The Periodic Table of Elements: What It Is and How It Works

The periodic table is a tool used in science to organize all the chemical elements. It arranges elements by their atomic number and shows repeating patterns in how they behave. These patterns come from the way electrons are arranged in atoms. The periodic table helps explain why elements react and how they form compounds.

What Is the Periodic Table?

The periodic table is a chart that organizes all known chemical elements. The elements are arranged based on their atomic number, which is the number of protons in an atom’s nucleus.

Key Historical Point-

• Dmitri Mendeleev, a Russian chemist, first organized elements by atomic mass in 1869.
  
• He noticed repeating (periodic) chemical properties.
  
• Later, scientists discovered that arranging elements by atomic number (not mass) showed even clearer repeating patterns. This became the modern periodic table.
  

What Each Element Block Shows-

• Chemical symbol (e.g., H for Hydrogen, O for Oxygen)
  
• Atomic number (e.g., 1 for Hydrogen, 8 for Oxygen)
  
• Often, the atomic mass is also shown (e.g., 1.008 for Hydrogen)
  

Purpose-

The table shows how elements are related, how they react and helps predict properties of unknown or new elements.

The Structure of the Periodic Table

The periodic table is arranged into-

• 7 periods (horizontal rows)
  
• 18 groups (vertical columns)
  

Group = Same Number of Valence Electrons

Elements in the same group have the same number of electrons in their outer shell, so they behave similarly in chemical reactions.

Important Groups-

• Group 1 – Alkali Metals–
  1 valence electron. Very reactive, especially with water (e.g., Lithium, Sodium).
  
• Group 2 – Alkaline Earth Metals–
  2 valence electrons. Also reactive, but less than Group 1 (e.g., Calcium, Magnesium).
  
• Groups 3–12 – Transition Metals–
  Varied properties. Often form colorful compounds. Good conductors (e.g., Iron, Copper).
  
• Group 17 – Halogens–
  7 valence electrons. Very reactive nonmetals. Need 1 more electron to complete shell (e.g., Fluorine, Chlorine).
  
• Group 18 – Noble Gases–
  Full valence shells. Very stable and nonreactive (e.g., Helium, Neon, Argon).
  

Periodicity- Why Patterns Repeat

Periodicity means that certain properties of elements repeat at regular intervals. This happens because of how electrons fill energy levels (shells) in atoms.

Atomic Structure-

• Each atom has-
  
  • A nucleus (with protons and neutrons)
    
  • Surrounding shells of electrons
    
• As atomic number increases, electrons are added in a specific order to different shells.
  

Why Properties Repeat-

• Noble gases (like Neon and Argon) have complete outer shells. This makes them chemically stable.
  
• Other elements try to gain, lose, or share electrons to get a similar full-shell structure.
  

Examples-

• Sodium (Na) has 1 extra electron. It loses it to get the stable structure of Neon → becomes Na⁺.
  
• Chlorine (Cl) needs 1 electron. It gains it to become like Argon → becomes Cl⁻.
  

These behaviors explain chemical reactivity and bond formation.

Also Check – What Are Chemical Elements? A Complete Guide for Students

Chemical Properties- Ionic and Covalent Behavior

Atoms interact to become more stable, often by forming chemical bonds. This helps them achieve a full outer shell of electrons.

Ionic Bonds

Atoms transfer electrons to each other-

• Alkali metals (Group 1) lose 1 electron → form +1 ions (e.g., Na⁺)
  
• Alkaline earth metals (Group 2) lose 2 electrons → form +2 ions (e.g., Ca²⁺)
  
• Halogens (Group 17) gain 1 electron → form –1 ions (e.g., Cl⁻)
  

Covalent Bonds

Atoms share electrons–

• Common among nonmetals
  
• Fluorine (F) has 7 valence electrons → needs 1 more
  
• It shares 1 electron with another atom → both atoms complete their outer shell
  

These bonding types follow patterns related to the group number, because the number of valence electrons controls how atoms bond.

What are The Groups in The Periodic Table?

Trends Across Periods and Down Groups

The periodic table helps us understand how the properties of elements change as we move-

Across a Period (Left to Right)-

• Atomic number increases by 1 for each element.
  
• Electrons are added to the same outer shell.
  
• The nucleus becomes more positive, pulling electrons closer.
  
• This makes the atomic radius (size) smaller.
  
• Ionization energy increases – more energy is needed to remove an electron.
  
• Electronegativity increases – atoms attract shared electrons more strongly.
  
• Metallic character decreases – elements become more nonmetallic.
  

Down a Group (Top to Bottom)-

• Each element has one more electron shell than the one above.
  
• This makes atoms larger.
  
• Valence electrons are farther from the nucleus and more loosely held.
  
• Ionization energy decreases – it is easier to remove an electron.
  
• Reactivity increases for metals (like alkali metals) and decreases for nonmetals (like halogens).
  

These trends help predict how elements react chemically and how strongly they attract electrons.

Special Element Groups and Classifications

Beyond standard groups, some elements are grouped based on shared properties, even if they are not in the same column.

Transition Elements (Groups 3–12)-

• Found in the center block of the table.
  
• Show similar sizes across a period.
  
• Can form multiple types of ions (e.g., Fe²⁺, Fe³⁺).
  
• Often form colored compounds (e.g., CuSO₄ is blue).
  
• Are good conductors of heat and electricity.
  

Also Check – What Are Transition Elements? Properties, Periodic Table Location, and Atomisation Enthalpy

Lanthanoids and Actinoids-

• Shown in two rows below the main table.
  
• Also called the f-block elements.
  
• Have very similar chemical properties.
  
• Actinoids (like Uranium) are mostly radioactive.
  

Noble Metals-

• Include gold (Au), silver (Ag), platinum (Pt) and others.
  
• Very chemically stable.
  
• Do not react easily with air or water.
  
• Used in coins, jewelry and electronics because they resist corrosion.
  

These groups help us understand special behaviors that don’t follow the regular vertical columns.

States of Elements and Physical Properties

At room temperature, elements exist in different states of matter–

• Gases– 11 elements (e.g., Oxygen, Nitrogen, Neon)
  
• Liquids– 3 elements (Mercury, Bromine and sometimes Gallium)
  
• Solids– All remaining elements (e.g., Iron, Sulfur, Silicon)
  

Each element has unique physical properties-

• Melting point– Temperature where it turns from solid to liquid
  
• Boiling point– Temperature where it turns from liquid to gas
  
• Electrical conductivity– Ability to carry electric current
  
• Thermal conductivity– Ability to conduct heat
  
• Spectral lines– Pattern of light an element gives off when excited
  

These properties can be used to identify and analyze elements in a lab or in nature.

Radioactive Elements and Transuranium Elements

Radioactive elements have unstable nuclei. Over time, they decay by releasing particles and energy.

• All elements with atomic numbers ≥ 84 (like Polonium and Uranium) are radioactive.
  
• Some lighter elements have radioactive isotopes (e.g., Carbon-14).
  
• Each radioactive isotope has a half-life – the time it takes for half of it to decay.
  

Types of Radioactive Decay-

• Alpha decay– Atom loses 2 protons and 2 neutrons → atomic number decreases by 2
  
• Beta decay– A neutron becomes a proton → atomic number increases by 1
  
• Electron capture– An inner electron is pulled into the nucleus → atomic number decreases by 1
  

Transuranium elements-

• Elements with atomic numbers > 92 (e.g., Plutonium, Americium)
  
• Do not exist naturally in large amounts
  
• Must be made in laboratories
  
• All are radioactive
  

Some radioactive elements, like Technetium and Promethium, were first made artificially and occur in nature only in tiny amounts.

Frequently Asked Questions (FAQs)

1. What is the periodic table?

The periodic table is a chart that organizes all known chemical elements based on their atomic number (number of protons). It helps scientists understand and predict how elements behave chemically and physically.

2. Who created the periodic table?

The periodic table was first arranged by Dmitri Mendeleev in 1869. He organized elements by atomic mass and noticed repeating chemical properties. The modern version is arranged by atomic number for better accuracy.

3. What is shown for each element in the periodic table?

Each element block usually shows-

• The chemical symbol (e.g., H for Hydrogen)
  
• The atomic number (e.g., 1 for Hydrogen)
  
• The atomic mass (average mass of atoms)
  

4. What are groups and periods?

• Groups are vertical columns. Elements in the same group have the same number of valence electrons and show similar properties.
  
• Periods are horizontal rows. They show how elements change in properties as atomic number increases.
  

5. Why do elements in the same group behave similarly?

Because they have the same number of electrons in their outermost shell. This outer shell controls how an atom reacts.

6. What are the alkali metals?

They are in Group 1. They have 1 valence electron and are very reactive, especially with water. Examples- Lithium, Sodium, Potassium.

7. What are the alkaline earth metals?

They are in Group 2. They have 2 valence electrons and are also reactive, but less than alkali metals. Examples- Magnesium, Calcium.

8. What are transition metals?

These are elements in Groups 3–12. They-

• Form colored compounds
  
• Can have more than one ion
  
• Are good conductors of electricity
  

Examples- Iron, Copper, Zinc

9. What are halogens?

They are in Group 17. Halogens have 7 valence electrons and are very reactive nonmetals. They gain 1 electron to become stable. Examples- Fluorine, Chlorine, Iodine.

10. What are noble gases?

They are in Group 18. These gases have full outer electron shells, so they are very stable and unreactive. Examples- Helium, Neon, Argon.

11. What does periodicity mean in chemistry?

Periodicity means that chemical properties repeat at regular intervals when elements are arranged by atomic number. This happens because of repeating patterns in electron configuration.

12. Why do some atoms form ions?

Atoms form ions to get a full outer shell of electrons-

• Metals lose electrons → form positive ions (cations)
  
• Nonmetals gain electrons → form negative ions (anions)
  

13. What are ionic and covalent bonds?

• Ionic bond– Electrons are transferred between atoms (usually metal to nonmetal)
  
• Covalent bond– Electrons are shared between atoms (usually nonmetals)
  

14. What are periodic trends across a period?

Across a row (left to right)-

• Atomic size decreases
  
• Ionization energy increases
  
• Electronegativity increases
  
• Elements go from metals to nonmetals
  

15. What are trends down a group?

As you go down a group-

• Atomic size increases
  
• Reactivity increases for metals (like alkali metals)
  
• Ionization energy decreases (easier to remove electrons)
  

16. What are transition elements, lanthanoids and actinoids?

• Transition elements– Middle block; form colorful compounds, good conductors.
  
• Lanthanoids– Rare-earth metals; similar behavior; in f-block.
  
• Actinoids– All are radioactive; also in f-block.
  

17. What are noble metals?

These are unreactive metals such as Gold, Silver and Platinum. They resist corrosion and are used in jewelry and electronics.

18. What states of matter are elements in at room temperature?

• Solids– Most elements
  
• Liquids– Mercury, Bromine, (sometimes Gallium)
  
• Gases– Hydrogen, Oxygen, Nitrogen and noble gases
  

19. What physical properties help identify elements?

• Melting/boiling points
  
• Electrical conductivity
  
• Thermal conductivity
  
• Light spectrum (emission/absorption) – unique for each element
  

20. What is a radioactive element?

A radioactive element has an unstable nucleus that breaks down over time, releasing energy and particles.

21. What are common types of radioactive decay?

• Alpha decay– Loses 2 protons, 2 neutrons
  
• Beta decay– A neutron becomes a proton
  
• Electron capture– A proton becomes a neutron
  

22. What are transuranium elements?

Elements with atomic numbers greater than 92. They are not naturally abundant and are synthetically produced in labs. All are radioactive.

23. What is a half-life?

Half-life is the time required for half of a radioactive isotope to decay. It helps measure the age of materials and the stability of isotopes.

Glossary of Terms

Term	Definition
Atom	The basic unit of a chemical element, made of protons, neutrons and electrons.
Atomic Number	The number of protons in the nucleus of an atom; it determines the element.
Atomic Mass	The average mass of atoms of an element, usually including all isotopes.
Period	A horizontal row in the periodic table where atomic number increases from left to right.
Group	A vertical column in the periodic table; elements here have similar properties and valence electrons.
Valence Electrons	Electrons in the outermost shell of an atom; they influence bonding and reactivity.
Ion	An atom or group of atoms that has gained or lost electrons, becoming charged.
Cation	A positively charged ion formed by losing electrons.
Anion	A negatively charged ion formed by gaining electrons.
Electron Shell	A level or orbit where electrons are located around the nucleus of an atom.
Ionic Bond	A chemical bond formed when electrons are transferred between a metal and a nonmetal.
Covalent Bond	A bond formed when two nonmetals share electrons.
Reactivity	The ability of an element to undergo chemical reactions with other substances.
Transition Metals	Elements in the center of the periodic table known for forming colored compounds and variable ions.
Lanthanoids	Rare-earth elements with similar chemical properties, found in the f-block.
Actinoids	Radioactive elements in the f-block; some are naturally occurring, others synthetic.
Noble Gases	Inert gases in Group 18 with full outer electron shells.
Halogens	Highly reactive nonmetals in Group 17 that need one electron to complete their outer shell.
Periodicity	The repeating pattern of chemical and physical properties in the periodic table.
Spectral Line	A specific wavelength of light emitted or absorbed by an element; used to identify elements.
Radioactive Decay	The process by which unstable atomic nuclei lose energy by emitting particles.
Half-Life	The time it takes for half of a radioactive substance to decay.
Transuranium Elements	Elements with atomic numbers higher than 92; all are man-made and radioactive.
Electron Configuration	The arrangement of electrons in an atom’s electron shells.
Metalloids	Elements with properties between metals and nonmetals.
Ionization Energy	The energy needed to remove an electron from an atom.
Electronegativity	A measure of how strongly an atom attracts electrons in a bond.