Samacheer Class 9 Science - Atomic Structure

• (a) ion
• (b) isotope
• (c) isobar
• (d) another element

Answer:
(b) isotope

• (a) protons and electrons
• (b) only neutrons
• (c) electrons and neutrons
• (d) protons and neutrons

Answer:
(d) protons and neutrons

• (a) 80, 80, 35
• (b) 35, 55, 80
• (c) 35, 35, 80
• (d) 35, 45, 35

Answer:
(d) 35, 45, 35

• (a) 2, 8, 9
• (b) 2, 8, 1
• (c) 2, 8, 8, 1
• (d) 2, 8, 8, 3

Answer:
(c) 2, 8, 8, 1

This statement is true according to Bohr's model of the atom. Electrons revolve around the nucleus in fixed, discrete orbits or energy levels. Each orbit corresponds to a specific energy level, and electrons can only occupy these fixed orbits and not exist between them. The electrons in these orbits experience the attractive force of the positively charged nucleus, which provides the centripetal force needed to keep them in circular motion. The innermost orbit, closest to the nucleus, has the lowest energy, while orbits farther from the nucleus have progressively higher energy levels.

This statement is false. Isotopes are atoms of the same element that have the same atomic number but different mass numbers. Since atomic number is determined by the number of protons in the nucleus, isotopes must have the same number of protons and therefore the same atomic number. The difference between isotopes lies in the number of neutrons they possess, which causes them to have different mass numbers. For example, carbon-12 and carbon-14 are isotopes of carbon; both have atomic number 6, but they have different numbers of neutrons and thus different mass numbers.

This statement is false. While electrons do have negligible mass compared to protons and neutrons, they do possess a definite negative electric charge. The mass of an electron is approximately 1/1836 times the mass of a proton, which is why it is considered negligible in most calculations of atomic mass. However, the negative charge of the electron is a fundamental property and is not negligible; it is equal in magnitude but opposite in sign to the charge of a proton. This charge is essential for the chemical behavior of atoms and the formation of chemical bonds.

This statement is true. According to Bohr's model of the atom, electrons in smaller orbits closer to the nucleus have lower energy levels compared to electrons in larger orbits farther from the nucleus. The energy of an electron is inversely related to the size of its orbit. Electrons in the first orbit (n=1) have the lowest energy, those in the second orbit (n=2) have higher energy, and so on. This is why electrons naturally occupy the lowest available energy levels, filling inner orbits before moving to outer orbits.

Answer:

Beryllium

Atomic number = 4

Electronic configuration:

Potassium (K)

Atomic number = 19

Chlorine (Cl)

Atomic number = 17

(,^{1}_{1}H)

Hydrogen atom

• Atomic number = 1
• Mass number = 1

(,^{1}_{0}n)

Neutron

• Charge = 0
• Mass = 1 amu

(,^{0}_{-1}e)

Electron

• Charge = (-1)
• Negligible mass

Using:

• K = 2
• L = 8
• M = 18

Total:

Answer:

28 electrons

(a) Lithium, Sodium, Potassium

All have:

(b) Beryllium, Magnesium, Calcium

All have:

Answer:

Rutherford bombarded thin gold foil with alpha particles.

Observations:

• Most particles passed through foil.
• Few deflected.
• Very few bounced back.

Conclusion:

Most of atom is empty space.

Answer:

Both isotopes have:

• same atomic number
• same electronic configuration

Hence chemical properties are same.

They differ in number of neutrons.

Oxygen

Atomic number = 8

Electronic configuration:

Sulphur

Atomic number = 16

Electronic configuration:

(i) Atomic Number = 3, Mass Number = 7

Protons:

Electrons:

Neutrons:

(ii) Atomic Number = 92, Mass Number = 238

Protons:

Electrons:

Neutrons:

Answer:

Particles present inside nucleus are called nucleons.

They include:

• protons
• neutrons

Nucleons in Phosphorus

Mass number of phosphorus:

Hence phosphorus has:

Conclusions

• Atom contains very small nucleus.
• Most of atom is empty space.
• Entire positive charge concentrated in nucleus.
• Most mass concentrated in nucleus.
• Electrons revolve around nucleus.