Structure of atoms

Who proposed the first atomic theory?

John Dalton in 1808.

What are the main postulates of Dalton's Atomic Theory?

Matter is made of indivisible atoms, all atoms of an element are identical, atoms combine in whole-number ratios, and chemical reactions involve rearrangement of atoms.

What experiment led to the discovery of the electron?

Cathode ray tube experiment by J.J. Thomson.

What are cathode rays?

Streams of negatively charged particles (electrons) emitted from the cathode in a discharge tube.

How did J.J. Thomson discover electrons?

By observing deflection of cathode rays in electric and magnetic fields.

What charge-to-mass ratio did J.J. Thomson determine for electrons?

e/m = 1.758 × 10¹¹ C/kg

Who measured the charge on the electron?

Robert Millikan using the Oil Drop Experiment.

What was the result of Millikan's oil drop experiment?

Electron charge = −1.602 × 10⁻¹⁹ C

Who discovered protons and how?

Goldstein, using perforated cathode in a discharge tube, observing canal rays.

What are canal rays?

Streams of positively charged ions (protons) observed in gas discharge tubes.

What experiment did Rutherford conduct?

Alpha particle scattering experiment.

What was the setup of Rutherford's experiment?

Alpha particles were directed at a thin gold foil, and their scattering pattern was observed.

What were the observations from Rutherford's experiment?

Most particles passed through, some deflected, a few bounced back.

What conclusion did Rutherford draw?

Atom has a small, dense, positively charged nucleus; most of the atom is empty space.

What are the flaws in Rutherford's model?

It couldn't explain atomic stability or discrete spectral lines.

What holds electrons in orbit in Rutherford's model?

Electrostatic attraction between negatively charged electrons and positively charged nucleus.

What is the size of the nucleus compared to the atom?

Nucleus is 1/10,000 the size of the atom.

Why was Rutherford's model unstable?

According to classical physics, revolving electrons should lose energy and spiral into the nucleus.

What subatomic particle did Chadwick discover?

Neutron in 1932.

What is the charge and mass of a neutron?

Charge = 0; Mass = 1.675 × 10⁻²⁷ kg

Who proposed the quantum model of the atom?

Niels Bohr in 1913.

State Bohr's first postulate.

Electrons revolve in fixed energy levels or orbits without radiating energy.

Bohr's second postulate?

Angular momentum of electrons is quantized: mvr = nh/2π

What is 'n' in Bohr's equation?

Principal quantum number (1, 2, 3…)

Formula for radius of nth orbit in hydrogen?

rₙ = 0.529 × n² Å

Formula for energy of nth orbit in hydrogen?

Eₙ = −13.6/n² eV

Formula for velocity of electron in nth orbit?

vₙ = 2.18 × 10⁶ × (1/n) m/s

Energy levels are…?

Quantized and negative (E < 0)

Transition from higher to lower orbit releases energy in the form of?

Photon (light)

Formula for energy of photon emitted?

ΔE = E₂ − E₁ = hν = hc/λ

What value of Rydberg constant (R_H) is used?

R_H = 1.097 × 10⁷ m⁻¹

What does Bohr's model explain well?

Hydrogen atom spectrum.

Limitations of Bohr's model?

Can't explain multi-electron atoms, Zeeman effect, or fine structure.

What is the Lyman series?

Transitions to n=1; lies in UV region.

What is the Balmer series?

Transitions to n=2; lies in visible region.

What is an atomic spectrum?

Series of lines formed by transitions between energy levels.

What does line spectrum indicate?

Energy levels in atom are quantized.

What is wave-particle duality?

Electrons exhibit both particle and wave nature (de Broglie).

What is de Broglie's equation?

λ = h/mv

Who developed quantum mechanical model?

Erwin Schrödinger in 1926.

Schrödinger's equation gives what?

Probability distribution of electrons (orbitals).

What are orbitals?

Regions of space with high probability of finding an electron.

What is Heisenberg's uncertainty principle?

It's impossible to simultaneously know exact position and momentum of a particle.

Formula for uncertainty principle?

Δx·Δp ≥ h/4π

What is quantum number 'n'?

Principal quantum number - indicates size and energy level.

Azimuthal quantum number (l)?

Shape of orbital (0 to n−1); s, p, d, f

Magnetic quantum number (m)?

Orientation of orbital (−l to +l)

Spin quantum number (s)?

Spin direction (+½ or −½)

What is Pauli's Exclusion Principle?

No two electrons in an atom can have the same set of 4 quantum numbers.

What is Hund's Rule?

Every orbital in a subshell is singly occupied before pairing.

What is Aufbau principle?

Electrons fill orbitals in order of increasing energy.

Order of energy for orbitals (up to 4p)?

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p

What is 1s² 2s² 2p⁶ 3s²?

Electron configuration of magnesium (Z = 12)

Maximum electrons in s orbital?

2

In p orbital?

6

In d orbital?

10

In f orbital?

14

Shape of s orbital?

Spherical

Shape of p orbital?

Dumbbell

What does a node represent?

A point where the probability of finding an electron is zero.

How many nodes in 3s orbital?

2 nodes (n − 1)

What is shielding effect?

Inner electrons shield outer electrons from full nuclear charge.

What is effective nuclear charge (Z_eff)?

Actual nuclear charge felt by valence electron.

Formula for maximum electrons in shell?

2n²

Total number of orbitals in n = 3 shell?

9 orbitals (1s, 3p, 5d)

What is an isoelectronic species?

Different elements with same number of electrons.

Example of isoelectronic species?

Na⁺, Mg²⁺, Ne - all have 10 electrons.

What is ground state?

Lowest energy state of an atom.

What is excited state?

Any state with higher energy than ground state.

What is ionization energy?

Energy required to remove an electron from an atom in gaseous state.

What is Planck's constant?

h = 6.626 × 10⁻³⁴ J·s

What is the speed of light?

c = 3.00 × 10⁸ m/s

Formula relating energy and frequency?

E = hν

Formula for frequency and wavelength?

ν = c/λ

What is the energy of a photon with λ = 500 nm?

Use E = hc/λ

What does negative energy in atom mean?

Bound system; energy must be supplied to remove electron.

Value of Rydberg constant?

R_H = 1.097 × 10⁷ m⁻¹

Energy of nth orbit of H atom?

Eₙ = −13.6/n² eV

Wavelength of Balmer line from n=3 to n=2?

Use Rydberg formula.

What is photoelectric effect?

Emission of electrons when light shines on a metal surface.

What proves particle nature of light?

Photoelectric effect

What proves wave nature of electrons?

Electron diffraction

What is quantum?

Discrete packet of energy

What is a photon?

A quantum of light

What is dual nature of radiation?

Exhibits both wave and particle properties

What is an emission spectrum?

Bright lines on a dark background

What is an absorption spectrum?

Dark lines on a bright background

What is shell?

Orbit followed by electrons

What is subshell?

Division of shell into s, p, d, f

Why do elements emit light?

Electrons jump to lower levels and release energy as light.

What is meant by quantization of angular momentum?

Only certain discrete values of angular momentum are allowed.

Which principle violates when electrons are paired before half-filling?

Hund's Rule

Which quantum number determines energy level?

Principal quantum number (n)

What determines shape of orbital?

Azimuthal quantum number (l)

Who proposed that electrons have wave nature?

Louis de Broglie