Correct option: (B) Parmanu.

Kanada proposed that repeated division of matter would eventually lead to particles that could not be divided further. He called them parmanus.

Correct option: (C).

Correct option: (B) John Dalton.

Correct option: (C).

These negatively charged particles were later called electrons.

Correct option: (B).

The same type of negatively charged particle was obtained regardless of the material of the cathode or the gas in the tube.

Correct option: (B).

This is commonly called the plum-pudding model.

Correct option: (B).

This led Rutherford to conclude that most of the volume of an atom is empty space.

Correct option: (C).

The tiny dense central region was called the nucleus.

Correct option: (C).

Correct option: (B).

A revolving charged electron was expected to lose energy, spiral inward and eventually fall into the nucleus, which contradicts the observed stability of atoms.

Correct option: (C).

In a neutral atom:

Correct option: (C).

Correct option: (B).

K is closest to the nucleus and has the lowest energy; energy increases outward.

Correct option: (C) Neutron.

Correct option: (B).

Correct option: (C) Cl.

The first letter is uppercase and the second letter, when present, is lowercase.

Correct option: (A).

Correct option: (C).

Correct option: (B).

For L-shell, n = 2.

Correct option: (B).

Correct option: (A) 2, 8, 2.

Correct option: (C).

It needs two more electrons to complete the octet.

Correct option: (B).

Correct option: (B) Isotopes.

Carbon has Z = 6.

Correct option: (C).

Correct option: (C).

Isobars have the same mass number but different atomic numbers.

Correct option: (B).

Correct option: (A).

Correct option: (A).

Correct option: (B).

A thicker foil contains more atomic layers, increasing the opportunities for scattering interactions.

Kanada’s concept of parmanu was a philosophical explanation of matter rather than one derived from controlled experiments.

Dalton’s theory, proposed much later, was based on scientific evidence and became the first experimental scientific description of atoms.

• Cathode rays consist of negatively charged particles called electrons.
• Their nature was independent of the cathode material and gas, suggesting that electrons are fundamental components of all atoms.

• Most alpha particles passed straight through the foil without deflection.
• A small fraction were deflected through large angles and a very few bounced backward.

• Most of an atom is empty space.
• Almost all its positive charge and most of its mass are concentrated in a tiny dense nucleus.

A revolving electron is continuously changing direction and is therefore accelerating. Under the classical picture used in discussing Rutherford’s model, such a charged particle should lose energy, spiral inward and fall into the nucleus.

Real atoms are stable, so the model was incomplete.

Bohr proposed that electrons can occupy only certain allowed stationary energy levels or shells.

While an electron remains in an allowed stationary state, it does not lose energy. This prevented the predicted continuous spiral into the nucleus.

For a neutral atom:

Since the atom is neutral:

Z = 12: 2, 8, 2

Z = 16: 2, 8, 6

Z = 18: 2, 8, 8

The valence shell is the outermost shell of an atom that contains electrons.

The electrons present in this shell are called valence electrons.

Carbon: 2,4. It has four valence electrons and commonly shares four electrons to complete an octet, giving valency 4.

Oxygen: 2,6. It needs two additional electrons to complete an octet, giving valency 2.

Isotopes have the same atomic number and hence the same number of electrons in neutral atoms.

They therefore have the same electronic configuration and the same number of valence electrons, which largely determine chemical behaviour.

Electronic configuration:

It can lose one valence electron to obtain a stable octet. Therefore:

The element is sodium, Na.

Neutral atom:

Configuration:

It needs two electrons for a complete octet:

The element is sulfur.

Atomic number 17 corresponds to chlorine.

It needs one electron to complete the octet:

For a neutral atom:

Element: phosphorus, P.

It requires three electrons to complete an octet:

A corresponds to magnesium and B to chlorine.

Both have:

Therefore they are atoms of the same element.

Their mass numbers are:

They have the same atomic number but different mass numbers, so they are isotopes.

They have the same mass number but different atomic numbers. Therefore X and Y are isobars.

Therefore the average atomic mass is approximately 80.0 u.

Equal abundance:

90% mass-20 and 10% mass-22:

Greater abundance of the lighter isotope shifts the average toward 20 u.

Mass number counts protons and neutrons and is therefore a whole number for an individual atom.

Natural chlorine contains atoms of different isotopes, mainly mass numbers 35 and 37. The value 35.5 u is a weighted statistical average over a large population of chlorine atoms.

• U-235: fuel in nuclear reactors for generation of electricity.
• Co-60: radioactive isotope used in radiation treatment for cancer.
• C-14: used in archaeology and geology to estimate the age of ancient fossils and artefacts.

Thomson proposed a diffuse distribution of positive charge. Such a diffuse charge could produce only relatively small deflections.

Rutherford’s experiment showed that a few alpha particles experienced very large deflections or even bounced backward.

This required a highly concentrated, massive, positively charged region: the nucleus.

2,8,8 has eight electrons in the outermost shell, giving a complete octet. Such a configuration is largely stable and unreactive.

2,8,7 has an incomplete valence shell and needs one electron to attain an octet. It therefore has a stronger tendency to participate in chemical combination.

Valence electrons = 3.

Since there are fewer than four valence electrons, the simple rule predicts loss of three electrons.

It has five valence electrons.

Convert 0.1 mm:

Approximately one million atoms would span the thickness in this simplified estimate.

1. Dalton: atoms treated as indivisible building blocks of matter.
2. Thomson: introduced internal positive and negative charge through the electron-containing positive sphere.
3. Rutherford: discovered the nuclear structure and large empty space.
4. Bohr: introduced fixed allowed energy levels or stationary shells.

Geiger and Marsden, working under Rutherford, directed a narrow beam of positively charged alpha particles at an extremely thin gold foil.

Observations:

• Most alpha particles passed straight through.
• Some were deflected through small or large angles.
• A very small number bounced back.

Conclusions:

• Most of an atom is empty space.
• Positive charge is not spread uniformly.
• A tiny dense positively charged nucleus is present at the centre.
• The nucleus contains most of the atomic mass.

• Electrons move in fixed allowed paths called shells, orbits or stationary states.
• Each shell has a definite energy and is therefore an energy level.
• Shells are represented as K, L, M, N… or n = 1,2,3,4…
• Electrons cannot occupy arbitrary positions between allowed shells.
• An electron in a stationary state does not lose energy.
• Energy increases as the shells become farther from the nucleus.
• An electron moves between levels only by absorbing or releasing a fixed amount of energy equal to the energy difference.

Atoms such as helium were much heavier than could be explained by their number of protons alone.

James Chadwick discovered the neutron in 1932. It has no electrical charge but a mass nearly equal to that of a proton.

Protons and neutrons together account for almost all atomic mass because electron mass is comparatively negligible.

Thus:

Z = 14 corresponds to silicon, Si.

Valence electrons = 4.

Silicon has four valence electrons; under the chapter’s simple combining-capacity treatment:

Row 1:

Row 2:

Row 3:

For X:

X is chlorine and has 17 protons and, when neutral, 17 electrons.

Adding two neutrons:

Atomic number and electron configuration remain unchanged because the number of protons does not change.

X and Y are therefore isotopes of chlorine: chlorine-35 and chlorine-37.

60% mass-24 and 40% mass-26:

Reversed abundances:

Let fraction of X-35 = x.

Then fraction of X-37 = 1−x.

Therefore:

Element identity is determined by atomic number, which equals the number of protons.

Changing only the neutron count changes the mass number but leaves the atomic number unchanged.

For a neutral atom the electron count and electronic configuration are therefore unchanged.

Since chemical properties depend mainly on valence electrons, isotopes usually have similar chemical properties.

• The theoretical maximum capacity of shell n is 2n².
• Thus K can hold 2, L can hold 8 and M can theoretically hold 18.
• The outermost shell can contain a maximum of eight electrons in the treatment used here, except K which can contain only two.
• Electrons fill shells stepwise from the innermost outward: K, then L, then M…
• For the first 18 elements this gives configurations from H = 1 through Ar = 2,8,8.

This illustrates how extraordinarily small the nucleus is compared with the atom.

Atomic number: unchanged at 12 because it depends on protons.

Mass number: unchanged because mass number counts protons and neutrons only.

Overall charge: remains neutral because the replacement particles have the same negative charge and their number still equals the proton count.

Actual atomic mass: would increase because the negatively charged particles are now far heavier than ordinary electrons.

Dalton’s model treated atoms as indivisible particles. Electron discovery showed internal structure, leading Thomson to a charged-sphere model.

Rutherford’s scattering evidence replaced diffuse positive charge with a tiny nucleus. Rutherford’s stability problem led Bohr to stationary energy levels.

Later evidence showed that electrons do not literally follow fixed classical orbits; modern descriptions use probability-based electron clouds.

The sequence demonstrates that scientific models are revised when new experimental evidence reveals limitations of earlier explanations.

Protons = 13.

The neutral atom initially had 13 electrons.

There are now three more protons than electrons:

Initial electron number = 11.

After loss of one electron:

The proton number has not changed, so there is one extra positive charge:

There are six valence electrons.

Gaining two electrons produces two additional negative charges:

The K-shell is the first shell and can accommodate a maximum of only two electrons.

Helium therefore already has a completely filled outermost shell and does not need to gain, lose or share electrons to obtain stability.

Volume of a sphere is proportional to r³.

The atom’s volume is approximately one quadrillion times the nuclear volume under this simplified spherical comparison.

On this enormous model scale, the nucleus would have a radius of only about 64 m.

If 1% show large deflection, then:

Let fraction of A-63 = x.

Therefore:

P:

Q:

R:

P and Q: same A = 40 but different Z → isobars.

P and R: same Z = 18 but different A → isotopes.

Q and R: both Z and A differ → neither isotopes nor isobars.

Three shells with seven valence electrons among the first 18 elements gives:

Therefore Z = 17 and the element is chlorine.

For neutral atoms, identical electronic configurations mean the same number of electrons, hence the same number of protons and the same atomic number.

Different neutron numbers cause different mass numbers.

Their valence-electron arrangements are identical, so their chemical properties are generally similar.

They are therefore isotopes of the same element.

X:

Y:

Same mass number:

X and Y have the same mass number but different atomic numbers, so they are isobars.

Let fraction of mass-10 isotope = x.

Therefore:

In 5000 atoms:

Atom X

Therefore:

Electronic configuration:

Atomic number 18 is argon.

Atom Y

Y has two more protons:

Since A remains 40:

For a neutral atom:

The detailed shell configuration of atomic number 20 extends beyond the first-18-element table emphasised in this chapter; its identity is calcium.

Calcium commonly has valency 2.

Relationship:

but:

Therefore X and Y are isobars.