The Chemical Basis of Life Chapter 2

Organisms are Composed of Elements Matter is anything w/mass that takes up space Matter is composed of Elements Elements cannot be broken down into other substances by ordinary chemical means Each element is composed of a unique type of atom Elements combine to form molecules and compounds

Compounds A compound is a substance made of two or more different elements combined in a fixed ratio Ex’s. NaCl, H2O, CH4, C6H12O6,

Atoms Consist of Protons, Neutrons and Electrons An atom is the smallest unit of an element that still retains the properties of an element Atoms have smaller particles Neutrons No charge; in nucleus 1 atomic mass unit (amu) or 1 dalton Protons Positive charge; in nucleus 1 amu Electrons Negative charge , orbiting nucleus Zero amu Equal in number to protons in a neutral atom

Atomic Structure Atoms of different elements contain different numbers of protons (compare hydrogen, helium and lithium) Proton Neutron Electron Hydrogen (H) Helium (He) Lithium (Li)

A lithium atom has 3 protons and 4 neutrons A lithium atom has 3 protons and 4 neutrons. What is its atomic mass in daltons? 3 daltons 4 daltons 7 daltons 12 daltons None of the above

Atomic Number and Atomic Mass Atomic number = number of protons in nucleus Mass number = mass of protons and neutrons Atoms of the same element can have different mass numbers (isotopes) Isotopes are atoms of the same element with different mass numbers due to varying #’s of neutrons Unstable isotopes are called radioactive isotopes

Isotopes Hydrogen (1H) Deuterium (2H) Tritium (3H) Proton Neutron Electron Hydrogen (1H) Deuterium (2H) Tritium (3H)

Radioactive Isotopes Have Useful Applications Treating some medical conditions, especially cancer provide diagnostic information Sterilization of medical equipment

The Formation of Molecules Depends on Chemical Bonding Between Atoms Molecules are substances consisting of two or more elements held together by a covalent bond Ex’s. CO2, H2O, C6H12O6, The distribution of electrons in the valence shell determines the reactivity of an atom Octet rule: Except for the first shell which is full with two electrons, atoms interact in order to have eight electrons in their outermost energy level (valence shell) Unreactive elements have a full valence shell Reactive elements lack a full valence shell and will gain, lose, or share electrons to achieve stability

Chemically inert elements Valence shell complete 8e 2e 2e Helium (He) Neon (Ne)

Chemically reactive elements Valence shell incomplete 4e 1e 2e Hydrogen (H) Carbon © 1e 6e 8e 2e 2e Oxygen (O) Sodium (Na)

Covalent Bonds Join Atoms Into Molecules Covalent Bonds are formed when atoms share two/more valence shell electrons Allows each atom to fill its valence shell at least part of the time Two types of Covalent Bonds When electrons are shared equally = non polar covalent bond (electrically balanced) (CO2 and O2 ) When electrons are not shared equally = polar covalent bond

Covalent Bonds Join Atoms Into Molecules How are polar covalent bonds produced? Electronegativity is the attraction for electrons being shared in a covalent bond Atoms with different electronegativities produce polar molecules Ex. H2O

Comparing the charge around Types of Chemical Bonds

2. What type of chemical bond is holding a molecule of water together ?

Ionic Bonds are Attractions Between Ions Ionic bonds are formed when oppositely charged ions attract First an atom transfers a valence shell electron to another atom When an atom loses one/more electrons ions are formed Anions (– ions): have gained one or more electrons Cations (+ ions): have lost one or more electrons – + Sodium atom (Na) Chlorine atom (Cl) Sodium ion (Na+) Chloride ion (Cl–) Sodium chloride (NaCl)

Hydrogen Bonds + A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom – Hydrogen bond + + – – – + + + – (a) The slightly positive ends (+) of the water molecules become aligned with the slightly negative ends (–) of other water molecules.

Properties of Water Exists as Solid, Liquid, Gas Charged Ionizes into an acid and base Exists as Solid, Liquid, Gas Adhesion and Cohesion (via H-bonds) Surface tension Great solvent

Properties of Water Cohesion Adhesion Water binding to water Water binding to anything that has a charge Remember, water has a charge and will bind to anything with a charge

Capillary Action