Four elements that make up 96 of living matter

CHAPTER 2 The Chemical Context of Life

Elements and Compounds__________________________________________________________

Prerequisite Knowledge (Sec 2.1 – 2.2)

This information should be reviewed before the scheduled class lecture on CH 2. This is information

which I presume you have studied in high school. There is no need to define each term below;

simply make sure you understand each term and can use/apply it. I recommend skimming these sections

of your text and taking notes as needed.)

Vocabulary/Important Terms:

· matter

· element

· compound

· trace elements Name a trace element required by humans:

· atom

· subatomic particle (list all 3)

· atomic number (symbol = Z)

· atomic mass (symbol = M)

· isotope

· radioactive isotope Name a useful application Describe a concern

READ: Overview, Skim Section 2.1 for answers

1.

Distinguish between an element and a compound.

Element = collection of one type of atom

Compound = two or more different elements combined in fixed ratio

2.

Identify the four elements that make up 96% of living matter.

Oxygen, carbon, hydrogen, nitrogen

3.

Define the term trace element and give an example.

Elements required by an organism in only very small amounts. Ex: Iodine (remember the Goiter!)

Atoms and Molecules__________________________________________________________

Vocab/Important Terms:

· energy

· potential energy - (positional energy; energy due to position in a force field)

· electron shells/Electron energy levels

· valence electrons

· orbital

· octet rule: Atoms are most stable w 8 valence electrons exceptions to this rule?

· reactivity

(also depends on other factors not mentioned in text)

READ: Skim Section 2.2 for answers

4.

Draw and label (or simply describe in words) a simplified model of an atom. Explain how this model

simplifies our understanding of atomic structure.

See figures in text.

This model simplifies our understanding of atomic structure because it is not drawn to scale. A better representation would be the protons and neutrons positioned at the center of a baseball stadium and the

electrons would circle around the outermost tier of seats. These electrons spin around the nucleus of the

atom so quickly that it is impossible to determine the position of an electron circling an atom. A better representation of electron position in the atom would be an electron cloud (see fig 2.4a and remember the

yard stick I spun around in a circle in class!)

4A. Discuss the Yankee Stadium metaphor of an atom:

Protons and neutrons are baseballs at the pitcher’s mound and the electrons are spinning around the outer

tiers of seats in the baseball stadium

4B. Which subatomic particle is involved in chemical reactions?

Electrons!

5.

Distinguish between each of the following pairs of terms:

a. neutron (neutral charge) and proton (positive charge) b. atomic number (number of protons) and mass number (number of protons + neutrons) c. atomic weight and mass number. (omit)

6.

Explain how the atomic number and mass number of an atom can be used to determine the number of neutrons. EX: you are given the atomic number of oxygen = 8 and the mass number =16. You know that the atomic number is the number of protons in the atom and the mass number is the number of protons + neutrons in the atom.

The calculation would be as follows: mass number (16) – atomic number (8) = 8 neutrons!

7.

Explain how two isotopes of an element are similar. Explain how they are different.

SIMILAR: Isotopes of an element always have the same number of protons because if they had a different number of protons they would not be atoms of the same element. Isotopes have the same number of electrons.

DIFFER: Isotopes of the same element differ only in the number of neutrons in their respective nuclei.

8.

Describe two biological applications that use radioactive isotopes.

1. Used as tracers in biological research (more on this in ch. 20!)

2. Used in medical diagnosis (ex: detection of cancerous growth)

9.

Define the terms energy and potential energy. Explain why electrons in the first electron shell have less potential energy than electrons in higher electron shells.

Electrons in the 1st energy level are closer to the nucleus than electron in the 2nd or 3rd energy levels. Being further away from the nucleus gives the electrons in the higher energy levels more potential energy because it takes work to move an electron (negative charge) away from a positive nucleus.

Think of the electrons in the higher energy levels as being on the top step of a staircase and the electrons in the first energy level on one of the bottom stairs in a staircase. Which electron will have more speed as it falls from its stair to the landing at the bottom of the staircase? Of course the electrons on the top steps. Therefore, the electrons in the higher energy levels that are positioned further from the nucleus have more potential energy.

Bonding_____________________________________________________________________

Sec 2.3 Read this section carefully as the information is foundational. Use the vocab list and

questions below to help you focus on the more important concepts.. Most of you will prefer to

complete this SG after the lecture on Chemistry.

I. Vocab/important Terms:

Chemical bonds: Covalent & Ionic (STRONG) These form compounds.

Covalent bonds:

· covalent bond

· molecule

· electronegativity

Ionic bonds

· ionic bond

· ion – charged particle

· cation

· anion

· ionic compound/salt

Bonds (attractions) between molecules (WEAK)

· H-bonds

· van der Waals Interactions

READ: Section 2.3, carefully read subsections “Covalent bonds”, Ionic Bonds”, “hydrogen bonds”

10.

Distinguish among nonpolar covalent, polar covalent and ionic bonds.

Nonpolar covalent- electrons are equally shared among atoms in the compound. This type of bonding occurs when two or more atoms of the same element combine, ex: 2 atoms of Oxygen (O2. Each atom pulls equally on the electrons being shared because the atoms have the same number of protons in their respective nuclei.

Polar covalent- electrons are Unequally shared among the atoms in the compound. This type of bonding occurs when two or more atoms of different elements combine ex: 1 atom of Oxygen + 2 atoms of hydrogen (H2O. The oxygen atom has 8 protons pulling on the electrons being shared with the hydrogen atoms and the hydrogen atoms have only one proton in their respective nuclei pulling on the shared electrons. Thus the shared electrons spend a greater amount of time around the oxygen atoms, giving it a partial positive charge, and less time around either hydrogen atom, giving each of these atoms in this water molecule a partial positive charge.

Ionic- An electron or electrons are transferred from one atom to another. Ex: one electron form sodium (Na) is transferred to a Chlorine (Cl) atom. Two charged species call ions are formed, one positive ion (Na+) and one negative ion (Cl-). The positive and negative ions attract and form an ionic compound (NaCl).

10.5 How many covalent bonds must each of the following form in order to be stable:

H - 1 O - 2 N- 3 C- 4 P-3

11.

Explain why strong covalent bonds and weak bonds are both essential in living organisms.

Covalent bonds are necessary for living organisms because these strong interactions among atoms are important to maintain the structure of essential molecules, such as oxygen and water, as well as the 4 macromolecule that make up all living things: carbohydrates, lipids, proteins, nucleic acids (more about these in chapter 5!)

Weak bonds, however, such as hydrogen bonds, are also essential to life. These bonds also participate in maintaining the structure of the 2 of the 4 classes of macromolecules that make up our bodies. Also weak hydrogen bonding among water molecules allows for life to exist on earth, as we will explore in chapter 3.

12.

Distinguish between hydrogen bonds and van der Waals interactions.

Hydrogen bonds- forms when a hydrogen atom polar covalently bonded to an electronegative atom is also attracted to another electronegative atom.

Ex: The hydrogen atoms in a water molecule are each bound to the very electronegative oxygen atom. As described in #10, this causes the oxygen to be partially negative and each hydrogen atom to be partially positive. The partial positive charge on each hydrogen atom is attracted to the partially negative oxygen atom of two other water molecules. This interaction is what is known as a hydrogen bond.

OMIT van der Waals intetractions

Chemical Reactions__________________________________________________________

READ: Skim section 2.4 for answer

13. Explain what is meant by a chemical equilibrium.

The rate of the forward reaction is equal to the rate of the reverse reaction.

Suggested End of Chapter SELF-QUIZ questions:

#1, 2, 4, 5, 6 & 8

See back of text for answers!