Chemistry Pt1 Matter, Atoms & Bonds

Focus on atoms with atomic number 11, indicating the presence of sodium (Na).
- Sodium has 11 protons and, typically, 11 electrons.
- The electrons occupy shells, where the outermost shell is referred to as the valence shell.
Valence Electrons:
- Electrons present in the outermost shell contribute to the chemical properties of the atom.
- For atoms to interact with other atoms, their outer shells must be considered.

If an atom has fewer electrons than the maximum allowed in a shell, it will likely react with other atoms to achieve stability.
Notable examples of elements that frequently interact include:
- Carbon (C)
- Nitrogen (N)
- Hydrogen (H)
These elements collectively account for approximately 96% of the body weight in humans.

Molecular Formation:
- Molecules result from the interactions of valence shells between different atoms.
- Sodium, with an atomic structure of 11, often gives up an electron during bonding.
When sodium interacts with chlorine (Cl), the process results in:
- Sodium (Na) gives up an electron, leading to the formation of a positively charged ion (Na⁺).
- Chlorine, having a strong electronegativity, accepts this electron, resulting in a negatively charged ion (Cl⁻).
- The attraction between Na⁺ and Cl⁻ forms an ionic bond, resulting in the creation of sodium chloride (NaCl).

Ionic Bond: Formed through the transfer of electrons, resulting in charged ions that attract each other.
Covalent Bond: Involves the sharing of electrons between atoms to achieve full valence shells.
- Example: Water (H₂O)
- Oxygen has an atomic number of 8 and can share electrons with up to two hydrogen atoms (atomic number 1) to form the molecule.
- Oxygen shares one of its electrons with each hydrogen atom, utilizing covalent bonding.
- The polarity of the water molecule arises due to the differing electronegativities of oxygen and hydrogen, leading to regions of partial positive and negative charge within the molecule.

Water's Polarity:
- Water molecules are polar due to the unequal sharing of electrons.
- Oxygen is more electronegative, pulling electrons closer and developing a region of partial negative charge, whereas hydrogen develops a partial positive charge.
Hydrogen Bonds:
- A type of bond formed by the attraction between the positive region of one water molecule and the negative region of another.
- Though weaker than ionic or covalent bonds, hydrogen bonds are essential in organizing water molecules in various states (liquid and solid).

Anomalous Expansion: Water expands upon freezing due to the structure maintained by hydrogen bonding.
- Ice has molecules held further apart compared to liquid water, which is an unusual property for substances, resulting in ice being less dense than water.
High Heat Capacity:
- Water absorbs significant amounts of heat before increasing in temperature, primarily due to the breaking of hydrogen bonds.
Universal Solvent:
- Water's ability to dissolve various substances is pivotal for biological reactions, earning it the title of a universal solvent.
- Polar substances dissolve well in water, while nonpolar substances (fats) do not dissolve efficiently.
- This property aids in nutrient transport and biochemical reactions within the body.

When salt (e.g., NaCl) is introduced in water, the polar water molecules interact with the ions, pulling them apart into Na⁺ and Cl⁻ ions, due to the hydration of ions, which facilitates the process of dissolving.
The hydration shell forms as water molecules surround the ions, stabilizing them in solution, ultimately impacting biological systems, including physiological processes such as nutrient absorption and cellular function.