The Horizontal Rows of the Periodic Table
What Exactly is a Period?
A period is a horizontal row in the periodic table. All elements in the same period have the same number of atomic orbitals, which are the regions around the nucleus where electrons are likely to be found. For every element in Period 1, its electrons occupy only the first energy level ($ n=1 $). For Period 2, the highest energy level is the second ($ n=2 $), and so on, up to Period 7 ($ n=7 $). This is the primary rule that defines a period.
The number of elements in each period varies. Periods 1 and 2 are short, containing 2 and 8 elements respectively. Periods 4 and 5 are longer, with 18 elements each. This variation occurs because of the maximum number of electrons each energy level and its sublevels can hold. As the principal quantum number $ n $ increases, more orbitals become available, allowing for more elements in the period.
A Journey Through the Seven Periods
Each period tells a unique story of the elements it contains. Let's take a brief tour from the top to the bottom of the table.
| Period Number | Number of Elements | Highest Energy Level | Key Elements and Characteristics |
|---|---|---|---|
| 1 | 2 | 1 ($ n=1 $) | Hydrogen (H) and Helium (He). The simplest and lightest elements. |
| 2 | 8 | 2 ($ n=2 $) | Lithium (Li) to Neon (Ne). Includes life-essential carbon (C) and oxygen (O). |
| 3 | 8 | 3 ($ n=3 $) | Sodium (Na) to Argon (Ar). Contains common metals like sodium and magnesium (Mg). |
| 4 | 18 | 4 ($ n=4 $) | Potassium (K) to Krypton (Kr). The first period to include transition metals like iron (Fe) and copper (Cu). |
| 5 | 18 | 5 ($ n=5 $) | Rubidium (Rb) to Xenon (Xe). Includes silver (Ag) and tin (Sn). |
| 6 | 32 | 6 ($ n=6 $) | Cesium (Cs) to Radon (Rn). Contains the lanthanides (also called rare earth elements) and heavy metals like gold (Au) and lead (Pb). |
| 7 | 32 (incomplete) | 7 ($ n=7 $) | Francium (Fr) to Oganesson (Og). Contains the actinides, many of which are radioactive and synthetic (human-made). |
Periodic Trends: The Patterns Within a Row
The most powerful feature of the periodic table is its ability to predict element properties. As you move from left to right across any period, several key properties change in a smooth and predictable manner. These are called periodic trends.
Let's look at three major trends across a period:
1. Atomic Radius: This is the size of an atom. The atomic radius decreases from left to right across a period. With more protons in the nucleus, the positive charge increases, pulling the electron cloud closer. Imagine a magnet pulling on a set of metal filings; the stronger the magnet, the tighter the cluster. In Period 3, Sodium (Na) is a large atom, while Chlorine (Cl) is much smaller.
2. Ionization Energy: This is the energy needed to remove one electron from an atom. Ionization energy increases from left to right. Elements on the left (like metals) have loosely held electrons that are easy to remove. Elements on the right (like nonmetals) have tightly held electrons that require a lot of energy to remove. It is much easier to remove an electron from Magnesium (Mg) than from Sulfur (S).
3. Electronegativity: This is an atom's ability to attract electrons in a chemical bond. Electronegativity increases from left to right. Fluorine (F), the most electronegative element, is on the far right of Period 2. Lithium (Li), on the far left of the same period, has very low electronegativity.
A Closer Look: Period 3 in Action
To see these concepts come to life, let's examine Period 3 in detail. This period provides a perfect microcosm of the trends and property changes found throughout the table.
| Element (Symbol) | Atomic Number | Type of Element | Physical State at Room Temp. | Common Use or Property |
|---|---|---|---|---|
| Sodium (Na) | 11 | Reactive Metal | Solid | Table salt (Sodium Chloride, NaCl) |
| Magnesium (Mg) | 12 | Lightweight Metal | Solid | Used in alloys for airplanes and cars |
| Aluminum (Al) | 13 | Metal | Solid | Cans, foil, and structural materials |
| Silicon (Si) | 14 | Metalloid | Solid | Semiconductors in computer chips |
| Phosphorus (P) | 15 | Nonmetal | Solid | Fertilizers and matches |
| Sulfur (S) | 16 | Nonmetal | Solid | Sulfuric acid, gunpowder |
| Chlorine (Cl) | 17 | Reactive Nonmetal | Gas | Water purification, disinfectants |
| Argon (Ar) | 18 | Noble Gas | Gas | Inert gas used in light bulbs |
Notice the clear progression: we start with shiny, conductive metals (Na, Mg, Al), move through a metalloid (Si) which has properties of both metals and nonmetals, then to nonmetallic solids (P, S), a reactive nonmetallic gas (Cl), and finally end with an inert, non-reactive noble gas (Ar). This pattern of metals -> metalloids -> nonmetals -> noble gases is repeated in other periods, making the table a powerful predictive tool.
Common Mistakes and Important Questions
A: No, this is a common point of confusion. Periods are the horizontal rows, and elements in a period share the same highest electron shell. Groups (or families) are the vertical columns, and elements in a group share similar chemical properties and the same number of valence electrons. For example, all elements in Group 1 (the alkali metals) have one valence electron and are highly reactive.
A: The number of elements is determined by the maximum number of electrons that can fit in the energy levels being filled. The first shell ($ n=1 $) can only hold 2 electrons, so Period 1 has 2 elements. The second shell ($ n=2 $) can hold 8 electrons, so Period 2 has 8 elements. Starting from Period 4, the $ d $ and $ f $ orbitals begin to fill, which can hold 10 and 14 electrons respectively, leading to the longer periods.
A: For the most part, yes, the decrease is smooth. However, there are tiny, subtle deviations that are studied at a higher level, often related to the specific way electron subshells are filled (e.g., the $ p $ orbitals have slightly higher energy than $ s $ orbitals). For introductory chemistry, it is perfectly acceptable to state that the atomic radius decreases smoothly from left to right.
Footnote
1 Atomic Orbital: A region around an atom's nucleus where there is a high probability of finding an electron. Think of it as a "home" for electrons.
2 Valence Electrons: The electrons in the outermost shell of an atom. These are the electrons involved in forming chemical bonds.
3 Effective Nuclear Charge (Z_eff): The net positive charge experienced by an electron in an atom. It is approximately equal to the number of protons in the nucleus minus the number of inner-shell (shielding) electrons.
4 Metalloid: An element that has properties intermediate between those of metals and nonmetals (e.g., Silicon, Germanium).