Matter and Its Chemical Basis

Definition and Composition of Matter

Matter is anything that takes up space and has mass. All living and non-living things are composed of matter, which can exist as pure elements, compounds, or mixtures.

• Element: A substance that cannot be broken down into other substances by chemical reactions.

• Compound: A substance consisting of two or more elements combined in a fixed ratio.

• Compounds have properties distinct from the elements that compose them.

• Example: Sodium chloride (NaCl) is a compound formed from sodium (a metal) and chlorine (a gas), resulting in edible table salt with properties different from either element.

Elements and Compounds

Elements

Elements are the fundamental substances of matter. There are 92 naturally occurring elements, each with unique properties.

• Iron (Fe): An example of a naturally occurring element.

Compounds

Compounds are substances composed of two or more elements in a fixed ratio. The properties of a compound differ from those of its constituent elements.

• Example: Water (H2O) is a compound of hydrogen and oxygen.

• Example: Sodium chloride (NaCl) contains equal numbers of sodium and chlorine atoms.

Elements Essential to Life

Major and Trace Elements

Of the 92 naturally occurring elements, about 20–25% are essential for life. Four elements—carbon, hydrogen, oxygen, and nitrogen—make up approximately 96% of living matter. The remaining 4% consists mainly of calcium, phosphorus, potassium, and sulfur. Trace elements are required in minute quantities.

• Essential elements: Required for an organism to survive, grow, and reproduce.

• Trace elements: Required in very small amounts (less than 0.01% of body mass).

Table: Elements in the Human Body

Trace elements (less than 0.01% of mass): Boron (B), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), zinc (Zn).

Atomic Structure

Atoms and Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles:

• Protons (p+): Positively charged particles found in the nucleus.

• Neutrons (n0): Electrically neutral particles found in the nucleus.

• Electrons (e-): Negatively charged particles that form a cloud around the nucleus.

Each element has a unique number of protons, known as its atomic number.

Atomic Number and Mass Number

• Atomic number (Z): Number of protons in the nucleus of an atom. Determines the element's identity.

• Mass number (A): Total number of protons and neutrons in the nucleus.

• Atoms are electrically neutral when they have equal numbers of protons and electrons.

Isotopes: Atoms of the same element with different numbers of neutrons. Some isotopes are stable, while others are radioactive (unstable and decay over time).

Applications of Isotopes

• Radioactive isotopes can be used for dating fossils (radiometric dating), as tracers in metabolic processes, and in medical imaging.

• Half-life: The time required for half of the atoms in a radioactive sample to decay.

Electron Arrangement and Chemical Properties

Electron Energy Levels and Shells

Electrons have potential energy due to their position relative to the nucleus. Electrons are arranged in energy levels or shells around the nucleus. The energy of an electron increases with its distance from the nucleus.

• First shell: Closest to the nucleus, lowest energy, holds up to 2 electrons.

• Second shell: Holds up to 8 electrons.

• Third shell: Holds up to 18 electrons (in larger atoms).

Orbitals

Orbitals are three-dimensional regions around the nucleus where electrons are likely to be found. Each orbital can hold up to two electrons.

• s orbital: Spherical shape, one per energy level.

• p orbitals: Dumbbell-shaped, three per energy level (starting from the second shell).

• Atoms are most stable when their valence (outermost) shell is full.

Valence Electrons and Chemical Reactivity

The chemical behavior of an atom is determined by the number of electrons in its valence shell. Atoms with incomplete valence shells are chemically reactive, while those with full valence shells are inert (unreactive).

Chemical Bonds and Molecular Structure

Chemical Bonds

Atoms with incomplete valence shells can interact by sharing or transferring electrons, forming chemical bonds. The strongest chemical bonds are covalent and ionic bonds.

• Covalent bond: A pair of valence electrons is shared by two atoms.

• Ionic bond: One atom donates an electron to another, resulting in oppositely charged ions that attract each other.

Covalent Bonds

• Single bond: Sharing of one pair of electrons (e.g., H–H).

• Double bond: Sharing of two pairs of electrons (e.g., O=O).

• Structural formula: Shows the arrangement of atoms and bonds (e.g., H–O–H for water).

• Molecular formula: Indicates the number and type of atoms (e.g., H2O).

Bonding Capacity

• Oxygen: Forms two covalent bonds to complete its valence shell.

• Carbon: Forms four covalent bonds (e.g., methane, CH4).

Electronegativity and Bond Polarity

• Electronegativity: The attraction of an atom for the electrons in a covalent bond.

• Nonpolar covalent bond: Electrons are shared equally (e.g., H2, O2).

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Ionic Bonds and Compounds

• Ionic bond: Formed when one atom strips an electron from another, creating ions (e.g., Na+ and Cl- in NaCl).

• Ionic compounds: Form crystalline structures rather than discrete molecules.

• The strength of ionic bonds depends on environmental conditions (e.g., water can weaken ionic bonds).

Weak Chemical Bonds

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.

• Van der Waals interactions: Weak attractions due to transient local partial charges in molecules or atoms in close proximity.

• Although weak individually, these bonds can be collectively strong and are important in biological systems (e.g., gecko's toe hairs adhering to surfaces).

Molecular Shape and Biological Function

The three-dimensional shape of a molecule determines its function in a cell. Molecular shape is determined by the positions of the atoms' orbitals involved in bonding. Molecules with similar shapes can have similar biological effects (e.g., natural endorphins and morphine binding to the same brain receptors).

Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the breaking and forming of chemical bonds, leading to new arrangements of atoms. The starting substances are called reactants, and the resulting substances are products.

• All atoms in the reactants must be accounted for in the products (law of conservation of mass).

• Chemical reactions are reversible; products of the forward reaction can become reactants in the reverse reaction.

• Chemical equilibrium: The point at which the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.

Example of a Chemical Reaction

• Formation of water:

Additional info: The study of chemical principles is fundamental to understanding biological processes, as all living organisms are composed of and depend on chemical interactions.