Post on 12-Jan-2016
transcript
Chapter 2
Study Guide
Dr. Joseph Silver
this chapter deals with- structure of atoms
- how atoms form molecules- the periodic table
- the control of chemical reactions
anything that has mass (made up of atoms)
andoccupies space (no matter how small)
is calledmatter
the word atomwas used 2000 years ago in Greece and
we still are learning more about atoms today
we have not yet been able to actuallyclearly see an atom
butscientists have done many experiments
whichshow that one or more electrons (-)
circle about a nucleuscontaining protons(+) and neutrons(0 no
charge)
atomic number = number of protons in an atom
atomic mass = mass of protons+neutrons+electrons
electron mass is so small that it contributesvery little to the mass of an atom
Atomic mass is also referred to asdaltons
the mass of a proton = 1 daltonthe mass of a neutron = 1 dalton
the mass of an electron = 1/2000th of a Daltonso atomic mass is mainly
the mass of protons and neutrons
remember mass represents the amount of matter in any atom, molecule or anything
else
weight represents the amount of force gravity
exerts on a substance
if you go to Mars your weight changesbut your mass does not change
the nucleus of an atom has protons and neutrons packed into the center of the atom
andthe electrons are located in circular orbits
(orbitals) or layers about the nucleus
-electrons travel so quickly that we cannot see one -they do not fly off due to their attraction to the
nucleus -atoms with large atomic numbers have many
orbital layers
when protons are equal to electrons - neutral atom
when protons do not equal electrons – atom is an ion
if there are more protons than electrons – positive cation
if there are more electrons than protons – negative anion
C12 = 6P + 6E + 6 neutrons C13 = 6P +6E + 7 neutronsC14 = 6P + 6E + 8 neutrons
atoms of an element with different number of neutrons are called isotopes
some isotopes release radioactive energy as the extra neutrons are released
isotopes have many uses in industry, medicine, chemistry, and biology
one use is to date the age of fossils
isotopes have a half lifewe know how long it takes for one-half of the
energy in an isotope to disappear and we use this information to compare the amount of an isotope in new material and in a fossil
and arrive at the age of the fossil
electrons determine the chemical behavior of atoms
how
when the outer electron shell of an atom is not full
it will give away electrons, or get electrons from other atoms, or share electrons with other atoms in order to have the outer shell filled
the farther an electron is from the nucleus the greater is the energy of the electron
likewisethe closer to the nucleus
the less energy in an electron
look at the drawings on pages 34the first electron layer can have 2 electrons
the second layer can have 8 electronsthese 8 are made up 4 orbitals each with 2
electrons(see fig 2.8)
for the purpose of this class any layer or orbital past the second orbital is
filled by 8 electrons
the electrons in the outer shell of an atom are referred to as valence electrons
and theseare the electrons available for chemical
reactions
-electrons are first added to the first shell (level)
-when it is full electrons are added to the next shell
-when the 2nd shell is full electrons are added to the 3rd
and so on for the 90 naturally occurring elements
as well as the laboratory manufactured elements
an atom with 1 electron in its first outer shell
would love to give it away or gain one so that
the outer shell a would be complete
an atom with 7 electrons in the outer shellwould love to find an atom to give it 1
electronand
an atom with 6 electrons in the outer shellwould love to find an atom to give it 2
electronsor 2 atoms to each give it 1 electron
in atoms with multiple shellsgaining electrons can fill the last
orbital levelor giving away electrons can empty the
last shell and
the lower level shell when it is filledbecomes the outer layer
see fig 2.12 page 38
on page B-1there is a full periodic table
showing the 90 natural elements as well as 28 man made elements
and onpage 34 there is a drawing showing the first
18 elements
in the periodic tablethe elements are arranged
according to their atomic number(the number of protons or electrons)
they are arranged in columns according to the number of electrons
in the outermost orbital shell
the electrons in the outer shellare called
valance electronsand they are located in the valence shell
these are the electrons whichare involved in chemical reactions
energy provides the ability to do workwhen an electron is released from its valence shell
the next lower orbit becomes the outer orbital shell
and the released electronreleases electron or atomic energy which is
used to power chemical reactions
look at the periodic table in the textbook-put a 1 over the H
-put a 2 over Be-put a 3 over B-put a 4 over C-put a 5 over N-put a 6 over O-put a 7 over F
-put an 8 over Hethe number represents the number of electrons present
in the outer shell of the elements in the column (H,Li,Na,K,Rb,Cs,Fr all have 1 electron in the
outer shell)(Be,Mg,Ca,Sr,Ba, Ra all have 2 electrons in the
outer shell)
an element with 1 electron in the outer shell can readily interact with an element having
7 valence electrons
an element with 1 valence electron can also readily interact with elements having 6
valence electrons
by exchanging or sharing electrons they fill
the outer shells and form compounds with stable atoms
ionic bonds form when atoms exchange electrons
and become positive and negative ionswhich are attracted to each other and form
a new molecule or compoundlook at fig 2.9
sodium gives up its 1 outer electron chlorine adds 1 electron to its 7 and fills the
shellsodium becomes positive by giving away an
electronchlorine adds an electron and becomes negative
the + and – ions attract to become sodium chloride
carbon has 4 electrons in its outer shelltwo carbons share their outer 4 electronsmaking an outer shell in each atom with 8
electronsthus each has a full outer shell with 8 electrons
forming a covalent bond
the electrons travel so fast that it is as ifthe 8 electrons are in both atoms at the
same time
when the atoms forming covalent bondsare similar in size the bonds may
be neutralbut
if one atom is much larger than the other then the covalent bond may show a degree of polarity
(one end of the molecule may be more positive or negative than the other end of
the molecule)
the ability of atoms to form chemical bonds is influenced by
- temperature (2x for every 10C)- concentration
-catalysts (enzymes)- pressure (keeps atoms closer together)
I will on many occasions mention the shape of a molecule
why
because molecules cannot talk to each othermolecules cannot use a phone to tell you
something
so
molecules react according to their shapethey fit together like pieces of a puzzle