Periodic Table Groups and Periods

Groups and periods organize elements on the periodic table of the elements. A group is a vertical column down the periodic table, while a period is a horizontal row across the table. Both groups and periods reflect the organization of electrons in atoms. Element atomic number increases as you move down a group from top to bottom or across a period from left to right.

• An element group is a vertical column on the periodic table. Atoms in a group share the same number of valence electrons. There are 18 element groups.
• An element period is a horizontal row on the periodic table. Atoms in a period have the same number of electron shells. There are 7 element periods.
• There are 18 groups and 7 periods.

Element Groups

Elements within the same group share the same number of valence electrons. The number of valence electrons depends on the octet rule. For example, elements in group 1 have 1 valence electron, elements in groups 3-12 have a variable number of valence electrons, and elements in group 17 have 7 valence electrons. The lanthanides and actinides, located below the main table, all fit within group 3.

There are 18 element groups. Elements in the same group share common chemical and physical properties. For example, the group 1 elements are all soft, reactive metals. The group 17 elements are highly reactive, colorful nonmetals.

Alternate Group Classification System

Sometimes chemists classify element groups according to shared properties, which do not strictly adhere to individual columns. These groups go by the names alkali metals, alkaline earth metals, transition metals, basic metals (post-transition metals), nonmetals, halogens, noble gases, lanthanides, and actinides. Under this system, hydrogen is a nonmetal. The nonmetals, halogens, and noble gases are all types of nonmetals. The metalloids have properties intermediate between metals and nonmetals. The alkali metals, alkaline earths, lanthanides, actinides, transition metals, and basic metals are all groups of metals.

Element Periods

Elements within a period share the same number of electron shells and the same highest unexcited electron energy level. Elements within a period display periodic table trends, moving from left to right, involving atomic and ionic radius, electronegativity, ionization energy, and metallic character.

There are seven element periods. Some periods contain more elements than others because the number of included elements depends on the number of electrons allowed in an energy sublevel. Note that the lanthanides are within period 6 and the actinides are in period 7. These two rows make up the f-block, where electrons fill f-orbitals. For formatting reasons, the f-block appears below the main body of the periodic table, but these elements actually fit within group 3 of the table.

• Period 1: H, He (does not follow the octet rule)
• Period 2: Li, Be, B, C, N, O, F, Ne (involves s and p orbitals)
• Period 3: Na, Mg, Al, Si, P, S, Cl, Ar (all have at least 1 stable isotope)
• Period 4: K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr (first period with d-block elements)
• Period 5: Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe (same number of elements as period 4, same general structure, and includes the first exclusively radioactive element, Tc)
• Period 6: Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At, Rn (first period with f-block elements)
• Period 7: Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rd, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, Fl, Mc, Lv, Ts, Og (all elements are radioactive; contains heaviest natural elements and many synthesized elements)

Electron Configuration Trends

The layout of the periodic table reflects patterns in electron configurations. As you move across a period (left to right), electrons fill orbitals in the same principal energy level (or shell). As you move down a group, electrons occupy higher energy levels, adding new shells.

• Across a period: Electrons fill orbitals in a specific order—first s, then p (and for longer periods, d and f). For example, in period 2, elements fill the 2s and then the 2p orbitals.
  • Example:
    • Li (Z = 3): 1s² 2s¹
    • Ne (Z = 10): 1s² 2s² 2p⁶
• Down a group: Elements in the same group have similar valence electron configurations, even though they are in different energy levels.
  • Example: Group 1 elements:
    • Li: [He] 2s¹
    • Na: [Ne] 3s¹
    • K: [Ar] 4s¹

This pattern explains why elements in the same group share chemical properties—they have the same number of valence electrons, just in different shells.

Periodic Trends

As you move across a period or down a group, certain element properties follow predictable trends. These patterns result from increasing atomic number and the arrangement of electrons.

Across a Period (left to right):

• Atomic radius decreases: Increased nuclear charge pulls electrons closer.
• Electronegativity increases: Atoms more strongly attract bonding electrons.
• Ionization energy increases: It takes more energy to remove an electron.
• Metallic character decreases: Elements become less metallic and more nonmetallic.

Down a Group (top to bottom):

• Atomic radius increases: Additional shells increase the size of the atom.
• Electronegativity decreases: Valence electrons are farther from the nucleus.
• Ionization energy decreases: Outer electrons are more easily removed.
• Metallic character increases: Elements become more metallic and reactive (especially for groups 1 and 2).

These trends help predict element behavior, reactivity, and bonding types.

❓FAQs About Periodic Table Groups and Periods

Q: Why is hydrogen sometimes not grouped with the alkali metals (group 1)?
A: Hydrogen has one valence electron like group 1 metals, but it’s a nonmetal and behaves very differently. Some periodic tables place it separately or above group 17.

Q: Why are the lanthanides and actinides below the main table?
A: This is just for space. The lanthanides (period 6) and actinides (period 7) actually fit within group 3. Including them in the main table makes it too wide.

Q: What’s the difference between a group and a period?
A: A group is a vertical column where elements have the same number of valence electrons. A period is a horizontal row where elements share the same number of electron shells.

Q: Do elements in the same period have similar properties?
A: No. Elements in the same period have different properties because they have different numbers of valence electrons. However, their properties change in a predictable way across the period.

Q: What are the s, p, d, and f blocks on the table?
A: These blocks refer to the type of atomic orbital being filled with electrons:

• s-block: groups 1 and 2 (plus hydrogen and helium)
• p-block: groups 13–18
• d-block: transition metals (groups 3–12)
• f-block: lanthanides and actinides

References

• Fluck, E. (1988). “New Notations in the Periodic Table” . Pure Appl. Chem. IUPAC. 60 (3): 431–436. doi:10.1351/pac198860030431
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
• Scerri, E. R. (2007). The periodic table, its story and its significance. Oxford University Press. ISBN 978-0-19-530573-9.