Class 9 Science Exploration Chapter 8 MCQ – Journey Inside the Atom

Dalton proposed that matter consists of indivisible atoms.

Later discoveries of electrons, protons and neutrons showed that atoms have smaller components and led to revised atomic models.

Cathode rays were found to be streams of negatively charged particles much lighter than atoms.

These particles were identified as electrons, showing that atoms are divisible.

Cathode rays had the same nature with different cathode materials and gases.

This indicated that their particles, electrons, are common constituents of atoms.

Thomson pictured positive charge spread throughout a sphere, with negatively charged electrons embedded in it.

The positive and negative charges balanced to make the atom neutral.

Most alpha particles crossed the foil without encountering a region that strongly deflected them.

This showed that most of the atom’s volume is empty space.

Strong deflection of a few positive alpha particles required concentrated positive charge.

Rutherford concluded that positive charge and most atomic mass occupy a very small nucleus.

An alpha particle contains two protons and two neutrons.

It is positively charged because it has protons but no electrons to balance their charge.

An orbiting electron is accelerating and, under the classical explanation, should lose energy and spiral into the nucleus.

Rutherford’s model therefore could not explain why atoms remain stable.

Bohr proposed allowed shells with definite energies.

An electron in such a stationary state does not continuously lose energy, preventing the collapse predicted by the earlier model.

The K-shell is the first energy level and lies closest to the nucleus.

In the shell model, levels farther from the nucleus have higher energies.

Each allowed level has a definite energy.

To move to a higher level, the electron must absorb energy equal to the difference between the two levels.

A neutron has no electric charge and nearly the same mass as a proton.

Neutrons contribute to nuclear mass without increasing the nucleus’s positive charge.

James Chadwick discovered the neutron in 1932.

The discovery helped explain why nuclear mass is greater than the mass contributed by protons alone.

A proton’s positive charge equals an electron’s negative charge in magnitude.

Equal numbers of protons and electrons therefore produce zero total charge.

A chemical symbol begins with an uppercase letter.

If it contains a second letter, that letter is lowercase, so aluminium is written Al.

Atomic number Z equals the number of protons in the nucleus.

It identifies the element; changing the proton number changes the element’s identity.

Mass number A counts the nucleons in the nucleus.

Nucleons are protons and neutrons, so A = number of protons + number of neutrons.

Number of neutrons = mass number − atomic number.

Therefore, the atom contains 56 − 26 = 30 neutrons.

Atomic number gives 12 protons, and neutrality requires 12 electrons.

Neutrons = 24 − 12 = 12.

The maximum shell capacity is calculated using 2n².

For the L-shell, n = 2, so 2 × 2² = 8 electrons.

A neutral sodium atom contains 11 electrons.

The first shell holds 2, the second holds 8, and the remaining electron occupies the third shell.

Valence electrons occupy the atom’s outermost occupied shell, called the valence shell.

They play a major role in determining how atoms combine.

Oxygen has six valence electrons and needs two more to complete an octet.

Its combining capacity, or usual valency, is therefore 2.

The K-shell can hold a maximum of two electrons, and helium already has both.

This complete outer shell gives helium a stable electronic configuration.

Isotopes have the same number of protons and therefore the same atomic number.

Different neutron numbers give them different mass numbers.

All hydrogen isotopes have one proton.

Tritium has mass number 3, so its neutron number is 3 − 1 = 2.

Isotopes have the same proton number and the same electron arrangement in their neutral atoms.

Chemical behaviour depends mainly on valence electrons, so their chemical properties are similar.

The isotope abundances must be included in the average.

Average mass = (35 × ¾) + (37 × ¼) = 35.5 u.

Isobars are atoms of different elements with the same mass number.

Argon-40 and calcium-40 both have mass number 40 but different atomic numbers.

Uranium-235 is an isotope used as nuclear fuel.

Energy released through nuclear processes is used in nuclear power plants to generate electricity.