Udisii

Mastering Concepts


2.1

1. Which chemical elements do organisms require in large amounts?

Carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus are the chemical elements that organisms require in large amounts.

2. Where in an atom are protons, neutrons, and electrons located?

An atom’s protons and neutrons are in its nucleus. A cloud of electrons surrounds the nucleus.

3.What does an element’s atomic number indicate?

An atom’s atomic number indicates the number of protons in its nucleus.

4. What is the relationship between an atom’s mass number and an element’s atomic mass?

An atom’s mass number is the sum of protons and neutrons in its nucleus. The atomic mass is the average mass of all of the isotopes of that particular element.

5. How are differentisotopes of the same element different from one another?

Isotopes of the same element differ in the number of the neutrons in their nuclei.

2.2

1. How are atoms, molecules, and compounds related?

Molecules and compounds are formed of atoms. Molecules are joined atoms, while compounds are molecules formed of two or more different elements.

2. How does the number of valence electronsdetermine an atom’s tendency to form bonds?

The number of valence electrons in the valence shell determines how many “vacancies” an atom has to fill before it is stable. If its valence shell is filled, it is chemically inert; if it has one or more vacancies it tends to be chemically reactive.

3. Explain how electronegativity differences between atoms result in nonpolar covalent bonds, polarcovalent bonds, and ionic bonds.

The difference in electronegativity between atoms results in different kinds of bonds:
- nonpolar covalent bonds form between atoms that have similar electronegativity and share bond electrons equally;
- polar covalent bonds form between atoms that have different levels of electronegativity and share bond electrons unequally;
- ionic bonds formbetween pairs of atoms that have extremely different levels of electronegativity. Rather than sharing electrons, one atom takes an electron from the other..

4. What is the relationship between polar covalent bonds and hydrogen bonds?

Hydrogen bonds form between molecules with polar covalent bonds. In water, for example, hydrogen bonds form between the partial positive charge of one of a watermolecule’s hydrogen atoms and the partial negative charge of the oxygen atom of an adjacent water molecule.


2.3

1. How are cohesion and adhesion important to life?

Cohesion decreases the rate of evaporative water loss and helps water move from roots to leaves in plants; it also allows small insects to move on water. Adhesion helps with the movement of water in plants as the watermolecules adhere to the walls of the vessels.

2. What is the difference between hydrophilic and hydrophobic molecules?

Hydrophilic molecules will dissolve into and mix with water, while hydrophobic molecules will not. Hydrophilic molecules are often polar, while hydrophobic molecules are nonpolar.

3. How does water help an organism regulate its body temperature?

The hydrogen bonds in waterkeep it from changing temperatures quickly. Water also evaporates and helps bring down high temperatures.

4. How does the density difference between ice and water affect life?

Because the density of ice is less than water, a layer of ice forms at the top of water sources in cold temperatures. This locks in warmer temperatures below.

5. What happens in a chemical reaction?

In achemical reaction, the chemicals involved (the reactants) swap atoms, forming different chemicals (the products). As atoms are swapped, chemical bonds are broken, energy is released or used in the reaction, and new chemical bonds are formed.

6. How does water participate in the chemistry of life?

Because cells are mostly made of water and because most cells are surrounded by water, water is the...