Horizontal Rows On The Periodic Table

Horizontal Rows on the Periodic Table

Horizontal rows on the periodic table are called periods, and they organize the elements according to the order in which their electron shells are filled. In practice, each period begins with an element whose atomic number is the next whole number and ends with a noble gas. Although elements in the same row do not always have similar chemical properties, moving from left to right reveals clear patterns in atomic size, ionization energy, electronegativity, and metallic character. Understanding these horizontal rows provides a practical foundation for predicting how atoms interact and form compounds And that's really what it comes down to..

What Is a Period?

A period is one of the seven horizontal rows that make up the modern periodic table. A period contains a sequence of elements in which electrons are added to the same principal energy level, or electron shell, as the atomic number increases.

For example:

• Hydrogen and helium occupy the first period.
• Lithium through neon make up the second period.
• Sodium through argon belong to the third period.
• The fourth period runs from potassium to krypton.
• The fifth period contains rubidium to xenon.
• The sixth period includes cesium to radon.
• The seventh period extends from francium to oganesson.

The period number generally tells the highest principal electron shell occupied by an element. Hydrogen and helium use the first shell, while elements in the second period use shells with the principal quantum number n equal to two. In the sixth period, for instance, the outermost electrons occupy the sixth shell, even though some inner electrons are found in the fourth and fifth shells.

The atomic number of an element is the total number of protons in its nucleus. Think about it: since an electrically neutral atom has the same number of protons and electrons, atomic numbers also show the order in which electrons are added. Beginning at the left side of a period, each successive element has one more proton and one more electron than the element before it Nothing fancy..

Why Do the Rows Have Different Lengths?

The number of elements in each period depends on the available subshells within the relevant electron energy levels. The seven periods contain 2, 8, 8, 18, 18, 32, and 32 elements, respectively.

The first period is short because only the 1s subshell is filled. The 1s subshell can hold two electrons, so the first row contains only hydrogen and helium.

The second and third periods each include an s subshell and a p subshell. Which means the 2s and 3s subshells can each hold two electrons, while the 2p and 3p subshells can each hold six. Adding these capacities gives eight elements per row.

This changes depending on context. Keep that in mind Simple, but easy to overlook..

The fourth and fifth periods become longer because they include a d subshell, which can hold ten electrons. The sixth and seventh periods are longer still because they include an f subshell, capable of holding fourteen electrons Most people skip this — try not to..

The sixth period, for example, includes:

1. The filling of the 6s subshell with two electrons.
2. The filling of the 4f subshell with fourteen electrons.
3. The filling of the 5d subshell with ten electrons.
4. The filling of the 6p subshell with six electrons