mse 101 - lecture 2 - atomic structure

8/11/2019 MSE 101 - Lecture 2 - Atomic Structure

http://slidepdf.com/reader/full/mse-101-lecture-2-atomic-structure 1/18

Atomic structure

Three elementary particles for a free atom:

1. Protons

– particles with a positive charge (1.60 x 10-19

C) – 1.673 x 10-24 g

2. Electrons

– particles with a negative charge (-1.60 x 10-19 C)

– 9.11 x 10-28 g

3. Neutrons

– electrically neutral particles

– 1.675 x 10-24 g



Definitions

• Atomic Number, Z

– the number of protons in the nucleus.

*For a neutral atom, Z = # of protons = # ofelectrons.

• Atomic Mass, A – sum of the mass of the proton and neutron.

*Isotopes – atoms of the same element but

having different atomic masses due tovariation in number of neutrons



Definitions

• Atomic Weight

– weighted average of the atomic masses of an

atom’s naturally occurring isotope amu per atom (molecule)

*atomic mass unit, amu or dalton – 1/12 of the

atomic mass of the most common isotope of carbon grams per mole

*mole – the quantity of a substance

corresponding to 6.023 x 1023

(Avogadro’s number)atoms/molecules/ions

1 amu/atom = 1 g/mol



Atomic models

1. Bohr Atomic Model electrons are assumed to revolve around the

atomic nucleus in discrete orbitals

the position of any electron is more or less welldefined in terms of its orbitals

the energies of electrons are quantized

electrons may jump to a higher or lower energylevel

describes the electrons in terms of position(electron orbitals) and energy (quantized energylevels)

weakness: unable to explain several phenomenainvolving electrons



The Bohr Atom Electron states for the Bohr and

wave-mechanical model



Atomic models

2. Wave-mechanical Model electrons have wave and particle characteristics

the position of an electron is described by a

probability the consequence is that electrons are no longer

moving in fixed orbits but the motion is described

by a wave function/equation



Comparison of the (a) Bohrand (b) wave-mechanical

atom models in terms of

electron distribution.



Quantum numbers

characterizes the electrons in an atom three of the quantum numbers characterizes the

size, shape, and spatial orientation of an electronsprobability density

Principal Quantum Number, n

mainly responsible for determining the total energyof the electron

n = 1,2,3,4,… ( the lower the value of n, the morestable is the state)

sometimes denoted by K, L, M, N, O

the only quantum number associated with the Bohrmodel

related to the distance of an electron from the

nucleus (size of orbit/shell)



Quantum numbers

Orbital Quantum Number, l signifies the subshell

related to the the shape of the orbit

associated with the angular momentum of therevolving electron

restricted by the principal quantum number, n : l = 0

to (n-1) the letters s,p,d,f,g and h have also been used to

signify l = 0,1,2,3,4,5. Thus, the energy level

corresponding to n = 1 and l = 0 is called the 1slevel, that for n = 2 and l =1 is called the 2p leveletc.



Quantum numbers

Magnetic Quantum Number, ml

determines the number of energy states for eachsubshell

related to the component of the angular momentumin a specified direction

can have values from +l to –l including zero

Spin Quantum Number, m l

related to the spin of the electron about its own axis

can only have a value of ½



Schematic representation of the relative energies of the

electrons for the various shells and subshells.

The smaller the n, the

lower the energy level.

Within each shell,subshell energy

increases with l.

Overlap of energy

levels within one shell ispossible with states in an

adjacent shell.



Pauli Exclusion Principle

determines the manner in which the statesare filled

“Each electron state can hold no more than 2

electrons, which must have opposite spins.”*Ground State – when electrons occupy the

lowest possible energies

*Electron/ic Configuration – represents themanner in which the states are occupied

*Valence Electrons – electrons that occupy the

outermost shell- participate in the bonding between atoms

- determine many of the physical and

chemical properties of materials



Schematic representation

of the filled and lowest

unfilled energy states fora sodium atom.



The Periodic Table

was introduced by D. Mendeleev, a

Russian scientist, in 1869

elements are situated, with increasingatomic number, in seven horizontal rows

called periods

elements of any given family which show a

similarity in chemical properties are

arranged in the same column or group



electropositive elements: readily give up their valence electrons

electronegative elements: readily accept electrons

mse 101 - lecture 2 - atomic structure

Documents