Presently about 22% of the Earth's atmosphere is oxygen but at the time of
the first appearance of life there was virtually no oxygen. What caused the
increase in oxygen concentration? Why is it important for organisms that the
oxygen concentration is at its present level?
-The evolution of cyanobacteria, photosynthetic prokaryotes led to the
build of oxygen in the atmosphere because oxygen is a product of
photosynthesis. Most organisms utilize aerobic respiration which
Define the terms atomic number
, mass number
, atomic mass
. What elements
are represented by the chemical symbols C, H, O, N, S, P, Na, K, Cl, Ca, Fe,
Mg Cu, B?
- Atomic number - the number of protons in an element. Mass number -
total number of protons and neutrons in an atom. Atomic mass-
approximately the mass number of an atom expressed in daltons.
Carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, sodium,
potassium, chlorine, calcium, magnesium, copper, boron.
What are isotopes and radioisotopes? What are the three naturally occurring
isotopes of carbon? Which isotope is most common and which isotope is
- Isotopes are atoms with the same atomic number but with different mass
numbers. Radioisotope is an isotope in which the nucleus is unstable
and is decaying with the release of energy and particles. Carbon
C is the most common and
C is a radioisotope.