Chapter 3Chapter 3

ATOMIC THEORY AND THE ATOMIC THEORY AND THE PERIODIC TABLEPERIODIC TABLE

Subatomic particle

Discoveries Properties

Electron (e) Thomson, 1887 Present in all atoms

Extermely light (1/1836 mass of H atom)

Posses negative charge, assigned -1

Proton (p)Thomson and Goldstein, 1907

Present in all atoms

About the same mass as H atom

Has positive charge equal in magnitude but oppisite in sign to electron, assigned +1

Neutron (n) Chadwick, 1932 About the same mass as a proton

Has no Charge (is electrically nuetral)

ATOMS

NucleusNucleus

The nucleus was found to be The nucleus was found to be composed of two kinds of particlescomposed of two kinds of particles

Some of these particles are called Some of these particles are called protonsprotons – charge = +1 charge = +1 – mass is about the same as a hydrogen atom mass is about the same as a hydrogen atom

The other particle is called a neutron The other particle is called a neutron – has no charge has no charge – has a mass slightly more than a proton has a mass slightly more than a proton

Basic facts about AtomsBasic facts about Atoms For an atom to be For an atom to be

neutral, neutral, # of Protons = # of Electrons# of Protons = # of Electrons

Atomic NumberAtomic Number - - The The number of number of protonsprotons in the in the nucleus of an atom. All nucleus of an atom. All atoms of particular atoms of particular element have the element have the same atomic number, same atomic number, which is indicated by a which is indicated by a subscript to the left of subscript to the left of the element symbolthe element symbol

Mass NumberMass Number - the - the number of protons number of protons plus neutrons in the plus neutrons in the nucleus of an atom.nucleus of an atom.

IsotopesIsotopes - Different - Different forms of an element forms of an element having the same having the same number of protons but number of protons but different numbers of different numbers of neutrons (and neutrons (and therefore different therefore different atomic weights).atomic weights).

Isotopes are

Isotopes are identified by their Mass NumberMass Number = Protons + Neutrons

Atomic MassAtomic Mass Atomic Weight (Mass)Atomic Weight (Mass) - The - The

mass of a particular atom mass of a particular atom relative to the mass of an relative to the mass of an atom or carbon-12 (atom or carbon-12 (12C12C), ), which is arbitrarily assigned a which is arbitrarily assigned a mass of exactly 12.mass of exactly 12.

Average Atomic WeightAverage Atomic Weight - - Average weight of an element Average weight of an element based on the naturally based on the naturally occurring isotopes and the occurring isotopes and the relative abundance of these relative abundance of these isotopes on Earth.isotopes on Earth.

A unit of A unit of massmass equal to the equal to the mass of a single atom of the mass of a single atom of the most common isotope of most common isotope of carbon, divided by 12  carbon, divided by 12  

The The atomic mass unitatomic mass unit, also , also called the called the daltondalton after the after the chemistchemist John Dalton, is a John Dalton, is a small unit of mass used to small unit of mass used to express atomic masses and express atomic masses and molecular masses. It is molecular masses. It is defined to be 1/12 of the defined to be 1/12 of the mass of one atom of Carbon-mass of one atom of Carbon-12. The abbreviations "u", 12. The abbreviations "u", "amu" and "Da" are used for "amu" and "Da" are used for this unit; often, atomic this unit; often, atomic masses are written without masses are written without any unit and then the amu is any unit and then the amu is implied. implied.

The value is 1 amu ≈ The value is 1 amu ≈ 1.6605387 × 10-27 kilograms.1.6605387 × 10-27 kilograms.

Periodic TablePeriodic Table

Components of the Periodic Components of the Periodic TableTable

Columns are called Columns are called GroupsGroups or Families or Families – Elements with similar chemical and physical properties are in the same Elements with similar chemical and physical properties are in the same

column column Rows are called Rows are called PeriodsPeriods

Each period shows the pattern of properties repeated in the next period Each period shows the pattern of properties repeated in the next period Main GroupMain Group

– (Representative Group) - Groups IA - VIIIA (Representative Group) - Groups IA - VIIIA Transition MetalsTransition Metals

– Groups IB - VIIIBGroups IB - VIIIB Rare Earth ElementsRare Earth Elements

– Lanthanides (Ce - Lu) and Actinides (Th - Lr)Lanthanides (Ce - Lu) and Actinides (Th - Lr) MetalsMetals

– about 75% of all the elements about 75% of all the elements – lustrous, malleable, ductile, conduct heat and electricitylustrous, malleable, ductile, conduct heat and electricity

NonmetalsNonmetals– dull, brittle, insulators dull, brittle, insulators

Metalloids Metalloids – also know as semi-metals also know as semi-metals – some properties of both metals & nonmetals some properties of both metals & nonmetals

LawsLaws

Law of Law of Mendeleev: Mendeleev: – Properties of the Properties of the

elements recur in elements recur in regular cycles regular cycles (periodically) when (periodically) when the elements are the elements are arranged in order of arranged in order of increasing atomic increasing atomic weight.weight.

  Periodic Law:Periodic Law:– The properties of The properties of

the elements are a the elements are a periodic function of periodic function of atomic numbers. atomic numbers.

ATOMIC ORBITALSATOMIC ORBITALS

OrbitalOrbital /áwrbit'l/ /áwrbit'l/ nounnoun. (Phys) . (Phys) Space Space in an atom occupied by an in an atom occupied by an electronelectron. A subdivision of the . A subdivision of the available space within an atom for an available space within an atom for an electron to orbit the nucleus. an electron to orbit the nucleus. an atom has many orbitals, each of atom has many orbitals, each of which has a fixed size and shape and which has a fixed size and shape and can hold up to two electrons. can hold up to two electrons.

s Orbitalss Orbitals Each orbital has a name. The orbital occupied by Each orbital has a name. The orbital occupied by

the hydrogen electron is called a the hydrogen electron is called a 1s orbital.1s orbital. The The "1""1" represents the fact that the orbital is in the represents the fact that the orbital is in the energy level closest to the nucleus. The energy level closest to the nucleus. The "s""s" tells tells you about the shape of the orbital. s orbitals are you about the shape of the orbital. s orbitals are spherically symmetric around the nucleus - in spherically symmetric around the nucleus - in each case, like a hollow ball made of rather each case, like a hollow ball made of rather chunky material with the nucleus at its centre.chunky material with the nucleus at its centre.

2s orbital.2s orbital. This is similar to a 1s orbital except This is similar to a 1s orbital except that the region where there is the greatest that the region where there is the greatest chance of finding the electron is further from the chance of finding the electron is further from the nucleus - this is an orbital at the second energy nucleus - this is an orbital at the second energy level.level.

p Orbitalp Orbital Not all electrons inhabit s orbitals (in fact, very few Not all electrons inhabit s orbitals (in fact, very few

electrons live in s orbitals). At the first energy level, electrons live in s orbitals). At the first energy level, the only orbital available to electrons is the 1s the only orbital available to electrons is the 1s orbital, but at the second level, as well as a 2s orbital, but at the second level, as well as a 2s orbital, there are also orbitals called orbital, there are also orbitals called 2p orbitals.2p orbitals.

A p orbital is rather like 2 identical balloons tied A p orbital is rather like 2 identical balloons tied together at the nucleus. The diagram on the right is together at the nucleus. The diagram on the right is a cross-section through that 3-dimensional region a cross-section through that 3-dimensional region of space. Once again, the orbital shows where there of space. Once again, the orbital shows where there is a 95% chance of finding a particular electron.is a 95% chance of finding a particular electron.

Unlike an s orbital, a p orbital points in a particular Unlike an s orbital, a p orbital points in a particular direction - the one drawn points up and down the direction - the one drawn points up and down the page.page.

d and f oribitald and f oribital In addition to s and p orbitals, there are two other sets of In addition to s and p orbitals, there are two other sets of

orbitals which become available for electrons to inhabit at orbitals which become available for electrons to inhabit at higher energy levels. At the third level, there is a set of five higher energy levels. At the third level, there is a set of five dd orbitals (with complicated shapes and names) as well as orbitals (with complicated shapes and names) as well as the 3s and 3p orbitals (3px, 3py, 3pz). At the third level the 3s and 3p orbitals (3px, 3py, 3pz). At the third level there are a total of nine orbitals altogether.there are a total of nine orbitals altogether.

At the fourth level, as well the 4s and 4p and 4d orbitals At the fourth level, as well the 4s and 4p and 4d orbitals there are an additional seven there are an additional seven ff orbitals - 16 orbitals in all. s, orbitals - 16 orbitals in all. s, p, d and f orbitals are then available at all higher energy p, d and f orbitals are then available at all higher energy levels as well.levels as well.

For A'level purposes, you have to be aware that there are For A'level purposes, you have to be aware that there are sets of five d orbitals at levels from the third level upwards, sets of five d orbitals at levels from the third level upwards, but you will not be expected to draw them or name them. but you will not be expected to draw them or name them. Apart from a passing reference, you won't come across f Apart from a passing reference, you won't come across f orbitals at all.orbitals at all.

Fitting electrons into Fitting electrons into orbitalsorbitals

You can think of an atom as a very bizarre house You can think of an atom as a very bizarre house (like an inverted pyramid!) - with the nucleus (like an inverted pyramid!) - with the nucleus living on the ground floor, and then various rooms living on the ground floor, and then various rooms (orbitals) on the higher floors occupied by the (orbitals) on the higher floors occupied by the electrons. On the first floor there is only 1 room electrons. On the first floor there is only 1 room (the 1s orbital); on the second floor there are 4 (the 1s orbital); on the second floor there are 4 rooms (the 2s, 2px, 2py and 2pz orbitals); on the rooms (the 2s, 2px, 2py and 2pz orbitals); on the third floor there are 9 rooms (one 3s orbital, three third floor there are 9 rooms (one 3s orbital, three 3p orbitals and five 3d orbitals); and so on. But 3p orbitals and five 3d orbitals); and so on. But the rooms aren't very big . . . the rooms aren't very big . . . Each orbital can Each orbital can only hold 2 electrons.only hold 2 electrons.

A convenient way of showing the orbitals that the A convenient way of showing the orbitals that the electrons live in is to draw "electrons-in-boxes".electrons live in is to draw "electrons-in-boxes".

The order of filling orbitalsThe order of filling orbitals

Electrons fill low energy orbitals Electrons fill low energy orbitals (closer to the nucleus) before they fill (closer to the nucleus) before they fill higher energy ones. Where there is a higher energy ones. Where there is a choice between orbitals of equal choice between orbitals of equal energy, they fill the orbitals singly as energy, they fill the orbitals singly as far as possible.far as possible.

The diagram (not to scale) summarises The diagram (not to scale) summarises the energies of the orbitals up to the the energies of the orbitals up to the

4p4p Notice that the s orbital always has a slightly lower energy than Notice that the s orbital always has a slightly lower energy than

the p orbitals at the same energy level, so the s orbital always fills the p orbitals at the same energy level, so the s orbital always fills with electrons before the corresponding p orbitals.with electrons before the corresponding p orbitals.

The real oddity is the position of the 3d orbitals. They are at a The real oddity is the position of the 3d orbitals. They are at a slightly higher level than the 4s - and so it is the 4s orbital, which slightly higher level than the 4s - and so it is the 4s orbital, which will fill first, followed by all the 3d orbitals and then the 4p orbitals. will fill first, followed by all the 3d orbitals and then the 4p orbitals. Similar confusion occurs at higher levels, with so much overlap Similar confusion occurs at higher levels, with so much overlap between the energy levels that the 4f orbitals don't fill until after between the energy levels that the 4f orbitals don't fill until after the 6s, for example the 6s, for example

Writing electronic Writing electronic configurationsconfigurations

The two electrons in He represent the complete The two electrons in He represent the complete filling of the first electronic shell. Thus, the electrons filling of the first electronic shell. Thus, the electrons in He are in a very stable configuration in He are in a very stable configuration

For Boron (5 electrons) the 5th electron must be For Boron (5 electrons) the 5th electron must be placed in a 2placed in a 2pp orbital because the 2 orbital because the 2s s orbital is filled. orbital is filled. Because the 2Because the 2pp orbitals are equal energy, it doesn't orbitals are equal energy, it doesn't matter which 2matter which 2pp orbital is filled orbital is filled

The The Heisenberg Uncertainty Heisenberg Uncertainty PrinciplePrinciple

says - loosely - that you can't know says - loosely - that you can't know with certainty both where an electron with certainty both where an electron is and where it's going next. That is and where it's going next. That makes it impossible to plot an orbit makes it impossible to plot an orbit for an electron around a nucleus. Is for an electron around a nucleus. Is this a big problem? No. If something this a big problem? No. If something is impossible, you have to accept it is impossible, you have to accept it and find a way around it.and find a way around it.

PrinciplesPrinciples

Hund's ruleHund's rule for degenerate orbitals, the for degenerate orbitals, the lowest energy is attained when the lowest energy is attained when the number of electrons with the same spin is number of electrons with the same spin is maximizedmaximized

Aufbau principleAufbau principle - Lowest energy orbitals - Lowest energy orbitals fill first fill first

Pauli exclusion principlePauli exclusion principle - No 2 - No 2 electrons can have the same set of electrons can have the same set of quantum numbers (maximum of 2 quantum numbers (maximum of 2 electrons per orbital) electrons per orbital)

A guide to the order of orbital A guide to the order of orbital energies energies

Order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f

FactsFacts Electrons repel each other, by occupying different orbitals Electrons repel each other, by occupying different orbitals

the electrons remain as far as possible from one another the electrons remain as far as possible from one another A carbon atom in its lowest energy (ground state) A carbon atom in its lowest energy (ground state) has two has two

unpaired electronsunpaired electrons Ne has Ne has filled up the n=2 shellfilled up the n=2 shell, and has a , and has a stable electronic stable electronic

configurationconfiguration Electronic configurations can also be written in a short hand Electronic configurations can also be written in a short hand

which references the which references the last completed orbital shelllast completed orbital shell (i.e. all (i.e. all orbitals with the same principle quantum number 'n' have orbitals with the same principle quantum number 'n' have been filled) been filled)

The electronic configuration of Na can be written as The electronic configuration of Na can be written as [Ne]3[Ne]3ss11

The electronic configuration of Li can be written as The electronic configuration of Li can be written as [He]2[He]2ss11 The electrons in the stable (Noble gas) configuration are The electrons in the stable (Noble gas) configuration are

termed termed the core electronsthe core electrons The electrons in the outer shell (beyond the stable core) are The electrons in the outer shell (beyond the stable core) are

called called thethe