Review atoms, bonds, water, pH, and the chemical properties that make living systems possible.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
Protons are positive, neutrons have no charge, and electrons are negative. The number of protons identifies the element. Changing neutrons makes an isotope; gaining or losing electrons makes an ion.
A carbon atom always has six protons. Carbon-12 and carbon-14 differ in neutron number. An atom that loses an electron becomes more positively charged.
An atom has 11 protons and 10 electrons. What is its net charge?
+1.
Eleven positive charges and ten negative charges leave one extra positive charge. Neutrons do not affect the net electric charge.
Covalent bonds share electrons. Ionic attractions occur between oppositely charged ions. Hydrogen bonds are attractions involving a partly positive hydrogen and a nearby electronegative atom.
Water molecules have strong covalent bonds within each molecule and hydrogen bonds between neighboring molecules. Do not call the attraction between two water molecules a covalent bond.
Oxygen pulls shared electrons more strongly than hydrogen. This gives water a partly negative oxygen end and partly positive hydrogen ends. The molecule is polar even though its total charge is zero.
Polarity lets water surround many ions and polar molecules. Nonpolar substances, such as oils, do not mix readily with water.
Which water property helps a water strider stand on the surface, and what causes it?
High surface tension, caused by cohesion between water molecules through hydrogen bonding.
At the surface, water molecules are pulled toward neighboring water molecules. This creates a surface that resists being broken. Adhesion is water sticking to another kind of material.
Water absorbs or releases a lot of heat while changing temperature relatively slowly. Evaporation cools a surface because faster-moving molecules leave it.
Hydrogen bonds absorb energy as they break and release energy as they form. This helps moderate body temperature and climate. Sweat must evaporate to provide much of its cooling effect.
Why does ice float, and why can that help organisms survive winter?
Ice is less dense than liquid water. Floating ice forms an insulating layer over liquid water below.
Hydrogen bonding holds water molecules farther apart in ice. Most substances become denser as solids, so water’s behavior is unusual and biologically useful.
A solution changes from pH 6 to pH 4. How does its hydrogen ion concentration change?
It becomes 100 times greater.
Each one-unit decrease in pH means ten times more H+ in the usual classroom model. Two units means 10 × 10 = 100. Lower pH means more acidic.
A buffer reduces sudden pH changes by accepting H+ when too much is present or releasing H+ when too little is present. It does not make every solution neutral.
Many proteins work only within a narrow pH range. Buffers help protect those conditions, but a buffer has a limited capacity and can be overwhelmed.