Concept: Composition of an atom: • An atom consists of protons (positively charged particles) and neutrons in its nucleus. • The total number of protons in a nucleus is equal to the number of electrons (negatively charged particles) orbiting the nucleus. Therefore, an atom is electrically neutral. • A chemical element is always represented using its symbol as follows: Z X A • Where X is the symbol for the element, A is its mass number and Z is the atomic number. The mass number A is the sum of the number of neutrons and protons present in the nucleus of the atom. The atomic number Z is the number of protons present in the nucleus of the atom. Explanation: • As asked 11Na23 the atomic number is 11 and due to the atom being electrically balanced hence the number of electrons in the atoms are also the same as the number of protons so the number of electrons is also 11. Again, Since the electrons are present in the atom but not in the nucleus hence the number of electrons present in the nucleus is equal to zero. • The mass number is 23 hence by removing the mass of the proton from 23 we obtain the number of the neutron. • Hence the neutrons are 23 - 11 = 12. So the Number of protons, neutrons, and electrons in the nucleus of 11Na23 are 11,12, 0 respectively. Common Pitfalls: • The number of protons, neutrons, and electrons is asked in the context of nucleus of the atom. Since electrons are not present in the nucleus therefore the correct answer is 11, 12, 0.