The Chemical Context of Life. How Did Life Originate? Understanding what life is and how it originated from non-living matter requires some understanding of basic chemistry. In Terms of How They Function, Living Things Are Literally Biochemical Machines.
How Did Life Originate?Understanding what life is and how it originated from non-living matter requires some understanding of basic chemistry
There are 92 kinds of atoms in nature, called elements. Each element has a characteristic set of protons, neutrons, and electrons (indicated by the atomic number in each block).
Electrons are distributed in concentric regions outside the nucleus called electron shells. Each electron shell has a limited capacity, as shown by the numbers below:An atom’s inner shells fill first because negatively charged electrons are attracted to the positively charged nucleus
Atoms whose outer shells are filled because they have just the right number of electrons will be stable and not interact with other atoms. These are called the inert elements, or noble gases.
2+8 = 10
2+8+8 = 18
2+8+8+8 = 26
Non-inert elements will interact with one another in ways that fill their outer
electron shells or cancel charges
For example, consider the two smallest elements: hydrogen (1) and helium (2).(a) Hydrogen atoms have an unfilled shell and will interact with other atoms in ways that fill the shell.(b) Helium atoms are non-reactive (and therefore solitary) because their only (and therefore outer) electron shell is filled to capacity.
lost an electron
has a filled outer shell
outer shells filled
Ionization: transfer of electron creates ions
stole an electron
has a filled outer shell
cations (+) form
outer shells almost full
anions (-) form
An unfilled outer electron shell; capacity of 2 electrons
A non-polar covalent bond: 2 equally shared electrons
The electrons are shared equally because the two atoms are identical and therefore have the same ability to attract electrons (electronegativity)
Because the electrons are shared equally, the positive charges of each nucleus are cancelled out by the electrons. Therefore hydrogen molecules are uncharged or non-polar.
Atoms that are 2 or more electrons short of a filled outer shell are likely to fill the shell by sharing electrons from other atoms, forming covalent bonds.
Carbon has 4 electrons in its outer shell, needing 4 more
Oxygen has 6 electrons in its outer shell, needing 2 more
Atoms with different electronegativities share electrons unequally. The more electronegative atom gets more than its fair share of the electron’s time and therefore has a slightly negative charge. The other atom will have a partial + charge because it gets less of the electron’s time than is required to cancel the positive charges in its nucleus.
Attraction between oppositely charged atoms engaged in polar covalent bonding with
water tension: numerous hydrogen bonds can be very strong
Electrons are constantly on the move, mostly in random directions around the atom’s nucleus.
Even when the number of electrons equals the number of protons, there will be brief events where an atom’s electrons tend to be bunched together a little.
This leaves one side of those atoms + charged and the other - charged until the electrons spread out again.
3 charged atoms (“dipoles”)
The two atoms will attract one another until their electron clouds begin to overlap and repel the two atoms …which remain suspended at a distance where attraction and repulsion are balanced
Van der Walls interactions are very weak. The movement of atoms at room temperature breaks them apart almost immediately, except...
Biological molecules that interact often have complementary shapes, which brings large numbers of atoms into position for Van der Waals interaction …the cumulative effect of which is very strong
Many molecules have polar (charges) and non-polar (uncharged) regions. The polar regions tend to orient toward the water that the molecule is suspended in, whereas the no-polar parts fold up within the interior of the molecules, hidden from water.