Basic Chemistry Ch. 2

Organisms are Composed of Elements Elements cannot be broken down by ordinary chemical means Elements are composed of atoms or unique building blocks for each element Compounds Substances consisting of two or more different elements combined in a fixed ratio (C6H12O6 and NaCl) Molecules Substances consisting of two or more elements; elements can be the same or different

Atoms and Atomic Structure Three important subatomic particles exist: Neutrons No charge in the nucleus, 1 atomic mass unit (amu) Protons Positive charge, in the nucleus, 1 amu Electrons Negative charge, orbit nucleus, 0 amu Equal to the number of protons in an atom Atoms of different elements contain different numbers of subatomic particles Compare hydrogen, helium and lithium

Atoms Have a Unique Number of Protons Neutron Electron Hydrogen (H) Helium (He) Lithium (Li) Figure 2.2

Identifying Atoms Using Atomic Number and Atomic Mass Atomic number = number of protons in nucleus Mass number = mass of protons and neutrons in the atoms nucleus Atoms of the same element can have different mass numbers Isotopes are atoms of the same element that differ in their number of neutrons

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 (unreactive) elements have a full valence shell Unstable (reactive) elements lack a full valence shell and will gain, lose, or share electrons with other atoms to achieve stability, & thereby forming bonds

(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 Electrons are shared Allows each atom to fill its valence shell part of the time Sharing may be equal or unequal Equal sharing produces nonpolar molecules (Ex. CO2) Unequal sharing by atoms with different electron-attracting abilities produces polar molecules (H2O) Atoms with six or seven valence shell electrons are electronegative, e.g., oxygen

+ Reacting atoms Resulting molecules or Hydrogen atoms Carbon atom Molecule of methane gas (CH4) (a) Formation of four single covalent bonds:

+ Reacting atoms Resulting molecules or Oxygen atom Oxygen atom Molecule of oxygen gas (O2) (b) Formation of a double covalent bond: Figure 2.7b

+ Reacting atoms Resulting molecules or Nitrogen atom Nitrogen atom Molecule of nitrogen gas (N2) (c) Formation of a triple covalent bond:.

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 + An electrostatic attraction between a positive hydrogen and an 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

Capillary Action