Chemical Elements and Compounds

Distinguishing Elements from Compounds

Understanding the difference between elements and compounds is fundamental in biology and chemistry. Elements are pure substances consisting of only one type of atom, while compounds are substances formed from two or more elements chemically bonded together.

• Element: A substance that cannot be broken down into other substances by ordinary chemical procedures. Examples: Oxygen (O), Hydrogen (H).

• Compound: A substance consisting of two or more elements in a fixed ratio. Example: Water (H2O).

Exploring Life at Many Levels

Major Elements in Living Matter

Living organisms are primarily composed of a few key elements, which are essential for life processes.

• Four elements make up 96% of living matter: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N).

• Trace elements: Elements required in minute amounts, such as Iron (Fe) and Iodine (I).

Atoms and Molecules

Structure of the Atom

Atoms are the basic units of matter, defined by their atomic number, mass number, atomic weight, and valence.

• Atomic number: Number of protons in the nucleus; defines the element.

• Mass number: Sum of protons and neutrons in the nucleus.

• Atomic weight: Average mass of atoms of an element, accounting for isotopes.

• Valence: The combining capacity of an atom, determined by the number of electrons in the outer shell.

• Isotopes: Atoms of the same element with different numbers of neutrons.

Example: Carbon-12 and Carbon-14 are isotopes of carbon, differing in neutron number.

Determining Number of Subatomic Particles

The atomic number and mass number of an atom allow us to determine the number of protons, neutrons, and electrons.

• Number of protons: Equal to atomic number.

• Number of neutrons: Mass number minus atomic number.

• Number of electrons: Equal to number of protons in a neutral atom.

Electron Configuration and Chemical Behavior

The arrangement of electrons in an atom's electron shells determines its chemical properties and reactivity.

• Electron shells: Energy levels where electrons reside; the outermost shell is most important for chemical bonding.

• Valence electrons: Electrons in the outermost shell; atoms tend to gain, lose, or share electrons to achieve a full valence shell (usually 8 electrons, known as the octet rule).

• Reactivity: Atoms with incomplete valence shells are more chemically reactive.

Example: Sodium (Na) has one valence electron and readily loses it to form Na+ ion.

Types of Chemical Bonds

Covalent Bonds

Covalent bonds involve the sharing of pairs of valence electrons between atoms, resulting in molecule formation.

• Single covalent bond: Sharing of one pair of electrons.

• Double covalent bond: Sharing of two pairs of electrons.

• Polar covalent bond: Unequal sharing of electrons, leading to partial charges (e.g., in water).

• Nonpolar covalent bond: Equal sharing of electrons.

Example: In a water molecule (H2O), oxygen and hydrogen share electrons, but oxygen attracts them more strongly, making the bond polar.

Ionic Bonds

Ionic bonds are formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other.

• Cation: Positively charged ion (loss of electron).

• Anion: Negatively charged ion (gain of electron).

Example: Sodium chloride (NaCl) forms when sodium donates an electron to chlorine.

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom.

• Importance: Hydrogen bonds stabilize the structure of proteins and DNA, and are responsible for many properties of water.

• Example: Water molecules are held together by hydrogen bonds, giving water its high cohesion and surface tension.

Van der Waals Interactions

These are weak attractions between molecules or parts of molecules that result from transient local partial charges.

• Importance: Significant in biological molecules, especially in the folding of proteins and the binding of substrates to enzymes.

Molecular Shape and Biological Function

Shape Influences Function

The three-dimensional shape of a molecule is critical to its function in biological systems. Shape determines how molecules interact with each other, such as enzyme-substrate specificity or hormone-receptor binding.

• Example: The neurotransmitter molecule fits into its receptor on a nerve cell due to complementary shapes.

Photosynthesis: Chemical Equation

Summary of Photosynthesis

Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen, using sunlight energy.

Chemical equation:

Example: This equation summarizes the transformation of solar energy into chemical energy in plants.