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Atoms are the indivisible units that compose all ordinary matter. Their name derives from the Greek word for "cannot be cut." Each atom contains a nucleus of protons and neutrons surrounded by a cloud of electrons. The simplest atom, hydrogen, contains one proton and one electron but no neutrons.
Commonly encountered atoms include hydrogen, oxygen, and carbon. Water, with the formula H₂O, consists of two hydrogen atoms bonded to one oxygen atom. Although water is a chemical compound, its constituent atoms are not identical, which distinguishes it from an element.
Atomic structure varies across elements. Hydrogen has one proton and one electron; all other atoms possess at least one proton, one neutron, and one electron. Protons and neutrons have similar masses (≈1.6726×10⁻²⁷ kg and 1.6749×10⁻²⁷ kg, respectively), while electrons are far lighter (≈9.1094×10⁻³¹ kg) and can often be neglected in mass calculations.
In a neutral atom, the number of protons equals the number of electrons, balancing the positive and negative charges. When this balance is disturbed, the atom becomes an ion, carrying a net charge (e.g., +3 or –2).
Atomic behavior mirrors the solar‑system model: the nucleus acts as the sun, with electrons orbiting due to electrostatic attraction. Quantum mechanics describes electron positions probabilistically, underpinning modern technologies such as semiconductors and quantum computing.
The periodic table is the definitive reference for atom names and key properties. It lists 103 elements—92 naturally occurring and 11 synthetically produced—each identified by a unique atomic number (the count of protons). The table’s entry typically displays the element’s symbol, atomic number, and average atomic mass.
For instance, sodium (Na) has atomic number 11 and an average mass of 22.99 amu. Subtracting the atomic number from the mass (≈23) yields 12 neutrons. This calculation demonstrates how neutron count can be inferred from atomic mass.
As you move across the periodic table from left to right or down from top to bottom, atoms generally become heavier due to the addition of protons and neutrons.
Atoms are categorized by their neutron count. Isotopes are atoms of the same element (identical atomic number) that differ in neutron number, leading to distinct atomic masses. For example, iron (Fe) naturally occurs as a mix of four isotopes: ⁵⁴Fe (5.845 %), ⁵⁶Fe (91.754 %), ⁵⁷Fe (2.119 %), and ⁵⁸Fe (0.282 %). Each isotope shares chemical properties but differs in physical attributes such as mass and melting point.
Other classifications include noble gases (group 18), metals (alkali, alkaline‑earth, transition, post‑transition, actinoids, lanthanoids), metalloids (boron, arsenic, silicon, germanium, antimony, tellurium, astatine), and non‑metals (oxygen, nitrogen, sulfur, phosphorus, etc.).
A compound consists of two or more different elements bonded together, such as water (H₂O). A molecule is the smallest discrete unit of a compound that retains its chemical identity. For example, a single sodium chloride molecule (NaCl) is the smallest fragment of table salt that still exhibits the salt’s properties.
Solutions arise when one or more substances (solutes) dissolve in another (solvent) without forming chemical bonds—sugar dissolved in water is a classic example.
Earth’s bulk composition is dominated by ten elements, accounting for ~99 % of its mass. Oxygen constitutes 46.6 %, silicon 27.7 %, aluminum 8.1 %, and iron 5.0 %. In the human body, calcium (3.6 %), sodium (2.8 %), potassium (2.6 %), and magnesium (2.1 %) are key electrolytes.
Even a single atom of an element—such as a gold atom—remains the same element, illustrating the concept of elemental identity at the microscopic level.
Isotopes, while chemically identical, exhibit unique physical properties. This difference enables applications ranging from medical diagnostics (e.g., ¹⁸F in PET scans) to geological dating (e.g., ²⁶Al). Recognizing isotopic variation is essential for fields like environmental science, nuclear physics, and bioanalysis.
Overall, a clear grasp of atoms, elements, and isotopes provides the foundation for chemistry, physics, and the emerging technologies that shape our world.
