Atomic number Z = 9
No. of neutrons n = l0
Mass number A = ?
No. of neutrons n = A - Z 
Mass number A = n + Z
= 10 + 9 = 19
The symbol of the element is ₉Fe¹⁹ Fluorine

(i) Atoms of the different elements can have the same number of neutrons are called "Isotones".
(ii) Ex : ₂₉Ca⁴⁰ and ₁₉K³⁹
(iii) Numbers of neutrons = A - Z = 20

(i) Proton (p): The proton is the positively charged particle and is located in the nucleus. Its positive charge is of the same magnitude as that of the electron's negative charge.
(ii) Neutron (n): Neutron is inside the nucleus. The neutron does not have any charge. Except hydrogen (protium), the nuclei of all atoms contain neutrons
(iii) Electron (e): This is a negatively charged particle. Electrons revolve around the nucleus of the atom in specific orbits. The mass of an electron is negligible as compared to that of a proton or neutron.

(i) Valency is defined as the combining capacity of an element.
(ii) The valency of the element with atomic number 8, Oxygen (2,6) is = 2.
(iii) The compound formed by oxygen with hydrogen is water. \(\mathrm{O}_{2}+2 \mathrm{H}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}\)

i) Protons and neutrons in the nucleus of an atom are called as nucleons.
ii) They are located in the nucleus of an atom and are called so.
Properties:
i) Atomic mass is only depends on the mass of nucleous.
ii) They make the atom electrically neutral. (Protons - Positive, Neutrons - Neutral)
iii) Nucleons are similar in properties like mass except charge.

Atomic number (Z) = 26, Mass number (A) = 56
No. of protons P = 26
No. of electrons e = 26
No. of neutrons n = A - Z
= 56 - 26 = 30
Atomic structure of ₂₆Fe⁵⁶ is

Sub-atomic particles:
(i) Proton (p): This is the positively charged particle and it is located at the nucleus.
(ii) Neutron (n): Neutron is inside the nucleus. The neutron does not have any charge. Except hydrogen (protium), the nucleus of all atoms contain neutrons.
(iii) Electron(e): This is a negatively charged particle. Electrons revolve around the nucleus of the atom in specific orbits.

(i) Common salt is sodium chloride.
(ii) Sodium and chlorine elements are present in it.
(iii) The molecular formula of sodium chloride is NaCl.

(i) Yes, an atom of an element has no electron will have the mass.
(ii) (Atomic mass, only depends on the mrmber of protons and the number of neutrons inside the nucleus. It is independent of number of electrons.)
(iii) Atom can exist with out electron and becomes a cation.
Ex: 
1) Hydrogen atoms (Z = 1) has only one electrons.
2) It loses one electron and becomes H⁺ ion
3) H → H⁺ + 1e^-
  (atom)    (Ion)