How would you classify the following elements as main group elements, transition metals or inner transition metals: 1. Ca 2. Fe 3. Ag 4. U 5. O 6. Pb?

Hint: By simply looking towards the modern periodic table, we can answer the given question. The sections in the table can help us determine the elements falling into the main group elements or transition metals or inner transition metals.

Complete answer:
Let us study the modern periodic table;
Main group elements-
These are the lightest elements of the periodic table from the groups 1 and 2; also, from groups 13 to 18.
Transition metals-
There are a total 38 transition metals in the periodic table. The elements from group 3 to 12 coming under the period 4 to 7 are transition metals. They are d-block elements.
Inner transition metals-
These are the f-block elements having valence electrons in f-orbitals. The lanthanides and actinides are the inner transition elements.
Now, looking towards the given illustration;
Ca is a main group element of period 4 and group 2 i.e. in the s-block.
Fe is the transition metal of period 4 and group 8 i.e. in the d-block.
Ag is the transition metal of period 5 and group 11 i.e. in the d-block.
U is the inner transition metal of period 2 and group 16 i.e. in the f-block.
O is the main group element of period 2 and group 16 i.e. p-block.
Pb is the main group element of period 6 and group 14 i.e. p-block.

Note:
Do note that the elements from the s-block and p-block will be considered as main group elements or metals.
Pb is the post transition metal.