Chemistry Chapter 2.1: Chemical Elements

2.1 Chemical Elements

• Matter exists as a solid, liquid, or gas and is made up of elements, which cannot be broken down into simpler substances.
• There are 92 known elements, and six are crucial for life: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS).
• An atom is the smallest unit of an element that still retains the properties of that element.
• Atoms are composed of three subatomic particles: protons, neutrons, and electrons.
• Protons are positively charged and reside in the nucleus of an atom.
• Neutrons have no charge and reside in the atom's nucleus.
• Electrons are negatively charged and orbit the nucleus.
• The atomic number represents the number of protons in an atom's nucleus.
• The mass number is the sum of protons and neutrons in an atom's nucleus.
• The periodic table organizes elements based on their chemical and physical properties.
• Elements in the same group (vertical column) share similar bonding characteristics.
• Isotopes of the same element have the same number of protons but differ in the number of neutrons.
• Some isotopes, like carbon-14, are unstable and radioactive.

2.2 Compounds and Molecules

• A compound is formed when two or more elements bond together.
• A molecule is the smallest unit of a compound.
• Water (H₂O) is a molecule composed of hydrogen and oxygen atoms.
• Molecules are often referred to as compounds in biological contexts.

2.2 Chemical Bonds

• Ionic Bonding:
  • Forms from electrostatic attraction between oppositely charged ions.
  • Example: Table salt (NaCl), formed by the attraction between sodium (Na⁺) and chloride (Cl⁻) ions.
  • Sodium loses an electron to become a positively charged ion (Na⁺), while chlorine gains an electron to become a negatively charged ion (Cl⁻).
• Covalent Bonding:
  • Atoms share electrons to complete their outer shell (octet).
  • Example: Hydrogen gas (H₂), where two hydrogen atoms share their electrons.
  • A double covalent bond involves two pairs of shared electrons, as in oxygen gas (O₂).
• Polar and Non-polar Covalent Bonds:
  • In a non-polar covalent bond, electrons are shared equally between atoms.
  • In a polar covalent bond, one atom is more electronegative (attracts electrons more strongly) than the other.
  • Example: Water (H₂O), where oxygen is more electronegative than hydrogen, creating a polar covalent bond.

2.3 Chemistry of Water

• Water makes up 70-90% of all living cells.
• The shape of the water molecule is essential for its properties and functions.
• Water is a polar molecule, with a bent shape due to the lone pairs of electrons on oxygen.
• Hydrogen bonding occurs between water molecules due to the attraction between a partially positive hydrogen atom on one molecule and a partially negative oxygen atom on another.

Properties of Water

• High Heat Capacity & Heat of Evaporation:
  • Due to hydrogen bonds, water absorbs significant heat without a large change in temperature.
  • This property helps regulate body temperature in organisms.
• Excellent Solvent:
  • Water's polarity allows it to dissolve many substances, making it a universal solvent.
  • Hydrophilic molecules can dissolve in water, while hydrophobic molecules cannot.
• Cohesive & Adhesive Nature:
  • Cohesion refers to the ability of water molecules to stick together due to hydrogen bonding.
  • Adhesion refers to the ability of water molecules to stick to other polar surfaces.
  • These properties facilitate water transport in organisms, for example, in blood vessels and plant vascular systems.
• Capillary Action:
  • A combination of adhesion and cohesion enables water to move up narrow tubes against gravity, known as capillary action.
  • This property plays a role in water transport in plants.

Explore the fundamentals of chemical elements in this quiz. Learn about the main building blocks of matter, the structure of atoms, and the organizing principles of the periodic table. Test your knowledge on crucial life elements and their properties.