Question

define atomic number,mass number,valency and molecular mass

Answer 1

atomic no. is. no. of proton. it is denoted by Z
mass no. (A) is sum of no. of proton and no. of neutrons.
valency is the combining capacity of element.it is the no. of electron in an atom that actually take part in bond formation.
molecular mass is is the mass of one molecule of substance.its unit is u

Answer 2

ATOMIC NO. :
The atomic no. uniquely identifies a chemical element. In an uncharged atom, the atomic no. is also equal to the no. of electrons.

MASS NO. :
The sum of atomic no. Z, and the no. of neutrons N, gives the mass no. A of an atom.

VALENCY :
The combining power of an element especially as measured by the no. of hydrogen atoms it can displace or combine with.

MOLECULAR MASS :
Molecular mass is a no. equal to sum of atomic masses of the atoms in a molecule.