Chapter 3

Atoms and the Periodic Table

Matter

• What accounts for matters secrets?– Atomists – Democritus– All things are made of Fire – Herclitus– Four Elements – Aristotle

Conservation of Mass and Law of Definite Proportions

• Robert Boyle (1627–1691): Provided evidence for the atomic nature of matter.– Element - A substance that

cannot be chemically broken down further

• Joseph Priestley (1733–1804):

Conservation of Mass and Law of Definite Proportions

• Antoine Lavoisier (1743-1794):

Conservation of Mass

• Law of Mass Conservation: Mass is neither

created nor destroyed in chemical reactions.

Hg(NO3)2 + 2 KI → HgI2 + 2KNO3

3.25g + 3.32g → 4.55g + 2.02g

6.57g = 6.57g

Law of Definite Proportions

• Law of Definite Proportions: In a unique

compound the elements will always be found in

the exact same ratio.

Dalton’s Atomic Theory

• John Dalton (1766–1844): Proposed explanations for

the laws of mass conservation and definite proportions.– Postulate 1: Elements are made of atoms– Postulate 2: Atoms of the same element have the same

properties (including mass). Atoms of different atoms have

different properties– Postulate 3: Compound are comprise of elements joined

together in small whole ratios– Postulate 4: Chemical reactions only rearrange the way the

atoms are combined

The Structure of Atoms: Electrons

• Elements are composed of atoms

• Compounds are composed of atoms of elements in a given ratio

• What does an atom look like?

The Structure of Atoms• Structure of the Atom:

The Structure of Atoms

• Comparison of Subatomic particles

Particle Grams amu Coulombs e

Electron 9.109382 x 10-28 0.0005485799 -1.602176 x 10-19 -1

Proton 1.672622 x 10-24 1.007276 1.602176 x 10-19 1

Neutron 1.674927 x 10-24 1.008665 0 0

Atomic Number

• # protons in an atom determines the element – atomic number (Z) - Atomic number is found

on the periodic table

• # electrons = # protons in a neutral atom

Mass Number

• mass of the atom ≈ # neutrons + # protons – Mass number = # protons + # neutrons

– An element may have more than one mass #• Isotopes: Atoms with identical atomic numbers,

but different mass numbers.• Due to different # of neutrons NOT protons

Isotopes and Atomic Weight

• Atoms of an element that have a different mass are called isotopes

Elements and Atomic Number

Symbol # protons # neutrons Mass # Atomic # # electrons

K 20

30 34

25 11

118 50

Atomic Mass

• Atomic Mass: A weighted average of the isotopic masses of an element’s naturally occurring isotopes.– Unit – atomic mass unit (amu)

Example

• Chlorine has two naturally occurring isotopes:

with an abundance of 75.77% and an isotopic

mass of 34.969 amu, and with an abundance of

24.23% and an isotopic mass of 36.966 amu.

What is the atomic mass of chlorine?

Isotopes and Atomic Weight

• Bromine has two naturally occurring isotopes: 79Br and 81Br, calculate the atomic mass of bromine.

Isotope Abundance%

79Br 50.69

81Br 49.31

Problem

• Atoms X, Y, Z, and R have the following nuclear compositions:

  

Which two are isotopes? 

– A.  X & Y– B.  X & R– C.  Y & R– D.  Z & R– E.  X & Z

Electronic Structure

• The Periodic Table Powerpoint– Elements in a group react similarly

• Electronic Structure– Atomic Size– Why metals for cations and non-metals anions

Electronic Structure of Atoms

• Structure based on an accepted quantum mechanical model– Mathematical description of the location in

which an electron can be found

– Quantum mechanical model – electron cloud can be divided into

• Shells• Subshells• Orbitals

Electronic Structure

Nucleus

Shell – Each FloorSubshell – Row of rooms

Orbital - Room2 beds per room – opposite direction

Electronic Structure of Atoms

• Shells – layers in which the electrons are grouped around the nucleus– As a layer is added the atomic size increases

• Shells agree with the period # on periodic table

– Differ in energy – requires more energy to remove an electron from a shell closer to the nucleus

– Electron capacity – increases with higher shells cause e- can spread out more

Shell # 1 2 3 4

e- capacity 2 8 18 32

Electronic Structure of Atoms

• Subshells– s, p, d, f

• 1st shell – only s• 2nd shell – s, p• 3rd shell – s, p, d• 4th shell – s, p, d, f• Note: shell has the # of subshells = the shell #

• 3s electron – an electron is found in the s subshell of the 3rd shell

Electronic Structure of Atoms

• Orbitals – Subshells can further be divided into orbitals– regions of space within an atom where the specific electrons are

more likely to be found.– Different orbitals have different shapes. Orbitals in s subshells

are spherical (a), while orbitals in p subshells are roughly dumbbell shaped (b).

Electronic Structure of Atoms

• 2 electrons in each orbital with opposite spins

Electronic Structure of Atoms

•The overall electron distribution within an atom is summarized in Table 3.2 below.

Electron Configurations

• Electron Configuration: The exact arrangement of electrons in atom’s shells and subshells. Rules to predict electron configuration:

1.Electrons occupy the lowest energy orbitals available first.

2.Each orbital can hold only two electrons, which must be of opposite spin.

3. If two or more orbitals have the same energy, each orbital gets one electron before any orbital gets two.

Electron Configurations

•Order of orbital energy levels:►Electrons fill orbitals from the lowest-energy orbitals upward.► Lower numbered shells fill before higher numbered shells at first. ►Some overlap in energy levels occurs starting with shell 3 and 4.

Electron Configurations►Electron configurations are described by

► write the shell number ► subshell letter in order of increasing energy. ► number of electrons actually occupying each subshell is

indicated by a superscript.

►A graphic representation can be made by indicating each orbital as a line and each electron as an arrow. The head of the arrow indicates the electron spin.

►A shorthand using noble gas configurations is very useful for large atoms.

Periodic Table and Electron Configurations

Electron Configurations

Electron Configurations

Problem

• Select the correct electron configuration for sulfur (Z = 16). 

– A.  1s21p62s22p6

– B.  1s22s22p83s23p4

– C.  1s22s22p83s23p2

– D.  1s22s22p63s23p4

– E.  1s22s22p63s23d4

Problem

• Select the correct electron configuration for Te (Z = 52). 

– A.  [Kr]5s25p64d8

– B.  [Kr]5s25d105p4

– C.  [Kr]5s24d105p6

– D.  [Kr]5s24f14

– E.  [Kr]5s24d105p4

Problem

• The electronic structure 1s22s22p63s23p64s23d8 refers to the ground state of 

– A.  Kr– B.  Ni– C.  Fe– D.  Pd– E.  none of these choices is correct

Electron Configuration

• Allows determination of valence e- and core e-

• Indicates why elements of a group have similar reactivity

• Determine which element is present by the sum of the superscripts– Ground state atom– Excited state atom

Optional Homework

• Text – 3.31, 3.32, 3.33, 3.44, 3.46, 3.48, 3.50, 3.52, 3.56, 3.70, 3.72, 3.74, 3.76, 3.78, 3.82, 3.84, 3.86, 3.96, 3.100, 3.106, 3.108, 3.110, 3.111

• Chapter 3 Homework – from website