The periodic table
Also known as: periodic table of the elements
The periodic table is an arrangement of the chemical elements in order of increasing atomic number, organized into rows and columns so that elements with similar properties line up. Its structure reflects the electron configurations of the atoms.
Elements are listed by atomic number, the count of protons in the nucleus, and arranged so that chemically similar elements fall into the same vertical column. Horizontal rows are called periods, and each one corresponds to filling a new principal energy level. Vertical columns are groups, and elements within a group share the same number of valence electrons, which is why they react in similar ways. Familiar groups include the alkali metals, alkaline earth metals, halogens, and noble gases, with the transition metals occupying the central block.
The shape of the table maps directly onto orbital filling. The first two columns make up the s-block, the last six the p-block, the transition metals the d-block, and the lanthanides and actinides the f-block. Reading an element's position therefore gives its electron configuration without memorization, and the table also separates metals on the left from nonmetals on the upper right, with metalloids along the dividing staircase.
Periodic trends are the payoff. Moving left to right across a period, effective nuclear charge increases while electrons enter the same shell, so atomic radius decreases while ionization energy and electronegativity increase. Moving down a group adds shells, so radius increases and ionization energy and electronegativity decrease. These trends explain why cesium loses an electron easily, why fluorine attracts electrons strongly, and why noble gases with filled valence shells are nearly inert. Mendeleev's original 1869 table was ordered by atomic mass and left gaps for elements he predicted; Moseley later showed that atomic number is the correct ordering principle.
The MCAT tests the periodic table in the chemical and physical foundations section, where questions rarely ask you to recite facts and instead expect you to use trends to compare reactivity, predict bonding, and rank ionization energies. On the SAT, the relevant skill is broader: reading structured tables and extracting the right value quickly, which the problem solving and data analysis section tests directly.
Key takeaways
- Elements are ordered by increasing atomic number, with periods as rows and groups as columns.
- Elements in the same group share valence electron counts and therefore similar chemical behavior.
- Atomic radius decreases across a period and increases down a group; ionization energy and electronegativity do the opposite.
- The s, p, d, and f blocks of the table correspond to the orbitals being filled.