chapter 02

Bio1151 Chapter 2 9

Matter consists of chemical in pure form and in combinations called compounds.
- An element is a substance that cannot be broken down to other substances by chemical means.
  Only a few elements are important in organisms.
- Common elements.
  Four elements make up 96% of living matter: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
Each element contains a single kind of atom made of , , and .
- In a helium atom, the nucleus contains 2 neutrons which have no electrical charge, and 2 positively charged protons.
  Two electrons, which are negatively charged, move around the nucleus.
  The atomic number of an element is the number of protons in the nucleus.
  The mass number (atomic mass) is the sum of protons plus neutrons.
  Review:
Electrons occupy different with different energy . The outermost, or , shell contains electrons.
- Electron shells.
  Electrons exist only at fixed levels of energy, which are also called electron shells.
  The outermost shell is called the valence shell, and contains valence electrons.
  Most chemical reactions involve valence electrons, since atoms are most stable with complete valence shells.
  Review:
- The first electron shell can contain up to 2 electrons; the 2nd and 3rd shells can contain up to 8 electrons each. Carbon has 4 valence electrons: 4 electrons in its outermost (valence) shell. An atom is most stable when its valence shell is filled with the maximum number of electrons.
A bond is the of a pair of electrons between different atoms of a . These bonds are strong and _ani_2,1 stable.
- Covalent bond.
  Each hydrogen atom has a single electron (represented as a cloud) in its valence shell - shell #1. The two valence electrons be shared in a strong covalent bond, forming an H[2] molecule. In the resulting molecule, the shared pair of electrons enable each atom to complete its valence shell.
  Two pairs of valence electrons can also be shared to form a double covalent bond.
  Review:
- Covalent bonds.
- Hydrogen (H[2]): two hydrogen atoms can form a single covalent bond by sharing one pair of electrons.
- Oxygen (O[2]): two oxygen atoms share two pairs of electrons to form a double covalent bond.
- Water (H[2]O): two hydrogen atoms and one oxygen atom are joined by single covalent bonds to produce a molecule of water.
- Methane (CH[4]): four hydrogen atoms can satisfy the valence of one carbon atom, forming methane.
- Q: What kind of bonds form between these hydrogen atoms and carbon?
  o A: Single covalent bonds.
In a covalent bond the atoms have different electronegativities and share the electrons .
- Electronegativity is the attraction of an atom for electrons.
  A water molecule is polar because oxygen is more electronegative than hydrogen.
  There is a partial negative charge on the oxygen and partial positive charges on the hydrogen atoms.
  The unequal sharing of electrons results in polar covalent bonds.
  Review:
In an bond, charged atoms called are formed.
- • In an ionic bond, one atom strips electrons away from another, forming oppositely charged ions.
  A positively charged ion is a cation. A negatively charged ion is an anion.
  Ionic compounds are often form crystals.
  Review:
- Compounds held together by ionic bonds may form crystals.
  The sodium cations (Na^+) and chloride anions (Cl^-) in table salt sodium chloride (NaCl) are each surrounded by 6 ions of the opposite charge, forming a crystal.
Electronegativity can also result in other weak chemical interactions.
- A bond forms from the attraction of charged regions of molecules.
  - A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.
    Review:
- interactions form when transiently charged regions of molecules attract each other.
_inv_2&A How Are Space Rocks Analyzed for Signs of Life?
- Van der Waals interactions are attractions between molecules whose electrons are distributed asymmetrically, resulting in "hot spots" of charged areas.
  A gecko toe is highly folded and contain tiny hairs that promote such interactions that enable it to crawl on smooth surfaces.
- A gecko toe has numerous tiny hairs, each with multiple projections that increase surface area.
  The van der Waals interactions between the hair tip and a surface allow a gecko to walk up a wall.