BIO 201 : lecture 3

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Since they are the same element, they have the same atomic number but different mass numbers.
Example: Hydrogen has three common isotopes:
1. Protium (\text{^1H}): 1 proton, 0 neutrons (most common).
2. Deuterium (\text{^2H}): 1 proton, 1 neutron.
3. Tritium (\text{^3H}): 1 proton, 2 neutrons (radioactive).
The chemical properties of isotopes are generally very similar because they depend primarily on the number of electrons (which is equal to the number of protons in a neutral atom), while physical properties like density can differ.

Chemical bonds are forces that hold atoms together to form molecules or compounds.
They involve the rearrangement of valence electrons between atoms.
There are primary types of chemical bonds:
1. Ionic Bonds: Formed when there is a complete transfer of electrons from one atom (typically a metal) to another (typically a non-metal), resulting in the formation of ions (charged atoms) that are attracted to each other.
  - Example: Sodium Chloride (\text{NaCl}). Na loses an electron to become Na^+, and Cl gains an electron to become Cl^-. The electrostatic attraction holds them together.
2. Covalent Bonds: Formed when atoms share electrons to achieve a stable electron configuration.
  - Example: Water (\text{H}_2 ext{O}). Oxygen shares electrons with two hydrogen atoms.

The polarity of a covalent bond depends on the difference in electronegativity between the two atoms involved.
Electronegativity: A measure of an atom's ability to attract electrons in a chemical bond.

Occur when electrons are shared unequally between atoms due to a significant (but not complete) difference in electronegativity.
This unequal sharing creates partial positive (\delta^+) and partial negative (\delta^-) charges on the atoms, forming a dipole.
Example: Hydrogen Chloride (\text{HCl}).
- Chlorine is more electronegative than hydrogen, so it pulls the shared electrons closer to itself.
- This gives chlorine a partial negative charge (\delta^-) and hydrogen a partial positive charge (\delta^+).

Occur when electrons are shared equally between atoms, typically when the atoms have similar or identical electronegativities.
No significant partial charges are formed.
Example 1: Diatomic molecules like Oxygen (\text{O}2) or Hydrogen (\text{H}2).
- The two identical atoms have the same electronegativity, so electrons are shared perfectly equally.
Example 2: Methane (\text{CH}_4).
- Although carbon is slightly more electronegative than hydrogen, the symmetrical tetrahedral structure of methane causes the individual bond dipoles to cancel each other out, resulting in a nonpolar molecule overall. (While individual C-H bonds are slightly polar, the molecule as a whole is nonpolar due to symmetry.)