The Horizontal Rows On The Periodic Table Are Called

The horizontal rows on the periodic table are called periods. These periods are fundamental in understanding the organization and properties of elements. Each period signifies the principal quantum number, or energy level, that electrons occupy in the atoms of the elements within that row.

The Periodic Table: A Quick Overview

The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are arranged in increasing order of atomic number, which is the number of protons in the nucleus of an atom.

The periodic table consists of:

Periods: The seven horizontal rows.
Groups: The 18 vertical columns.

What are Periods?

Periods are the horizontal rows that run across the periodic table. There are seven periods in total, each starting with an element that has one electron in a new principal energy level. As you move from left to right across a period, each element has one more proton and one more electron than the element before it. This increase in the number of electrons affects the chemical properties of the elements, leading to trends in atomic size, ionization energy, electronegativity, and metallic character.

Each period corresponds to the filling of electron shells around the atomic nucleus. The number of electron shells an atom has is equal to its period number. For example, elements in Period 1 have one electron shell, elements in Period 2 have two electron shells, and so on.

Understanding Each Period

Let's explore each period in more detail:

Period 1

Period 1 contains only two elements: hydrogen (H) and helium (He). Hydrogen is unique and does not neatly fit into any particular group, as it exhibits properties similar to both alkali metals and halogens. Helium, on the other hand, is a noble gas and has a full valence shell, making it very stable and unreactive.

Hydrogen (H): Atomic number 1, electron configuration 1s¹.
Helium (He): Atomic number 2, electron configuration 1s².

Period 2

Period 2 consists of eight elements, starting with lithium (Li) and ending with neon (Ne). This period introduces the filling of the 2s and 2p orbitals.

Lithium (Li): Atomic number 3, electron configuration 1s² 2s¹.
Beryllium (Be): Atomic number 4, electron configuration 1s² 2s².
Boron (B): Atomic number 5, electron configuration 1s² 2s² 2p¹.
Carbon (C): Atomic number 6, electron configuration 1s² 2s² 2p².
Nitrogen (N): Atomic number 7, electron configuration 1s² 2s² 2p³.
Oxygen (O): Atomic number 8, electron configuration 1s² 2s² 2p⁴.
Fluorine (F): Atomic number 9, electron configuration 1s² 2s² 2p⁵.
Neon (Ne): Atomic number 10, electron configuration 1s² 2s² 2p⁶.

Period 3

Period 3 also contains eight elements, starting with sodium (Na) and ending with argon (Ar). This period involves the filling of the 3s and 3p orbitals.

Sodium (Na): Atomic number 11, electron configuration 1s² 2s² 2p⁶ 3s¹.
Magnesium (Mg): Atomic number 12, electron configuration 1s² 2s² 2p⁶ 3s².
Aluminum (Al): Atomic number 13, electron configuration 1s² 2s² 2p⁶ 3s² 3p¹.
Silicon (Si): Atomic number 14, electron configuration 1s² 2s² 2p⁶ 3s² 3p².
Phosphorus (P): Atomic number 15, electron configuration 1s² 2s² 2p⁶ 3s² 3p³.
Sulfur (S): Atomic number 16, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴.
Chlorine (Cl): Atomic number 17, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁵.
Argon (Ar): Atomic number 18, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶.

Period 4

Period 4 includes 18 elements, starting with potassium (K) and ending with krypton (Kr). This period introduces the transition metals, which involve the filling of the 3d orbitals.

Potassium (K): Atomic number 19, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
Calcium (Ca): Atomic number 20, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
Scandium (Sc): Atomic number 21, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹.
Titanium (Ti): Atomic number 22, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d².
Vanadium (V): Atomic number 23, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³.
Chromium (Cr): Atomic number 24, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵.
Manganese (Mn): Atomic number 25, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵.
Iron (Fe): Atomic number 26, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
Cobalt (Co): Atomic number 27, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷.
Nickel (Ni): Atomic number 28, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁸.
Copper (Cu): Atomic number 29, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰.
Zinc (Zn): Atomic number 30, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰.
Gallium (Ga): Atomic number 31, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹.
Germanium (Ge): Atomic number 32, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p².
Arsenic (As): Atomic number 33, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³.
Selenium (Se): Atomic number 34, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.
Bromine (Br): Atomic number 35, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.
Krypton (Kr): Atomic number 36, electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶.

Period 5

Period 5 also contains 18 elements, starting with rubidium (Rb) and ending with xenon (Xe). Similar to Period 4, it includes transition metals, which involve the filling of the 4d orbitals.

Rubidium (Rb): Atomic number 37, electron configuration [Kr] 5s¹.
Strontium (Sr): Atomic number 38, electron configuration [Kr] 5s².
Yttrium (Y): Atomic number 39, electron configuration [Kr] 5s² 4d¹.
Zirconium (Zr): Atomic number 40, electron configuration [Kr] 5s² 4d².
Niobium (Nb): Atomic number 41, electron configuration [Kr] 5s¹ 4d⁴.
Molybdenum (Mo): Atomic number 42, electron configuration [Kr] 5s¹ 4d⁵.
Technetium (Tc): Atomic number 43, electron configuration [Kr] 5s² 4d⁵.
Ruthenium (Ru): Atomic number 44, electron configuration [Kr] 5s¹ 4d⁷.
Rhodium (Rh): Atomic number 45, electron configuration [Kr] 5s¹ 4d⁸.
Palladium (Pd): Atomic number 46, electron configuration [Kr] 4d¹⁰.
Silver (Ag): Atomic number 47, electron configuration [Kr] 5s¹ 4d¹⁰.
Cadmium (Cd): Atomic number 48, electron configuration [Kr] 5s² 4d¹⁰.
Indium (In): Atomic number 49, electron configuration [Kr] 5s² 4d¹⁰ 5p¹.
Tin (Sn): Atomic number 50, electron configuration [Kr] 5s² 4d¹⁰ 5p².
Antimony (Sb): Atomic number 51, electron configuration [Kr] 5s² 4d¹⁰ 5p³.
Tellurium (Te): Atomic number 52, electron configuration [Kr] 5s² 4d¹⁰ 5p⁴.
Iodine (I): Atomic number 53, electron configuration [Kr] 5s² 4d¹⁰ 5p⁵.
Xenon (Xe): Atomic number 54, electron configuration [Kr] 5s² 4d¹⁰ 5p⁶.

Period 6

Period 6 is a long period containing 32 elements, starting with cesium (Cs) and ending with radon (Rn). It includes the lanthanides, which are placed separately below the main body of the periodic table. The filling of the 4f orbitals occurs in the lanthanides.

Cesium (Cs): Atomic number 55, electron configuration [Xe] 6s¹.
Barium (Ba): Atomic number 56, electron configuration [Xe] 6s².
Lanthanum (La): Atomic number 57, electron configuration [Xe] 5d¹ 6s².
Cerium (Ce): Atomic number 58, electron configuration [Xe] 4f¹ 5d¹ 6s².
Praseodymium (Pr): Atomic number 59, electron configuration [Xe] 4f³ 6s².
Neodymium (Nd): Atomic number 60, electron configuration [Xe] 4f⁴ 6s².
Promethium (Pm): Atomic number 61, electron configuration [Xe] 4f⁵ 6s².
Samarium (Sm): Atomic number 62, electron configuration [Xe] 4f⁶ 6s².
Europium (Eu): Atomic number 63, electron configuration [Xe] 4f⁷ 6s².
Gadolinium (Gd): Atomic number 64, electron configuration [Xe] 4f⁷ 5d¹ 6s².
Terbium (Tb): Atomic number 65, electron configuration [Xe] 4f⁹ 6s².
Dysprosium (Dy): Atomic number 66, electron configuration [Xe] 4f¹⁰ 6s².
Holmium (Ho): Atomic number 67, electron configuration [Xe] 4f¹¹ 6s².
Erbium (Er): Atomic number 68, electron configuration [Xe] 4f¹² 6s².
Thulium (Tm): Atomic number 69, electron configuration [Xe] 4f¹³ 6s².
Ytterbium (Yb): Atomic number 70, electron configuration [Xe] 4f¹⁴ 6s².
Lutetium (Lu): Atomic number 71, electron configuration [Xe] 4f¹⁴ 5d¹ 6s².
Hafnium (Hf): Atomic number 72, electron configuration [Xe] 4f¹⁴ 5d² 6s².
Tantalum (Ta): Atomic number 73, electron configuration [Xe] 4f¹⁴ 5d³ 6s².
Tungsten (W): Atomic number 74, electron configuration [Xe] 4f¹⁴ 5d⁴ 6s².
Rhenium (Re): Atomic number 75, electron configuration [Xe] 4f¹⁴ 5d⁵ 6s².
Osmium (Os): Atomic number 76, electron configuration [Xe] 4f¹⁴ 5d⁶ 6s².
Iridium (Ir): Atomic number 77, electron configuration [Xe] 4f¹⁴ 5d⁷ 6s².
Platinum (Pt): Atomic number 78, electron configuration [Xe] 4f¹⁴ 5d⁹ 6s¹.
Gold (Au): Atomic number 79, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s¹.
Mercury (Hg): Atomic number 80, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s².
Thallium (Tl): Atomic number 81, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹.
Lead (Pb): Atomic number 82, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p².
Bismuth (Bi): Atomic number 83, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p³.
Polonium (Po): Atomic number 84, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴.
Astatine (At): Atomic number 85, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵.
Radon (Rn): Atomic number 86, electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶.

Period 7

Period 7 is also a long period, starting with francium (Fr) and potentially ending with oganesson (Og), though many elements in this period are synthetic and highly radioactive. It includes the actinides, which are placed separately below the main body of the periodic table. The filling of the 5f orbitals occurs in the actinides.

Francium (Fr): Atomic number 87, electron configuration [Rn] 7s¹.
Radium (Ra): Atomic number 88, electron configuration [Rn] 7s².
Actinium (Ac): Atomic number 89, electron configuration [Rn] 6d¹ 7s².
Thorium (Th): Atomic number 90, electron configuration [Rn] 6d² 7s².
Protactinium (Pa): Atomic number 91, electron configuration [Rn] 5f² 6d¹ 7s².
Uranium (U): Atomic number 92, electron configuration [Rn] 5f³ 6d¹ 7s².
Neptunium (Np): Atomic number 93, electron configuration [Rn] 5f⁴ 6d¹ 7s².
Plutonium (Pu): Atomic number 94, electron configuration [Rn] 5f⁶ 7s².
Americium (Am): Atomic number 95, electron configuration [Rn] 5f⁷ 7s².
Curium (Cm): Atomic number 96, electron configuration [Rn] 5f⁷ 6d¹ 7s².
Berkelium (Bk): Atomic number 97, electron configuration [Rn] 5f⁹ 7s².
Californium (Cf): Atomic number 98, electron configuration [Rn] 5f¹⁰ 7s².
Einsteinium (Es): Atomic number 99, electron configuration [Rn] 5f¹¹ 7s².
Fermium (Fm): Atomic number 100, electron configuration [Rn] 5f¹² 7s².
Mendelevium (Md): Atomic number 101, electron configuration [Rn] 5f¹³ 7s².
Nobelium (No): Atomic number 102, electron configuration [Rn] 5f¹⁴ 7s².
Lawrencium (Lr): Atomic number 103, electron configuration [Rn] 5f¹⁴ 6d¹ 7s².
Rutherfordium (Rf): Atomic number 104, electron configuration [Rn] 5f¹⁴ 6d² 7s².
Dubnium (Db): Atomic number 105, electron configuration [Rn] 5f¹⁴ 6d³ 7s².
Seaborgium (Sg): Atomic number 106, electron configuration [Rn] 5f¹⁴ 6d⁴ 7s².
Bohrium (Bh): Atomic number 107, electron configuration [Rn] 5f¹⁴ 6d⁵ 7s².
Hassium (Hs): Atomic number 108, electron configuration [Rn] 5f¹⁴ 6d⁶ 7s².
Meitnerium (Mt): Atomic number 109, electron configuration [Rn] 5f¹⁴ 6d⁷ 7s².
Darmstadtium (Ds): Atomic number 110, electron configuration [Rn] 5f¹⁴ 6d⁹ 7s¹.
Roentgenium (Rg): Atomic number 111, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s¹.
Copernicium (Cn): Atomic number 112, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s².
Nihonium (Nh): Atomic number 113, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p¹.
Flerovium (Fl): Atomic number 114, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p².
Moscovium (Mc): Atomic number 115, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p³.
Livermorium (Lv): Atomic number 116, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁴.
Tennessine (Ts): Atomic number 117, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁵.
Oganesson (Og): Atomic number 118, electron configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶.

Trends Within Periods

As you move across a period from left to right, several trends in elemental properties can be observed. These trends are primarily due to the increasing number of protons in the nucleus, which increases the effective nuclear charge experienced by the valence electrons.

Atomic Radius

Atomic radius generally decreases from left to right across a period. This is because the increasing nuclear charge pulls the electrons closer to the nucleus, resulting in a smaller atomic size.

Ionization Energy

Ionization energy, which is the energy required to remove an electron from an atom, generally increases from left to right across a period. This is because the increasing nuclear charge makes it more difficult to remove an electron.

Electronegativity

Electronegativity, which is a measure of an atom's ability to attract electrons in a chemical bond, generally increases from left to right across a period. This is because the increasing nuclear charge makes the atom more attractive to electrons.

Metallic Character

Metallic character generally decreases from left to right across a period. Elements on the left side of the periodic table are typically metals, while elements on the right side are nonmetals. The transition from metallic to nonmetallic character is gradual and depends on the specific properties of the elements.

Importance of Periods

Understanding the periods of the periodic table is crucial for several reasons:

Predicting Properties: Knowing the period of an element helps predict its chemical and physical properties.
Understanding Electron Configuration: Periods indicate the highest energy level occupied by electrons in an atom.
Explaining Trends: Periods help explain the trends in atomic size, ionization energy, electronegativity, and metallic character.
Organizing Elements: Periods provide a systematic way to organize elements based on their atomic structure and properties.

The Role of Electron Configuration

Electron configuration plays a vital role in determining the properties of elements within each period. The electron configuration describes the arrangement of electrons in different energy levels and orbitals around the nucleus of an atom. As you move across a period, electrons are added to the same principal energy level, leading to changes in the electron configuration and, consequently, the properties of the elements.

For example, in Period 2, the electron configuration changes from 1s² 2s¹ for lithium to 1s² 2s² 2p⁶ for neon. This change in electron configuration results in significant differences in the chemical behavior of these elements. Lithium is a reactive alkali metal, while neon is an inert noble gas.

Periods and Quantum Mechanics

The concept of periods is closely linked to quantum mechanics. Each period corresponds to a specific principal quantum number (n), which determines the energy level of the electrons. The principal quantum number can have integer values of 1, 2, 3, and so on.

n = 1: Period 1, filling the 1s orbital.
n = 2: Period 2, filling the 2s and 2p orbitals.
n = 3: Period 3, filling the 3s and 3p orbitals.
n = 4: Period 4, filling the 4s, 3d, and 4p orbitals.
n = 5: Period 5, filling the 5s, 4d, and 5p orbitals.
n = 6: Period 6, filling the 6s, 4f, 5d, and 6p orbitals.
n = 7: Period 7, filling the 7s, 5f, 6d, and 7p orbitals.

The filling of these orbitals follows the Aufbau principle, Hund's rule, and the Pauli exclusion principle, which govern the electron configurations of atoms.

FAQs About Periods

Q1: How many periods are there in the periodic table?

There are seven periods in the periodic table, each corresponding to a different principal energy level.

Q2: What is the significance of the periods in the periodic table?

Periods indicate the highest energy level occupied by electrons in an atom and help explain the trends in atomic size, ionization energy, electronegativity, and metallic character.

Q3: What trends can be observed as you move across a period?

As you move across a period from left to right, atomic radius generally decreases, while ionization energy and electronegativity generally increase. Metallic character decreases.

Q4: Why do elements in the same period have different properties?

Elements in the same period have different properties due to the increasing number of protons and electrons, which affects the electron configuration and the interactions between the nucleus and the electrons.

Q5: What role does electron configuration play in determining the properties of elements within a period?

Electron configuration determines the arrangement of electrons in different energy levels and orbitals, which influences the chemical and physical properties of the elements.

Q6: How are periods related to quantum mechanics?

Periods are related to quantum mechanics through the principal quantum number (n), which determines the energy level of the electrons. Each period corresponds to a specific value of n.

Conclusion

In summary, the horizontal rows on the periodic table are called periods. They are fundamental to understanding the organization, properties, and trends of elements. Each period signifies the principal quantum number, or energy level, that electrons occupy in the atoms of the elements within that row. By understanding periods, we can predict elemental properties, explain trends, and appreciate the systematic organization of the periodic table. The periodic table, with its periods and groups, remains an indispensable tool in the field of chemistry, providing a framework for understanding the behavior of matter.