When elements are arranged in order of atomic number, their chemical properties appear periodically. The table showing this is the periodic table, the heart of basic chemistry.
Basic Knowledge
Periodic law: A law stating that when elements are arranged in order of atomic number, properties such as the number of valence electrons, atomic radius, and ionization energy change periodically. Mendeleev (1869) created its prototype.
Structure of the periodic table:
・Periods (horizontal rows, 1–7): In the same period, the outermost shell is the same (the nth period has electrons up to the nth shell).
・Groups (vertical columns, 1–18): Elements in the same group have the same number of valence electrons and similar chemical properties.
Main group characteristics:
・Group 1 (alkali metals): Except H. One valence electron. Form cations (Na+). React with water.
・Group 2 (alkaline-earth metals): Two valence electrons. Form 2+ cations (Ca2+).
・Group 17 (halogens): Seven valence electrons. Form 1− anions (Cl-).
・Group 18 (noble gases): Zero valence electrons. Hard to react.
・Groups 3–12 (transition elements): Even neighboring elements have similar properties. Most are metals.
Groups 1, 2, and 12–18 are called representative elements.
Periodic law（The law that element properties appear periodically in atomic-number order）
Period（A horizontal row of the periodic table. It matches the number of the outermost shell）
Group（A vertical column of the periodic table. The number of valence electrons is the same）
Alkali metals（Group 1 except H. Li, Na, K, Rb, Cs, Fr）
Alkaline-earth metals（Group 2. Whether Be and Mg are included varies by convention. In the narrow sense: Ca, Sr, Ba, Ra）
Halogens（Group 17. F, Cl, Br, I, At. The name means salt-forming elements）
Noble gases（Group 18. He, Ne, Ar, Kr, Xe, Rn. Stable）
Representative elements（Groups 1, 2, and 12–18. Number of valence electrons = final digit of group number）
Transition elements（Groups 3–11. Electrons enter shells after the M shell. Many form colored ions）
Further Study (Background and Significance)
The periodic table is more useful when understood through patterns rather than memorized.
① Atomic radius: Within the same group, it becomes larger down the group (more electron shells). Within the same period, it becomes smaller toward the right (greater nuclear charge attracts electrons more strongly).
② Ionization energy (energy required to remove the first electron): Within the same period, it becomes larger toward the right (largest for noble gases). Within the same group, it becomes smaller down the group.
③ Electron affinity: Energy released when one electron is received. It is large for Group 17 (halogens).
④ Metallic character: Stronger toward the lower left of the periodic table. Nonmetallic character is stronger toward the upper right (excluding noble gases).
Carbon C, important in organic chemistry, is in Group 14 and has four valence electrons. It is a basic element that forms diverse molecules through four covalent bonds. Nitrogen N (Group 15, five valence electrons), oxygen O (Group 16, six valence electrons), and halogens (Group 17, seven valence electrons) are also located in easy-to-remember positions. IUPAC (International Union of Pure and Applied Chemistry) standardizes group numbering, element names, and nomenclature.
- Mendeleev discovered the periodic law in 1869
- Group = same number of valence electrons (similar chemical properties)
- Period = number of the outermost shell
- Group 1 alkali metals, Group 2 alkaline-earth metals, Group 17 halogens, Group 18 noble gases
- Representative elements = Groups 1, 2, and 12–18. Transition elements = Groups 3–11
- Atomic radius is large in the lower left and small in the upper right
- Ionization energy is large in the upper right (noble gases)
- Metallic character is strong in the lower left; nonmetallic character is strong in the upper right
- IUPAC standardizes nomenclature
Points to Note (Common Confusions)
① Group number and number of valence electrons: Group 1=1, 2=2, 13=3, 14=4, 15=5, 16=6, 17=7, 18=0 (or 8). For Groups 13–17, the number of valence electrons is the group number −10. ② Hydrogen H is formally in Group 1, but its properties differ from those of alkali metals. It can form either 1+ cations or 1− anions. ③ Transition elements have similar properties even when neighboring each other (representative elements resemble others in the same group). ④ Although noble gases are often said not to react, xenon (Xe), for example, forms xenon tetrafluoride XeF4. They are not completely inert.
Practice
- What is the element in Period 3 and Group 17? Also give its element symbol and number of valence electrons.
- In which direction in the periodic table does atomic radius generally increase?
- What are the element groups consisting of Group 1 (except H), Group 2, and Group 17 called?