The Chemical Basis of Life

1
The Chemical Basis of Life

• Mr. Gawles Biology I Class
• September 2005

2
Life Requires 25 elements

• Life is composed of matter
• Matter is composed of chemical elements (a
  substance that can not be chemically broken down
  into other substances)
• 92 elements occur in nature
• 25 are essential for life (O, C, H, N96 of us)
• Trace elements are essential to life but in
  minute quantitiesi.e., iodine, iron, magnesium,
  calcium etc
• Compounds are substances containing two or more
  elements in a fixed ratio (i.e., water, table
  salt)

3
Elements and Atoms

• Each element consists of one kind of atom, which
  is different from the atoms of other elements
  (how?)
• An atom is the smallest unit of matter that still
  retains the properties of an element
• Physicists have split the atom into dozens of
  subatomic particles, but we need only consider
  three

4
Atomic Structure

• Three key subatomic particles
• Proton single positive charge
• Electron single negative charge
• Neutron neutral (no) charge
• Protons and Neutrons are tightly packed in the
  core or nucleus of the atom
• Electrons orbit the nucleus (in an electron
  cloud) at nearly the speed of light
• Electrons stay in orbit b/c of the attractive
  forces of the positively charged protons of the
  nucleus

5
So what makes one element different from another?

• Elements differ in the of PROTONS in their
  atoms
• All atoms of a particular element have the same
  unique of PROTONS (this is called the atomic
  number)
• The number of protons as a rule is equal to the
  number of electrons tooso atoms tend not to
  carry a charge
• The of protons AND neutrons in the nucleus
  (almost all of the weight of the atom) is called
  the atomic mass (aka mass number)
• So, what is atomic and atomic mass of helium?
  Of carbon, of oxygen, of nitrogen, etc etc

6
Some elements have different forms called isotopes

• The different isotopes of an element have the
  same numbers of protons and electrons but have
  different numbers of NEUTRONS
• Example Isotopes of Carbon
• Carbon-12 (12C) w/ 6 neutrons99 of naturally
  occurring carbon
• Carbon-13 (13C) w/ _ neutrons0.99
• Carbon-14 (14C) w/ _ neutrons0.01
• Is unstable nucleus gives off particles of
  energy (radiation)?said to be radioactive
• Radioactive isotopes can be dangerous in nature
  but useful in medicine
• Dangerous because it can damage DNA and could
  lead to mutations and cancer
• Useful b/c all forms of an element are taken up
  by cells so doctors can use isotopes as
  biological spies to trace the fate of atoms in
  living things (kidney heart issues)

7
How and why do atoms interact the way they do?

• KEY IDEAS
• The structure of an atom dictates how it
  interacts with other atoms
• It is the number of electrons in the outermost
  shell that determines the chemical properties and
  reactivity of an atom

See Big Ideas and Questions on Shared Biology I
Website
8
Atoms and Chemical Reactions

• The arrangement of electrons determines the
  chemical properties of an atom (and by extension
  of an element) and how an atom behaves when it
  encounters other atoms
• Reality 1 Electrons vary in the amount of
  energy they possess
• Reality 2 The further away an electron is from
  its nucleus, the more energy it has
• Reality 3 Electrons in an atom occur only at
  certain energy levels called electron shells
• Depending upon the atomic number, an atom may
  have one, two, or many electron shells
• For atoms with more than one shell, electrons in
  the outermost shell are highest in energy (see 2
  above) and therefore are the most reactive
• Reality 4 Each shell can hold up to a specific
  of electrons innermost is full at 2
  outermost full at 8

See Worksheet 5
9
Atoms and Chemical Reactions II

• Take home lesson it is the number of electrons
  in the outermost shell that determines the
  chemical properties and reactivity of an atom
• Atoms whose outer shells are not full tend to
  interact with other atoms (via chemical
  reactions) until their outer shell needs are
  met.
• How many electrons in outer shell of
  hydrogen/carbon/nitrogen/oxygen? (WS5)
• Contrast this with helium, neon, argon etc
• These are called the inert gasses b/c they are
  unreactive

10
So how does a chemical reaction enable an atom to
fill its outer shell?

• When two (or more) atoms with incomplete outer
  shells react, each atom gives up or acquires
  electrons so that both partners end up with
  complete outer shells
• This can happen one of two ways1. Atoms may
  share electrons (COVALENT BONDING) with each
  other
• THIS IS A VERY STRONG BOND!
  OR2. Atoms may transfer (give or
  receive) electrons to and from each other (IONIC
  BONDING)
• THIS IS A WEAKER BOND THAN COVALENT

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Ionic Bonding involves transfer of electrons

• Ionic bonds are strong bonds resulting from
  oppositely charged IONS attracting each other
• An ION is an atom with an electrical charge
  resulting from the loss or gain of an electron in
  a chemical reaction
• Example Sodium (Na) Chlorine (Cl) form an
  ionic bond when they become sodium chloride
  (NaCl) or salt.

12
The making of saltPart I

• Na has only __ electron(s) in its outer shell
• Cl has __ electrons in its outer shell
• Which atom do you think will benefit more from
  losing an electron and which atom would benefit
  more from gaining an electron?
• See worksheet 4

13
The making of saltPart II

• When Na and Cl atoms collide, the chlorine atom
  strips the sodium atoms outer electron away
• This serves to fill chlorines third outer shell
  while sodium ends up with two complete shells
• Since electrons are passed from one atom to
  another, the charge of the electron is passed
  also
• Na lost an electron (lost negative charge) so it
  is ()
• Cl gained an electron (gained negative
  charge)?(-)
• In either case, both Na and Cl- are now known as
  IONS (an atom that has lost or gained or
  electron).
• The oppositely charged ions attract each other
  and form a strong attachment known as an IONIC
  BOND even though the resulting compound (NaCl) is
  neutral

See WS4
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Covalent Bonding involves sharing of electrons

• Two or more atoms held together by covalent bonds
  form a molecule
• TAKE HOMEThe number of covalent bonds an atom
  can form equals the number of additional
  electrons needed to fill its outer shell
• H can therefore form __covalent bond(s)
• Show bonding of H to H to form H2 on board
• Is H2 a molecule? Is it a compound?
• Is CH4 (methane) a molecule? A compound?
• O can therefore form __covalent bonds
• N can therefore form __covalent bonds
• C can therefore form __covalent bonds

See WS 3 Covalent Worksheet
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Practice Practice

• Determine the structural formula of the following
  molecules/compounds using the molecular formulas
  below
• WS 7
• H2O, CO2, O2, H2O2, CH4, C2H6, HCL
• Ionic or covalent WS
• How do you know if it is ionic or covalent?

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Take Home Messages for Chemical Reactions(stop)

• The structure of atoms and molecules determines
  the way they interact and behave
• While chemical reactions lead to changes in
  matter, they do not destroy nor create itthey
  simply re-arrange matter in various ways
• This is called the Law of Conservation of Matter
• Example 2 H2 O2?2H2O
• Notice same of H and O atoms appear on both
  sides of arrow, although they are grouped
  differently
• So, chemical reactions involve the breaking of
  old bonds so that a re-arrangement of the atoms
  can occur to form new bonds
• Using the lingo of chemistryreactants
  interact, atoms rearrange, and products result

17
Law of Conservation of Matter
Make this equation with the models.
18
Balancing Equations

• One simple rule the number of atoms of a given
  element on the reactant side MUST equal the
  number of atoms of that same element on the
  products side
• The only way to do it is to practice
• See WS 6 and give it a go!!!