The Chemical Basis of Life Chapter 2

Matter Consists of Elements Elements cannot be broken down by ordinary chemical means Atoms are unique building blocks for each element Atomic structure Compound A substance consisting of two or more different elements combined in a fixed ratio Ex. (C6H12O6)

Atomic Structure There are three important subatomic particles in atoms Neutrons No charge in the nucleus 1 atomic mass unit (amu) Protons Positive charge, in the nucleus 1 amu Electrons Negative charge , orbit nucleus Zero amu Equal in number to protons in atom Atoms of different elements contain different numbers of subatomic particles Compare hydrogen, helium and lithium

Hydrogen (H) Helium (He) Lithium (Li) Proton Neutron Electron Figure 2.2

Atomic Number and Atomic Mass Atomic number = number of protons in nucleus Mass number = mass of protons and neutrons Mass numbers of atoms of an element: are not all identical Isotopes: atoms of the same element that differ in the # of neutrons they contain

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

Radioactive Isotopes can Help or Harm Us

The Distribution of Electrons Determines an Atoms Chemical Properties Electrons occupy up to seven electron shells (energy levels) around nucleus 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) Stable elements have their valence shell fully occupied Reactive elements lack a full valence shell and tend to gain, lose, or share electrons (form bonds) with other atoms to achieve stability

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

(b) 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 through electron sharing Three Important Types of Chemical Bonds Covalent, Ionic and Hydrogen Covalent Bonds Formed when atoms share two or more valence shell electrons Molecule = two or more atoms held together by covalent bonds Allows each atom to fill its valence shell at least part of the time Sharing of electrons can be equal or unequal Equal sharing produces electrically balanced, nonpolar molecules Ex. CO2 Unequal sharing by atoms with different electron-attracting abilities produces polar molecules Ex. H2O Atoms with six or seven valence shell electrons are electronegative, e.g., oxygen

Ionic bonds are attractions between ions of opposite charge Ions are formed by: transfer of valence shell electrons between atoms Anions (– charge): have gained one or more electrons Cations (+ charge): have lost one or more electrons Attraction of opposite charges: results in an ionic bond – + Sodium atom (Na) Chlorine atom (Cl) Sodium ion (Na+) Chloride ion (Cl–) Sodium chloride (NaCl)

Hydrogen Bonds + Attractive force between electropositive hydrogen of one molecule and an electronegative atom of another molecule – 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 Charged Exists as Gas, Liquid and Solid Universal solvent Adhesion and Cohesion (via H-bonds) Surface tension Ionizes into acid and base

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

Capillary Action