Periodic Table of Elements

Q: What do elements in the same group have in common?

Elements in the same group (vertical column) have the same number of valence electrons, which are the electrons in the outermost energy shell. This gives them very similar chemical properties and reactivity patterns.

Q: How many elements are on the periodic table?

As of now, the periodic table has 118 confirmed elements, with the last one being Oganesson (Og) with atomic number 118. Scientists are continuously working on synthesizing new, heavier elements.

Understanding the Periodic Table

What Is the Periodic Table?

The periodic table of elements is a systematic arrangement of all known chemical elements based on their atomic number, electron configuration, and recurring chemical properties. It’s essentially a “map” of all elements that allows chemists to:

Predict properties of elements.
Understand trends (like reactivity, electronegativity, etc.).
See relationships between elements.

History of the Table

Dmitri Mendeleev (1869) created the first widely recognized table by arranging elements by increasing atomic mass and leaving blank spaces for undiscovered elements.

Later, Henry Moseley (1913) discovered that atomic number (the number of protons) was the key ordering factor, which led to the modern periodic table we use today.

Structure of the Periodic Table

Periods (Horizontal Rows)

There are 7 periods. As you go from left to right across a period, the atomic number increases, and elements become less metallic.

Groups (Vertical Columns)

There are 18 groups. Elements in the same group have similar chemical properties because they have the same number of valence electrons.

Group 1: Alkali Metals
Group 2: Alkaline Earth Metals
Group 17: Halogens
Group 18: Noble Gases

Blocks

Blocks are based on the subshell of the last electron:

s-block: Groups 1–2 (+ He)
p-block: Groups 13–18
d-block: Transition metals (Groups 3–12)
f-block: Inner transition metals (lanthanides & actinides)

Key Periodic Trends

Property	Across a Period (→)	Down a Group (↓)
Atomic radius	Decreases (nuclear charge ↑)	Increases (new shells added)
Ionization energy	Increases	Decreases
Electronegativity	Increases	Decreases
Metallic character	Decreases	Increases

Quick Facts

Smallest atomic number: Hydrogen (1)
Largest known atomic number: Oganesson (118)
Only liquid elements at room temp: Mercury (Hg) & Bromine (Br)
Most abundant element in Earth’s crust: Oxygen
As of now, the table has 118 confirmed elements.

Frequently Asked Questions

The first widely recognized periodic table was created by the Russian chemist Dmitri Mendeleev in 1869. He arranged the elements by increasing atomic mass and famously left gaps for elements that had not yet been discovered, correctly predicting their properties.

Elements in the modern periodic table are arranged in order of increasing atomic number (the number of protons in an atom's nucleus). This arrangement places elements with similar chemical properties into the same vertical columns, or groups.

Elements in the same group (vertical column) have the same number of valence electrons, which are the electrons in the outermost energy shell. This gives them very similar chemical properties and reactivity patterns.

As of now, the periodic table has 118 confirmed elements, with the last one being Oganesson (Og) with atomic number 118. Scientists are continuously working on synthesizing new, heavier elements.