It is a mixture
Hydrogen carbonate is a compound, not an element, and it therefore has a formula, not a symbol: H2CO3.
Ag is the chemical symbol for silver, which is an element. It is not a compound.
The subscript 4 should be written to the right of the symbol for fluorine in the compound dinitrogen tetrafluoride, which is N2F4.
Hl is an element. It is the chemical symbol for the element helium on the periodic table.
Solder is a metallic alloy
yes
It is a mixture
The symbol for the compound tetrahydride is TH4.
The covalent compound for the symbol N2O4 is dinitrogen tetroxide.
The symbol for nicotine (a compound) is C10H14N2.
NaOH
Symbol
This is an alloy, not ionic or covalent.
C is the chemical symbol for carbon.
Helium (He) is a chemical element, not a compound; the chemical symbol is He.
The compound element of wood is carbon. Its symbol is C