The Chemical Basis of Life Chapter 2

Organisms are Composed of Elements are substances that cannot be broken down to other substances by ordinary chemical means are composed of atoms

An Atom is the building block of an Element Atoms have smaller particles Neutrons no charge; in nucleus mass = 1 dalton Protons positive charge; in nucleus 1 dalton Electrons negative charge , orbiting nucleus zero dalton equal in number to protons of an 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 https://www.youtube.com/watch?v=E4B94zCY4ok https://www.youtube.com/watch?v=MS590Xtq9M4 Sterilization of medical equipment

Organisms are Composed of Elements can combine to form compounds

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

Organisms are Composed of Elements can also combine to form molecules A molecule is formed when two or more atoms are held together by covalent bonds https://www.youtube.com/watch?v=0gsrW0Vb5sw All compounds are molecules but not all molecules are compounds

Chemical Bonds The number of electrons in the valence shell determine the reactivity of an atom Octet rule: Except for the first shell (full with two electrons), atoms interact in order to have eight electrons in their valence shell Inert elements have a full valence shell and will not react to form a molecule Reactive elements lack a full valence shell and will react by gaining, losing, or sharing valence electrons (forming a molecule)

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 or more valence shell electrons Two Types of Covalent Bonds In non-polar covalent bonds valence electrons are equally shared (Ex’s. are CO2 and O2 ) In polar covalent bonds valence electrons are unequally shared (Ex’s. are H2O and )

Covalent Bonds Join Atoms Into Molecules Why are electrons shared equally/unequally? Electronegativity is the attraction for electrons being shared in a covalent bond When atoms in a bond have a significant difference in electronegativities polar covalent bonds are produced

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 A valence electron is lost from, or is added to another atom When an atom loses one/more electrons a cation is formed and when it gains one/more electron an anion is formed. – + 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