Chapter 4. Atoms, Elements and Chemical Reactions

Atoms consist of protons (positive), neutrons (neutral) and electrons (negative). The nucleus holds the protons and neutrons; electrons occupy the space around it. The number of protons (atomic number) defines the element [6][7].

The periodic table organizes the 118 known elements by atomic number. Columns (groups) share similar chemical properties; rows are periods. Broad classes are metals, nonmetals and metalloids.

Compounds and bonds. In ionic bonding, electrons transfer between atoms; in covalent bonding, electrons are shared. Water (H₂O) and carbon dioxide (CO₂) are covalent compounds; table salt (NaCl) is ionic.

Chemical reactions rearrange atoms into new substances. Mass is conserved, so equations must be balanced: 2H₂ + O₂ → 2H₂O.

Common reaction types: synthesis, decomposition, single replacement, double replacement and combustion.

Acids, bases and pH. The pH scale typically runs from 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is basic. Each unit change in pH is a tenfold change in hydrogen-ion concentration [7].

Rates of reaction increase with temperature, concentration, surface area and the presence of catalysts.

Review: How many protons, neutrons and electrons does a neutral atom of carbon-12 have? (Answer: 6, 6 and 6.)