HESI A2 · Chemistry · 6 min read
The Periodic Table and Chemical Symbols
- Describe the structural organization of the periodic table, including periods, groups, and the distinct classifications of metals, nonmetals, metalloids, and inner transition metals.
- Analyze periodic trends including atomic radius, ionic radius, first ionization energy, electronegativity, electron shielding, and metallic character.
- Interpret chemical symbols, elemental diatomic forms, and chemical formulas, distinguishing between empirical and molecular formulas while balancing charges in neutral ionic compounds.
Architecture of the Periodic Table
The modern periodic table is an organized arrangement of chemical elements placed in order of increasing atomic number. An element's atomic number represents the total number of protons located within an atom's nucleus. By organizing elements sequentially by their proton count, recurring chemical and physical patterns emerge across the rows and columns.
Horizontal rows on the periodic table are designated as periods. There are seven periods in the standard modern periodic table. The period number corresponds directly to the highest principal energy level occupied by ground-state electrons. For example, an element situated in the third period possesses ground-state electrons that reach the third principal energy level as their highest occupied level.
The periodic table is broadly partitioned into metals, nonmetals, and metalloids based on physical and chemical behavior. Metals are positioned across the left side and center of the periodic table. In contrast, nonmetals are situated on the upper-right portion of the periodic table, with the notable exception of hydrogen, which is located on the upper left. Lying along the diagonal boundary separating the metals from the nonmetals are the metalloids. Common metalloids include boron, silicon, germanium, arsenic, antimony, and tellurium. Beneath the main table, two specialized rows known as the lanthanides and actinides constitute the inner transition metals.
Metals and nonmetals exhibit contrasting physical characteristics that are easily recognized in everyday life. Metals are typically lustrous, malleable, ductile, and efficient conductors of heat and electricity. For instance, the shiny surface of metallic cookware or the ability of electrical wiring to conduct energy without snapping reflects these metallic properties. Nonmetals, conversely, are generally dull in appearance, brittle when solid, and poor conductors of heat and electricity.
| Class | Periodic Table Location | Distinguishing Physical Characteristics | Identified Elements / Subsets |
|---|---|---|---|
| Metals | Left side and center | Lustrous, malleable, ductile, efficient conductors of heat and electricity | Main-group metals, transition metals, lanthanides, and actinides |
| Metalloids | Along the diagonal boundary separating metals from nonmetals | Exhibit intermediate characteristics along the boundary | Boron, silicon, germanium, arsenic, antimony, and tellurium |
| Nonmetals | Upper-right portion (with the exception of hydrogen) | Generally dull, brittle when solid, poor conductors of heat and electricity | Hydrogen, halogens, noble gases, and other upper-right nonmetals |
Common mistakes
- Confusing periods with groups: Periods are the seven horizontal rows corresponding to occupied energy levels, whereas groups or families are the vertical columns containing elements with similar valence-electron configurations.
- Assuming noble gases have high electronegativity: Noble gases possess stable, full valence shells and rarely form chemical bonds, which is why they are generally omitted from electronegativity scales.
- Reversing ionic radius sizes: Losing electrons makes cations smaller than their neutral parent atoms, whereas gaining electrons makes anions larger than their neutral parent atoms.
- Confusing empirical formulas with molecular formulas: An empirical formula represents the simplest whole-number ratio of atoms, whereas a molecular formula provides the exact number of atoms of each element in a single molecule.
Key points
- The periodic table arranges elements by increasing atomic number (total nuclear protons) across seven horizontal periods and multiple vertical groups.
- The period number indicates the highest principal energy level occupied by ground-state electrons, while vertical groups contain elements with similar valence-electron configurations and properties.
- Key families include Group 1 alkali metals (form +1 cations), Group 2 alkaline earth metals (form +2 ions), Groups 3-12 transition metals (multiple oxidation states, colored compounds), Group 17 halogens (form -1 anions, most reactive nonmetals), and Group 18 noble gases (stable, unreactive; helium has two valence electrons, others have eight).
- Metals are lustrous, malleable, ductile, and conductive; nonmetals are dull, brittle, and poor conductors; metalloids (boron, silicon, germanium, arsenic, antimony, tellurium) lie along the diagonal boundary.
- Atomic radius decreases across a period and increases down a group; ionization energy and electronegativity generally increase across a period and decrease down a group, with fluorine being the most electronegative element.
- Cations are generally smaller than their parent atoms, anions are generally larger, and electron shielding changes relatively little across a period but increases down a group.
- Chemical symbols have one capital letter or a capital and lowercase letter (such as Na from natrium, K from kalium, and Fe from ferrum); the common elemental forms of seven elements are diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂); and neutral ionic compound formulas balance total positive and negative charge.
Practice this section
Explained HESI Chemistry questions, no account needed. HESI Chemistry practice test
The full lesson continues with 3 more sections with exam tips, audio in English and Spanish, 5 Check yourself questions and 20 flashcards.
- 1 month
$53.99$26.99 - 3 months
$158.99$79.49 - Lifetime
$323.99$161.99
One payment, no subscription.
See HESI plansAlready a member? Log in
Sample lesson: Atomic Structure and Isotopes25 practice questions on this exam
Each HESI practice test has 25 original questions with the answers explained. No account needed.
Open the HESI practice testsOriginal lesson written for HESI A2 practice · updated October 30, 2026. Exam names are trademarks of their owners; tests.global is not affiliated with them.