Early Atomic Theory and Structure

Empedocle (440 BC): all matter consists of 4 “elements” earth, fire, water and air.

Democritus (470-370 BC): matter is composed of indivisible particles (Greek atomos - not cuttable).

Aristotle (384-322 BC): endorsed Empedoclean theory, so that it dominated until 17th century.

John Dalton (1766-1844), 2000 years after Democritus, revived concept of atoms:

1. elements are composed of indivisible particles-atoms.2. atoms of the same element are alike in mass and size.3. atoms of different elements are not alike – they have different masses and sizes.4. compounds are union of two or more atoms of different elements.5. compounds contain simple numerical ratios of atoms.6. more than one compound can be formed of atoms of two elements.

Chapter 5

Dalton’s rules:

While most of the Dalton’s theory still holds, some modifications were necessary: An atom is composed of subatomic particles, it can be decomposed and may have isotopes of different masses.

Law of definite composition – a compound contains two or more elements in a definite proportion by mass.

Law of multiple proportions – atoms of two or more elements may combine in different ratios to produce more than one compound.

Water

Hydrogen peroxide

compound formula percent compositionwater H2O 11.2 % H, 88.8% Ohydrogen peroxide H2O2 5.9% H, 94.1% OCopper (I) chloride CuCl 64.2% Cu, 35.8% ClCopper(II) chloride CuCl2 47.3% Cu, 52.7% ClMethyl alcohol CH4O 37.5% C, 12.6% H, 49.9% OEthyl alcohol C2H6O 52.1%C, 13.1% H, 34.7% O

H2O and H2O2 have the same atoms, but different number.

Dalton rules (cont.)

Matter may have electric charge

1. charge may be of two types, positive and negative2. unlike charges attract, like repel3. can be transferred from one object to the other by contact or induction4. the smaller the distance, the stronger the force between two charges

(F – force, k - constant, q1, q2 - charges, r - distance) F = k q1 x q2

r2x

Michael Faraday (1791-1867) 1. Some compounds dissolved in water conduct electricity, others decompose 2. Particles of some elements are attracted to the positive or negative electrode.

3. Concluded that they are charged and called them ions (Greek-wanderer).

Svante Arhenius (1859-1927) 1. Melted NaCl also conduct electricity. NaCl breaks up to Na+ and Cl- ions.2. Ions move to oppositely charged electrode. Ion that goes towards positively charged cathode is called cation. The other goes to positive anode is called anion.

G.J. Stoney (1826-1911) named the unit of electricity electron.

Joseph Thompson (1856-1940) experimentally showed the existence of electron, which is: negative in charge, deflected by magnetic and electric fields, and capable of moving small paddle wheel.

Electric Charge

Ag+ Cd2+

Fe2+

Fe3+

Cu2+

Cu+ Zn2+

Cr2+

Cr3+

N3- O2- F-

Al3+ P3-

S2- Cl-

Br-

I-

Na+

K+

Ca2+

Cs+

Ba2+

Rb+

Sr2+

Mg2+

Be2+

H+

Li+

IA

IIBIVB ------ VIIIB ------- IB

IIIA IVA VA VIA VIIA

VIIBVIBVBIIIB

VIIIA

IIA

Group IA: always 1+

Group IIA: always 2+

Elements of B groups (transition metals) form ions of different charges

Group VA: often 3-

Group VIA: usually 2-

Group VIIA: always 1-

Invented by Dmitri Mendeleev, the Periodic Table shows recurring trends in properties of elements and their charges.

Charge = group number Charge = 8 - group number

Charge and Periodic Table

Group VIIIA: no charge-noble gases

Subatomic particles

Atom is extremely small, cannot be observed by optical microscope.

Number of other short-lived subatomic particles are detected in recent years.

Three subatomic particles are sufficiently long-lived to be used in experiments:

quarks, neutrinos, antineutrinos, positrons and antiprotons ... Their existence is still debated.

Atomic diameters range from 0.1 to 0.5 nm. Consider this dot on the right.

If its diameter is 1 mm, 10 million H atoms could be arranged in it.

Yet, there are particles smaller than H atom. They are called subatomic particles.

electron, proton and neutron.

Proton and electron have charges opposite in sign, identical in absolute value.

Proton and neutron have almost equal masses.

By 1907 Rutherford found that alpha particles emitted by certain radioactive elements were helium nuclei.

J.J. Thomson discovered electron in 1897. Shortly after, he proposed the first atomic ‘Plum-Pudding’ model where negatively-charged electrons (‘plums’) are embedded into a positively-charged sphere (‘pudding’).

Thomson and Goldstein discovered proton in 1907, and Chadwick discovered neutron in 1932.

The Evolution of Atomic Theory

Rutherford’s experiment

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

showed that the plum- pudding model is incorrect.

+ particles should pass through the sphere because it is effectively neutral.

Becquerel in 1896 discovered radioactivity. Radioactive elements spontaneously emit alpha particles, beta particles and gamma rays from their nuclei.

If Thomson’s model was correct, -particles would go straight through the foil.

2. Size of an atom is about 100,000 times bigger than that of its nucleus.

In order to avoid falling onto the nucleus, electron must be constantly rotating with great speed, as if it was everywhere at once.

Rutherford’s Conclusions:

Problem: classical physics says that the moving charge (electron) radiates energy.

Indeed, most of them did, but a few (1 in 20,000) were deflected, some in great angles. It was “as if you were firing 15” shell at a tissue paper, and it came back and hit you”.

The Evolution of Atomic Theory (cont.)

3. Atoms positive charge is concentrated in the nucleus.

Size of an atom: If the nucleus is as big as a marble, the atom is as big as football field.

Mass of p is 1840 x mass of e- (1.67 x 10-24 g).

4. Electron has negative charge, and proton has positive charge.

1. Atom consists of a nucleus and electrons.

Nucleus has both protons and neutrons.

Atoms are mostly empty space!

When one or more electrons are lost from an atom, a positive cation is formed.

Creation of Ions

When one or more electrons are added to a neutral atom, an anion is formed.

The word ‘cation’ comes from the fact that positive ion (cation) moves towards negatively-charged (cathode) in an electrolyte solution. The negative ion (anion) moves towards positively-charged electrode (anode).

An neutral atom has equal number of protons and electrons.

Remember that the proton has equal, but opposite, charge as an electron

Atomic Numbers of the Elements

The atomic number of an element is equal to the number of protons in the nucleus of that element. It gives the identity of the element.

Atomic number (Z) = number of protons in nucleus

XAZ

U23592

U23892

Mass Number

Atomic NumberElement Symbol

Two isotopes of uranium

Three isotopes of hydrogenHydrogen(Protium)

Deuterium Tritium (radioactive)

Protons: 92Neutrons: 235-92=143

Protons: 92Neutrons: 238-92=146

The mass of 12C is 12.000 g.There exists 1.11 % 13C and traces of 14C. The mass of C atom (12.01 g) is weighted average of all atoms.

Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutronsIsotopes: atoms of the same element with different numbers of neutrons in their nuclei. Neutrons can be envisioned as glue that holds nuclei together.

To overcome the problem in measuring extremely small masses, a system of relative atomic masses using “atomic mass units” was devised to express the masses of elements using simple numbers.

The standard to which the masses of all other atoms are compared to was chosen to be the most abundant isotope of carbon.

126CA mass of exactly 12 atomic mass

units (amu) was assigned to C.

1 amu is defined as exactly equal to 1/12 the mass of a carbon - 12 atom:

1 amu = 1.6606 x 10-24 gElement masses are average numbers, because of isotopes. Mass of H atom is 1.008 amu.

Masses of Elements and Isotopes

The mass of a single atom is too small to measure on a balance.

Using a mass spectrometer, the mass of the hydrogen atom was determined = 1.673 x 10-24 g.

126

Isotopes of the same element have different masses.

The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to the mass of carbon-12 (exactly 12.0000…amu).

To calculate the atomic mass, multiply the atomic mass of each isotope by its abundance and add the results.

IsotopeIsotopic

mass (amu)Abundance

(%)

Average atomic

mass (amu)

62.9298 69.09

64.9278 30.91 63.55

6329 Cu6529 Cu

(62.9298 amu) 0.6909 = 43.48 amu

(64.9278 amu) 0.3091 = 20.07 amu

63.55 amu

+

Example: calculate average atomic mass of copper.

x

x

Masses of elements and Isotopes (cont.)

Percent abundances are multiplied by 100!

ChlorineElement Symbol Z Mass# #p #n #e charge 36Cl Gold 197

56 79 +216 18 -2

80

Finding the Number of Protons, Neutrons and Electrons

Locate Cl in periodic table

Mass # is 36

Look up Atomic number (Z) for Cl

#n = 36 – #p = 19

1. From Periodic table, find: Element, symbol, atomic number and #protons.At least one of those must be given, the other three are read from Periodic Table.

Symbol Element Z = #p.

3. Find charge or #electrons (charge) = #p – #e #e = #p – (charge)

Charge = 0

2. Find mass number and #nMass# = #p + #n

17 17 1736 19 0ChlorineAu 7979 79118 0

Br -Bromine -13535 3645

Fill in the table below.

See if you can solve the other two examples. The solutions are: 135Ba2+, and 32S2-.

56 16

= #p + #n

4. You can get #e and charge. Use step 3 to find #p and from that identify element.

HW, Chapter 5 (p.100): 12, 15, 23, 27, 37 Chp. 10 (p.220): 13, 19, 33, 39

The Modern Atomic Theory and Periodic Table

Light moves with speed of 300,000 km/s.It is characterized with wavelength .

When passed through a prism, a familiar set of colors are seen. Spectrum is continuous as colors blend.

Visible spectrum is just a tiny fraction of the electromagnetic radiation.

Chapter 10

Rutherford model, based on classical (Newtonian) physics could not explain periodicity of the elements, nor how electrons stay in orbit around nucleus. A new model was needed.

Niels Bohr in 1913 postulated that an electron inside an atom can possess only certain values of energy. Those “packs” of energies are called quanta (pl. of quantum). Atom is in the lowest energy when its electrons are closest to the nucleus. When atom absorbs quantum of energy, electrons are moved to higher orbits.

Electrons can only have allowed energies, not the energies between the allowed ones. It is like a person on a ladder who can stand on a 1st, 2nd, 3rd rung, but not between the rungs.

Bohr’s Theory

Allowed orbits are called shells. They are characterized by the principal quantum number, n that goes from n = 1 (shell closest to the nucleus), to infinity.

The maximum number of electrons a shell can hold increases with n as the radius of the shell increases.

PeriodicityElements have similar chemical properties because their valence shell configurations are similar.

Li and Na have a single electron in the valence shell.

F and Cl have 7 electrons in the valence shell.

The dependence of chemical on valence shell occupancy holds for each group of elements.

The number of electrons in the valence shell is equal to the Roman-numeral group number for the representative elements.

Helium is an exception. It behaves as a noble gas although it has only 2 e- in the valence shell.

Group IA has one electron, Group IIA has two…

The lowest reactivity is found in the group VIIIA (noble gases). Elements of other groups can attain the valence shell of group VIIIA by producing compounds.

Line Spectra

Ground state Excited state

Transition n=3 n=2 produces 13.1 eV – 11.2 eV = 1.9 eV, i.e. red color. Similarly, n=4 n=2 produces 13.8 – 11.3 = 2.6, green color; other colors in the visible spectrum are also found.Bohr’s model correctly predicted all transitions of H atom; transition into n=1 produces a line in UV, into n=2 (visible) and into n3 (infrared). But the model failed to predict spectrum for any atom with more than one e-.

New, refined model includes subshells that are very close in energy and size. Every subshell is designated with a letter (s, p, d, f).

s subhell can hold two electrons. Each subshell after it holds four electrons more than the previous.

s: 2 e- p: 6 e-

d: 10 e- f: 14 e-

Filling of Subshells

Subshells exist for any n > 1. They increase in energy: s < p < d < f.

n s p d f

1 2

2 2 6

3 2 6 10

4 2 6 10 14

This crossing is the reason for the so-called ‘potassium problem’.

Present the electron configuration of 19K.

1s2 2s2 2p6 3s2 3p6 4s1

35Br is slightly more complicated.

1s2 2s2 2p6 3s2 3p6 4s2

3d10 4p5

Valence electron

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5

Rearrange:

Valence electronsOrbital diagram

Each ‘box’ contains 2 electrons with paired spins.

Subshells may cross each other; the crossing is more pronounced when spacing between shells reduces as the principle quantum number increases.

Subshell fill up

No need to memorize crossings. Just follow the periodic table.

Move He next to H to get four blocks, each with different width (s:2, p:6, d:10, f:14).

1s2 2s2 2p6 3s2 3p6 4s2 3d10

4p6 5s2 4d10 5p6 6s2 5d1 4f 7.

S

Try 26Fe

1s2 2s2 2p6

3s2 3p6 4s2 3d6

1st period: 1s2.2nd period: 2s2 2p6.3rd period: 3s2 3p4.

286+

16

64Gd:

Electron configuration of 16S:

Valence electrons

When a transition metal produces cation, first to go are its electrons in the s orbital of the valence shell !

Compound Formation

Neutral atoms:1 valence e- in Na7 valence e- in ClNote the size of atoms!

Valence electron jumps from Na to Cl

Ions: no electrons in n = 3 shell of Na.

Na+ has 8e- in n=2 shell.Cl- has 8e- in n=3 shell.

Note the size of ions!The best way for Mg to achieve octet is to lose two electrons from its 3rd shell.Theoretically, Mg could also gain 6 e-, but the 6 e- excess is too much for Mg nucleus to hold on to.

Each e- from Mg 3rd shell jumps into 2nd shell of an F atom.

The best way for F to achieve octet is to gain one e-.

Mg atom becomes Mg2+ cation.Each F atom becomes F- anion.All three ions have 8 valence e-.

Both ions satisfy octet rule.MgF2

NaCl

Atomic Size

One consequence: shells get larger as n increases.

Understanding of atomic size with Bohr’s model.

A combination of same valence shell and increased nuclear charge shrinks the atom in a group.

Quantum Mechanical Model

Electron has wavelike characteristics, it can tunnel to appear in ‘forbidden’ places. Uncertainty principle states that it is impossible to know at any given time where the electron is, or where it is going. Instead, e- travels around nucleus in an electron cloud or orbital.

Electron cloud exhibits wavelike motion, so that different orbitals have different shapes. It has a particular shape and energy determined by the principle quantum number n.

Allowed orbit must close in on itself.Nonallowed orbit does not.

The QM model also explains why e- stays in orbit around the nucleus: An e- orbital is allowed only if the electron wave closes in on itself.