Atoms: Structure, Elements, and Chemical Bonds

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Chapter 2: Atoms

Matter, Elements, Trace Elements, and Compounds

Matter is anything that occupies space and has mass. It exists in three physical states: solid, liquid, and gas. Elements are pure substances that cannot be broken down by chemical means and may exist in any of these states. Compounds are substances formed from two or more elements in fixed ratios.

Examples of Elements: Oxygen, Helium, and Hydrogen (gases); Gold, Copper, and Silver (solids); Mercury and Bromine (liquids).
Examples of Compounds: Water (H2O, liquid), Sodium Chloride (NaCl, solid), Carbon Dioxide (CO2, gas).

Trace Elements in the Human Body

Trace elements are required by living organisms in very small amounts (less than 0.01% of body weight) but are essential for health.

Examples: Boron, Chromium, Copper, Cobalt, Fluorine, Iodine, Iron, Manganese, Molybdenum, Tin, Selenium, Silicon, Vanadium, Zinc.
Iodine: Deficiency causes goiter (thyroid swelling); remedy is iodized salt.
Iron: Deficiency causes anemia (reduced oxygen-carrying capacity of hemoglobin); remedy is iron-fortified foods.
Fluoride: Excess causes fluorosis (brown teeth stains, but fewer cavities); used in toothpaste and water to prevent tooth decay.
Zinc: Often combined with Vitamin C to boost immunity.

Structure of the Atom

Atoms are composed of three subatomic particles: protons, neutrons, and electrons.

Protons: Positively charged, located in the nucleus, determine the element's identity.
Neutrons: No charge, located in the nucleus, determine the isotope.
Electrons: Negatively charged, orbit the nucleus in shells, determine chemical behavior.

Atomic Number, Mass Number, and Atomic Mass

Each element is defined by its number of protons (atomic number). The mass number is the sum of protons and neutrons. Atomic mass is approximately equal to the mass number, as electrons contribute negligibly to mass.

Atomic Number (Z): Number of protons in the nucleus.
Mass Number (A): Number of protons plus neutrons.
Atomic Mass Unit: 1 Dalton (Da) ≈ mass of one proton or neutron.

Equation:

Where = mass number, = number of protons, = number of neutrons.

Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons (thus different mass numbers). Some isotopes are radioactive, meaning their nuclei decay spontaneously, emitting radiation.

Examples: Hydrogen: H, H (deuterium), H (tritium); Carbon: C, C, C.
Applications: Radioactive isotopes are used in medical imaging (e.g., PET scans) and cancer treatment (e.g., chemotherapy).

Electron Shells and Distribution

Electrons occupy energy levels or shells around the nucleus. The arrangement of electrons determines an atom's chemical properties.

First shell: Maximum 2 electrons.
Second shell: Maximum 8 electrons.
Example: Carbon (atomic number 6): 2 electrons in the first shell, 4 in the second shell.

Ions and Chemical Bonds

Atoms become ions by gaining or losing electrons. Chemical bonds form when atoms interact to achieve stable electron configurations.

Ionic Bond: Formed by transfer of electrons from one atom to another (e.g., NaCl).
Covalent Bond: Formed by sharing electrons between atoms.
Polar Covalent Bond: Unequal sharing of electrons due to differences in electronegativity (e.g., H2O).
Non-polar Covalent Bond: Equal sharing of electrons (e.g., CH4).

Electronegativity and Chemical Reactions

Electronegativity is an atom's ability to attract shared electrons. Chemical reactions involve the making and breaking of bonds, often facilitated by catalysts such as enzymes.

High Electronegativity: Nonmetals like Fluorine and Chlorine.
Catalysts: Substances that speed up chemical reactions without being consumed (e.g., enzymes in biological systems).

pH and Acidity

pH measures the concentration of hydrogen ions (H+) in a solution. It is a logarithmic scale where each unit represents a tenfold change in H+ concentration.

Acids: Increase H+ concentration (pH < 7).
Bases: Decrease H+ concentration by providing OH- ions (pH > 7).
Neutral Solutions: pH = 7 (e.g., pure water).

Equation:

Example: A solution with [H+] = M has a pH of 7.

Table: Types of Chemical Bonds

Bond Type	How Formed	Example	Polarity
Ionic	Electron transfer	NaCl	High
Covalent (Non-polar)	Equal electron sharing	CH4	None
Covalent (Polar)	Unequal electron sharing	H2O	Moderate

Additional info: The Dalton is a standard unit of atomic mass used in biology and chemistry. Enzymes are biological catalysts that lower activation energy for reactions.