energy it
absorbed to go to the higher energy level.
• This admission of energy can be seen with an
instrument called a spectroscope.
Atomic Spectra
• Each element has its unique
atomic spectra.
• These spectra can be used
to identify elements
Electron Arrangements
• Although the electrons in an atom contribute little
to the mass of an atom, their arrangement
determines its chemical properties.
– The chemical properties of an
element are based on the number
of valance electrons, or outer
most electrons.
Orbitals and Electron Capacity of the First Four Principle Energy Levels
Principle
energy
level (n)
1
2
3
4
Type of
sublevel
Number of
orbitals per
type
Number of
orbitals per
level(n2)
Maximum number
of electrons (2n2)
s
s
p
s
p
d
s
p
d
f
1
1
3
1
3
5
1
3
5
7
1
2
4
8
9
18
16
32
Electron
Configuration
• Follow the following rules to write and electron
configuration
– Each added electron is placed into the sublevel of lowest
available energy level
– No more that two electrons can be placed in any orbital
– A single electron must be placed into each orbital of a
given sublevel before any pairing takes place (Empty Bus
seat Rule)
– The outermost principle energy level can only contain
electrons in the s and p orbitals
Orbital Notation
• While electron configuration notation can be
useful, it does not show how the electrons are
distributed in each sublevel
Types of Matter
Heterogeneous
Homogeneous
• Some matter looks uniform and doesn’t seem
to be made up of parts.
• When an material had uniform composition
throughout, the sample is said to be
homogeneous.
– It can contain more that one type of particle, but
particles are evenly mixed
Types of Mater
Heterogeneous
Homogeneous
• Other materials are
obviously made up of parts.
• Such materials, which have
varying compositions, are
said to be heterogeneous
– They are made up of parts
with different chemical and
physical properties.
– These parts are not
uniformly mixed or dispersed
Types of Matter
• Matter can be divided into major categories
– Pure substances
• All pure substances are homogeneous
– Mixture
• Can be either homogeneous or heterogeneous
Pure Substance
• A sample is a pure substance id its
composition is the same throughout the
sample.
• There are two types of pure substances
– Element
– Compound
Element
• Elements are substance that cannot be broken
down or decomposed into simpler substances
by chemical means.
– There are 91 naturally occurring elements
– The elements are arranged on the periodic table
of elements
Compound
• Compounds are composed of two or more
elements that are chemically combined in
definite proportions by mass.
– Although all compounds contain at lease 2
different types of atoms, the composition of the
compound is the same throughout.
– The Law Of Definite Proportions
Mixture
• A mixture is a combination of two or
more pure substances that can be
separated by physical means
Distinguishing between Mixtures and
Compounds