Development of the periodic table

How did Mendeleev arrange elements in his periodic table?

By increasing atomic mass and grouped by chemical properties.

Why did Mendeleev leave gaps in his periodic table?

To predict the properties of undiscovered elements.

How is the modern periodic table arranged?

By increasing atomic number.

What problem did early periodic tables face?

Elements with similar properties were not grouped together due to incorrect atomic masses.

What is periodicity in the periodic table?

The repeating pattern of chemical properties across periods.

Why was the discovery of isotopes important for the periodic table?

It showed elements could have different atomic masses but the same chemical properties.

How are elements grouped in the modern periodic table?

By the number of electrons in their outer shell.

Why are noble gases unreactive?

They have a full outer shell of electrons.

What is the significance of Group 1 in the periodic table?

It contains alkali metals, which are highly reactive.

What is the periodic law?

Properties of elements recur periodically when arranged by increasing atomic number.

How do the properties of elements change across a period?

They transition from metallic to non-metallic.

Why are transition metals placed in the center of the table?

They have variable oxidation states and form colored compounds.

What are halogens, and why are they in Group 7?

Halogens are reactive non-metals with 7 electrons in their outer shell.

What trend in reactivity is seen in Group 1 metals?

Reactivity increases down the group.

How does atomic radius change across a period?

It decreases due to increasing nuclear charge.