Density Formula and Measurement Techniques

Definition: Density is a physical property of matter that describes the mass per unit volume.
Formula:
- ( \text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)} )
- Units:
  - Common SI unit: kilograms per cubic meter (kg/m³)
  - Other units: grams per cubic centimeter (g/cm³), grams per liter (g/L)

Methods of Measurement:
1. For Solids:
  - Direct Method: Weigh the solid on a balance, measure its dimensions, and calculate volume (e.g., using geometric formulas for regular shapes).
  - Water Displacement Method: Submerge the solid in water, measure the volume of water displaced to find the solid's volume.
2. For Liquids:
  - Using a Hydrometer: A calibrated glass tube floats in the liquid; the level indicates the liquid's density.
  - Direct Measurement: Weigh a known volume using a graduated cylinder or volumetric flask.
3. For Gases:
  - Using a Gas Syringe: Measure the mass of the gas in a known volume container.
  - Ideal Gas Law Approach: Use the equation ( PV = nRT ) to calculate density from pressure (P), volume (V), temperature (T), and the ideal gas constant (R).
Temperature and Pressure Effects:
- Density can vary with changes in temperature and pressure, particularly for gases.
- Heavier substances generally have higher densities, while gases tend to have lower densities.

Density defines how much mass is packed into a given volume.
Density's formula is: Density (ρ) = Mass (m) / Volume (V)
The standard unit for density is kilograms per cubic meter (kg/m³).
Grams per cubic centimeter (g/cm³) and grams per liter (g/L) are also common density units.

Solids:
- Direct measurement of weight and volume using a balance and geometric formulas for regular shapes can determine density.
- The water displacement method calculates a solid's volume by measuring the water displaced when the solid is submerged.
Liquids:
- A hydrometer floats in the liquid, and its level indicates the liquid's density.
- Weighing a known volume of liquid using a graduated cylinder or volumetric flask determines its density.
Gases:
- Measuring the mass of gas within a container of known volume using a gas syringe can determine gas density.
- Using the "ideal gas law" (PV = nRT) allows for calculating gas density from pressure, volume, temperature, and the ideal gas constant.
- Density is influenced by temperature and pressure fluctuations, especially for gases.
- Heavier substances generally have higher densities, while gases tend to have lower densities.

Ionic Bonding: Formed when one atom transfers an electron to another. Metals typically lose electrons and become positively charged cations, while nonmetals gain electrons to become negatively charged anions.
Covalent Bonding: Formed when two atoms share one or more pairs of electrons. Typically happens between nonmetal atoms.

Types:
- Single Bonds: One pair of electrons shared between two atoms (e.g., H2, Cl2).
- Double Bonds: Two pairs of electrons shared between two atoms (e.g., O2, CO2).
- Triple Bonds: Three pairs of electrons shared between two atoms (e.g., N2).

Electronegativity: An atom's ability to attract and hold onto electrons. It is measured on a scale from 0.0 (least electronegative) to 4.0 (most electronegative). Fluorine is the most electronegative element.
Polar Molecules: Have a significant difference in electronegativity between their atoms, leading to partial charges.
Nonpolar Molecules: Have similar or identical electronegativities, resulting in an even distribution of electrons.

Formed from ionic bonds, where positively charged cations and negatively charged anions attract each other.
Typically solid at room temperature and have high melting and boiling points due to the strong electrostatic forces between ions.
Conduct electricity when dissolved in water or molten due to the movement of ions.
Examples: NaCl (table salt), MgO (magnesium oxide).

Predicts molecular shapes: Based on the repulsion between electron pairs in the central atom's valence shell.

Key Shapes:
- Linear: 180° bond angle (e.g., CO2)
- Trigonal Planar: 120° bond angle (e.g., BF3)
- Tetrahedral: 109.5° bond angle (e.g., CH4)
- Trigonal Bipyramidal: 90° and 120° bond angles (e.g., PCl5)
- Octahedral: 90° bond angle (e.g., SF6)
Shape is determined by:
- Number of bonding pairs
- Number of lone pairs
- Arrangement that minimizes electron repulsion

Lone pairs have a greater impact: They take up more space than bonding pairs, resulting in smaller bond angles.
Adjusted Angles:
- Trigonal pyramidal: Tetrahedral with one lone pair (e.g., NH3) resulting in ≈107° bond angles.
- Bent shape: Tetrahedral with two lone pairs (e.g., H2O) resulting in ≈104.5° bond angles.

Principle: Electron pairs arrange themselves to maximize their distance apart to minimize repulsion.
Types of Pairs:
- Bonding pairs: Shared electrons
- Lone pairs: Non-bonding electrons
Repulsion Order: Lone-lone > Lone-bonding > Bonding-bonding.

Concept: Atomic orbitals mix to form new hybrid orbitals used for bonding.
Types of Hybridization:
- sp: Linear geometry (e.g., BeCl2)
- sp²: Trigonal planar geometry (e.g., BF3)
- sp³: Tetrahedral geometry (e.g., CH4)
- sp³d: Trigonal bipyramidal geometry (e.g., PCl5)
- sp³d²: Octahedral geometry (e.g., SF6)

Determined by: Distribution of electrical charge, resulting in a dipole moment.
Influencing Factors:
- Electronegativity difference: Between bonded atoms
- Molecular shape symmetry:
  - Polar molecules: Asymmetric, with a net dipole moment (e.g., H2O)
  - Nonpolar molecules: Symmetrical, with no net dipole moment (e.g., CO2)