Atomic Structure (Tutoring)

e^- mass

9.109 × 10^{-31 kg}

e^- charge

- 1.602 × 10^-19 coulombs

p⁺ mass

1.673 × 10^-27 kg

p⁺ charge

+ 1.602 × 10^-19 coulombs

Thomson Model of Atom

• Plum Pudding Model

• Electrons randomly distributed throughout atom

• mass/ pos. charge uniformly distributed throughout atom

α (alpha particle)

He²⁺

Rutherford’s Model of Atom

• α- particle scattering experiment

Rutherford Observations

• Most α- particles passed undeflected

• Small fraction of α- particles deflected by some angles

• Very few particles (1/20000) bounced back at 180°

Rutherford Conclusion

• Most space in atom is empty

• pos. charge in atom is present in small volume

• pos. charge concentrated at centre of nucleus which has very small volume

Rutherford Results

• Nucleus surrounded by e- revolving around it in orbits (solar system resemblance)

• e- and nucleus held by electrostatic force of attraction (EFA)

• pos. charge/most mass of atom concentrated in nucleus at centre

• Nucleons: p+ and n⁰ in nucleus collectively

• size of nucleus very small than size of atom

  • r nucleus = 10^-15 m

  • r atom = 10^-10 m

r atom

10⁵ x r nucleus

Vol. atom

10¹⁵ x vol. nucleus

Radius of Nucleus

R = R_o(A)^1/3

R_o = 1.2 × 10^-15 m

A = mass number

Penetration Power

α < ß < γ

ß = 100x

γ = 1000x

Atomic Nuber (Z)

no. of p+

no of e-

Mass Number (A)

n + p

neutrons formula

A-Z (mass - atomic nos)

Charged atom

Z = no of e + charge on atom

Isotopes

• Same Z, diff. A

• change in neutrons

• has diff. physical properties

• same chemical properties (e- are same)

Isobars

• same A, diff Z

• diff. elements

• diff. chemical and physical properties

Average Atomic Mass (AAM)

Case 1 - Isotope %:

mass = m

% = x

AAM = (m₁x₁)+(m₂x₂)+(m₃x₃)/100

Case 2 - Isotope fraction:

mass = m

fraction = f

AAM = m₁f₁ + m₂f₂ + m₃f₃

Mass Spectrometry

• measures mass of isotopes

• see picture for sample pattern graph

Isoelectronic

Species w same no of e-

Isotones

Species w same no of n⁰

Isodiaphers

Species w same diff. of n⁰ and p+ (n-p)

• n-p ≠ 0

Isosters

Species w same no of atoms + same no of e-