Atoms and the Periodic Table

Chapter Three

Subatomic Particles

Atoms are composed of subatomic particlesChargeMass (amu)Mass (g)SymbolParticle

-15.485799 x 10-49.109328 x 10-28eElectron

01.0086651.674927 x 10-24nNeutron

+11.0072761.672622 x 10-24pProton

The Structure of an Atom

Attractive and Repulsive Forces inAtomic Structure

Structure of the Atom

• Protons and neutrons reside in a small, densenucleus.

• Electrons are spread out in space outside thenucleus. Most of the atom is empty space.

• Atoms are neutral overall, i.e., no net charge• Atomic Number (Z) - The number of protons in

the nucleus of an atom.– Determines the type of atom (which element it is)– Also number of electrons in a neutral atom

• Mass Number (A) - The sum of the protons plusneutrons in the nucleus of an atom.

• Therefore, # of neutrons = A - Z

Example Problem 3.2

• If Phosphorus has a mass number of A = 31, how manyprotons, neutrons and electrons are there?

Atoms and Ions

• An atom is electrically neutral.#e = #p#e = Z

• Ion - An atom or other chemical species thatcarries an overall positive or negative charge.– Monatomic ion - a single atom with a positive or negative

charge– Cation - a positive ion (#e < #p)– Anion - a negative ion (#e > #p)

Isotopes• Isotopes- Atoms of the same element that have different

mass numbers- Same number of protons- Different number of neutrons

• Nuclide - A nucleus with a particular massnumber

• Symbols of Nuclides– Write the symbol (Sy) of the element.– Put the atomic number (Z) at the lower left.– Put the mass number (A) at the upper left.

How to Indicate Isotopes

SyZ

A

Sample Isotope Problem

electrons

amu

neutrons

protons

14C13C12C

Atomic Weight (Mass)

• Atomic Weight (Mass) - The average mass of allof the isotopes of an element that occur in nature.

• Carbon-12 is assigned a mass of exactly 12 amu.– Standard

At. Wt. = fract. abund1 x mass1 +fract. abund2 x mass2 +fract. abund3 x mass3 + ….

Atomic Weight of Magnesium

11.01%25.9326Mg

10.00%24.9925Mg

78.99%23.9924Mg

AbundanceApproximate Mass(amu)

Isotope

The Periodic Table

• Periodic Table - A chart in which elements arearranged in order of increasing atomic numbersuch that elements having similar chemicalproperties lie beneath one another.

• The atomic number (Z), the symbol, and theatomic mass of each element is given.

• Groups(Families) - The columns in the table• Periods - The rows in the table

• Groups can be designated by numbers or names

• Representative Elements (Main Group Elements)– Elements of groups 1,2 and 13-18 (old 1A, 2A and 3A - 8A)– Alkali metals (1A), Alkaline earth metals (2A), Halogens (7A),

Noble gases (8A)

• Transition metals– Elements of groups 3-12 (old 3B - 8B and 1B & 2B)

The Periodic Table

The Periodic Table

The Periodic Table

• Atomic Radius versus Atomic Number

Periodic Properties

• Atomic Size– Decreases in going across a period– Increases in going down a group

• Ionization Energy– Energy required to remove an electron from an atom– Increases in going across a period– Decreases in going down a group

• Electron Affinity– Energy released when an electron is added to an atom– Increases in going across a period– Generally decreases in going down a group (some exceptions)

Properties of Electrons

• All moving objects have properties of bothparticles and waves- If the particle is large, e.g., a soccer ball, the associated wave is not ameasurable property of the object

• Electrons are sufficiently small that they can bedescribed as both particles and waves- wave-particle duality

• Current theories of the electronic structure ofatoms are based on the wave properties of theelectron

Quantum Mechanical Model of AtomicStructure

• Electrons are not perfectly free to move about in an atom• The energy of the electron is quantized• Quantized- only certain specific values are permitted

- Steps versus ramp analogy

• These values of energy are called energy levels- Electrons prefer to reside in the lowest possible energy level

• Electrons in an atom that are in the same energy level areat about the same average distance from the nucleus- Electrons that are closer to the nucleus have lower

energies because they are attracted mores strongly bythe nucleus

Location of An Electron

• Shells- like layers of an onion, as you progress from thecenter the larger it is, thus it can hold more electrons

• Subshells- each energy shell is further sub-divided intoenergy sublevels where the number of sublevels is equalto n which is the shell number- subshells (energy sublevels) are designated by letters- subshells increase in energy in the order of:

s < p < d < f

• Oribtals- regions in space within an atom where specificelectrons are most likely to be found

Orbitals

• The number of orbitals in a subshell increases- s subshells are composed of 1 orbital- p subshells are composed of 3 orbitals- d subshells are composed of 5 orbitals- f subshells are composed of 7 orbitals

• Each orbital can contain a maximum of 2electrons of opposite spin- s subshells contain 2 electrons- p subshells contain 6 electrons- d subshells contain 10 electrons- f subshells contain 14 electrons

Shapes of s and p Orbitals

Summary of Electron Distributionin Atoms

Energy Perspective of ElectronDistribution in Atoms

Electron Configurations

• Electron configurations- the exact arrangementof electrons in the shells and subshells of atoms

• 3 Rules apply for placement of the electrons- Rule 1: Electrons occupy the lowest-energy orbitals

available and are filled in the order shown in Fig 3.5- Rule 2: Each orbital can hold only 2 electrons which must

be of opposite spin- Rule 3: Two or more orbitals of the same energy (i.e., p,d,f

subshells) are each half filled by 1 electron with thesame spin before any one oribtal is filled by thesecond electron of opposite spin

Order of Filling Electron Orbitals

• Note that above the 3p level there is a crossover of energylevels, thus 4s orbital must filled before 3d orbitals, 5sorbital before 4d orbitals

Representation of ElectronConfigurations

• Written• Graphical

Electron Configuration and thePeriodic Table

• Relationship between rows (periods, shells) and columns(groups, subshells)

• Main group elements (s or p)• Transition group elements (d or f)

Valence Shell Electrons

• Valence electrons - electrons found in theoutermost shell (valence shell)

• Elements in the same group (column) of theperiodic table have similar electronconfigurations in their valence shell- Group 1 (1A) : ns1

- Group 2 (2a): ns2

- Group 13 (3A): ns2np1

- Group 17 (7A):ns2 np5

- Group 18 (8A): ns2np6

Where n is the shell number