Chapter+2

Chapter 2:

ELEMENTS OF LIFE Organisms are composed of **matter**: anything that takes up space and has mass - matter made up of **elements**: substance that cannot be broken down into other substances - **compound:** substance consisting of two or more elements in a fixed ratio

25 out of 92 chemical elements are essential to life - **Carbon (C), Oxygen (O), hydrogen (H) and Nitrogen (N)** make up 96% of all living things - **Phosphorus (P), sulfur (S), calcium (Ca) and potassium (K)** make up 4%

BONDING RULES + TYPES OF BONDS - atoms with incomplete valence shells can complete it valence shell by sharing or transferring valence electrons = **chemical bonds**
 * -** strongest types are **covalent** and **iconic**


 * COVALENT BONDS**: sharing of a pari of valence electrons by two atoms



examples: hydrogen, oxygen (double bond), water, methane

The attraction of an atom for the shared electrons of a covalent bond is called **ELECTRONEGATIVITY**
 * -** strong electronegative atoms attempt to pull the shared electrons toward themselves

- bond between two atoms of same element always nonpolar (ie H2, O2) - similar electronegativity also nonpolar (ie since carbon adn hydrogen are similar in electonegativity, CH4 is nonpolar)
 * Nonpolar covalent bond**: if electorns in a covalent bond are shared equally

ie. H2O, since O is more electronegative than H, electrons are pulled more toward oxygen --> results in partial negative charge on the oxygen and partial positive charge on hydrogen
 * Polar covalent bond**
 * -** if electonrs are not shared equally

Chemical reactions are the breaking and making of bonds


 * IONIC BONDS**
 * -** two atoms unequal in attraction for valence electron that one atom strips an electron from the other

- after transfer, atoms are no longer neutral and are **//ions//** //with charges//
 * cations:** positive charge
 * anions:** negative charge

- compounds formed by ionic bonds are called **ionic compounds/salts** ie **NaCl**
 * -** water can dissolve salts by reducing the attraction between the salt's anions and cations

Weak chemical bonds play important roles in chemistry of life - brief bonds are important to a variety of processes ie. signal molecule form one neuron use bonds to bind briefly to receptor molecules on the surface of a receiving neuron --> triggers a response by the recipient


 * HYDROGEN BONDS:** form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another strongly electronegative atom
 * ie.** usually oxygen or nitrogen atoms

CHEMICAL REACTIONS ARE THE BREAKING AND MAKING OF BONDS ie. photosynthesis: 6CO2 + 6H2O --> C6H12O6 + 6O2
 * reactants --> products**

- all chemical reactions are reversible: they merely rearrange matter, doesn't destroy anything

the energy consumed and released summed dictates endergonic (endothermic) and exergonic (exerthermic)

**Endergonic/endothermic**: free energy/heat absorbed from surroundings
 * Exergonic/exerthermic: ** net release of free energy