Biology Midterm Study Guide

Biology Midterm Review

Chapter 2- The Chemical Basis of Life 1:

Atoms - The smallest functional units of matter that form all chemical substances
- Cannot be further broken down into other substances by ordinary chemical or physical means
- Each specific type of atom is a chemical element

Three Subatomic Particles:
- Protons – Positive (+1), found in nucleus, same number as electrons
- Neutrons – Neutral (0), found in nucleus, number can vary
- Electrons – Negative (-1), found in orbitals, same number as protons

Atomic Nucleus: Center of atom where protons and neutrons are confined, entire atom has no electric charge

Orbitals:
- Areas in which the probability of finding an electron is high
- “S” orbitals are spherical
- “P” orbitals are propeller or dumbbell shaped
- Each orbital can only hold 2 electrons o Atoms with more then 2 electrons will have more then one orbital o Orbitals occupy energy shells that are associated with specific energy levels
- Atoms with progressively more electrons have orbitals within electron shells
- Are at greater distances from the centre of the nucleus o 1st shell is 1 spherical orbital (1s) which holds 2 electrons o 2nd shell is 1 spherical orbital (2s) and 3 dumbbell-shaped orbitals (2p)
• Holds 4 pairs of electrons
• Valence electrons: Electrons in the outer shell
• Available to combine with other atoms
Electron Energy:
An atom’s electrons:
- Vary in the amount of energy they possess
Energy:
- Is defined as the capacity to cause change
Potential energy
- Is the energy that matter possesses because of its location or structure

- An electron can move from one level to another only if the energy it gains or loses is exactly equal to the difference in energy between the two levels. Arrows indicate some of the step-wise changes in potential energy that are possible

Protons:
• Positive particles which distinguish one element from another
• Atomic number
• Number of protons in an atom
• Equal to the number of electrons in the atom so that the net charge is zero
• Useful criteria for organizing chemical elements into “The Periodic Table”

Atomic Mass:
• Protons and neutrons are nearly equal in mass
• Dalton
• Unit of measurement for atomic mass
• One dalton (Da) equals 1/12 the mass of a carbon atom
• Mole
• 1 mole of any element contains:
• The same number of atoms as in exactly 12g of carbon
• 6.022 x 1023 number of atoms – Avogadro’s constant

Atomic Mass:
• Indicates an atom’s mass relative to the mass of other atoms
• Most common form of carbon is used as the reference for atomic mass units
• Has six protons and six neutrons and is assigned an atomic mass of exactly 12
• Hydrogen atom has an atomic mass of 1/12 of a carbon atom
• Magnesium atom has twice the mass of a carbon atom

Isotopes:
• Multiple forms of an element that differ in the number of neutrons
• 12C contains 6 protons and 6 neutrons
• 14C contains 6 protons and 8 neutrons
• Radioisotopes: Unstable isotopes found in nature
• Lose energy through radiation
• Modern medicine makes use of the high energy level of radioisotopes
• Example - PET scan

Carbon, Hydrogen, Oxygen, Nitrogen
• Typically make up about 95% of the mass in living organisms
• Hydrogen and oxygen occur primarily in water
• Nitrogen is found in proteins and nucleic acids
• Carbon is the building block of all living matter
• Mineral elements – less than 1%
• Trace elements – less than 0.01%
• Essential for normal growth and function

Chemical Bonds and Molecules
• Molecule
• 2 or more atoms bonded together
• Molecular formula
• Contains chemical symbols of elements found in a molecule
• Subscript indicates how many atoms are present
• Compound
• Molecule composed of 2 or more elements
• Properties differ greatly from the elements that combine to form it

Covalent Bonds:
• Shared pairs of electrons between atoms
• Occur between atoms whose outer electron shells are not full
• Strong chemical bonds because the shared electrons behave as if they belong to each atom
• Can share
• 1 pair of electrons
• Example - Single bond H-F
• 2 pairs of electrons
• Example - Double bond O=O
• 3 pairs of electrons
Example - Triple bond

Octet Rule:
• Atoms are stable when their outer shell is full
• For many atoms, the outer shell fills with 8 electrons
• One exception is hydrogen, which fills its outer shell with 2 electrons

Electronegativity:
• Is the attraction of a particular kind of atom for the electrons in a covalent bond
• The more electronegative an atom,
• The more strongly it pulls shared electrons toward itself
• Oxygen is a pig for electrons and nitrogen is almost as bad! They don’t share equally with carbon or hydrogen.