Atomic Structure and the Periodic Table

Chapter 10

Introduction to ChemistryMr. R. Gilbertson

Atoms Viewed with a Scanning Tunneling

Microscope

These pictures are taken of the surface of several metals. Actually the atoms in the surface are pictured.

Elements – Early History

Aristotle believed in only 4 elements

Chemical Symbols

An abbreviation of the name of an element.

Uses the first letter of an elements name, if more than one element starts with the same letter then a second letter (the next important hard sound) is used.

Sometimes Greek or latin names are used instead of the English.

First letter is always capitalized, second letter is never capitalized.

Early Elemental Symbols

Matter and Atoms

Atom - Smallest particle of matter which maintains the properties of a substance.

The ultimate building blocks of matter. Composed of smaller particles (sub-atomic)

Electron – e-, first discovered, negative charge, 1/1827 of an atomic mass unit (considered negligible) , located on the outside of the atom.

Proton - P+ positive charge, 1826/1827 of an atomic mass unit, located in the nucleus.

Neutron – No, no charge, mass of 1 Atomic mass unit, located in the nucleus.

Nucleons – Protons and Neutrons in the nucleus.

Discovery of Electron

J.J. Thomson’s apparatus

Proton discovery

J.J Thomson used hydrogen in a cathode ray tube

Atomic Theory

Democritis – 2400yrs ago – first proposed the idea of atoms.

Atmos- Greek “indivisible” Dalton – 1800’s proposed atoms as

solid spheres having distinct mass and size.

Existence based on Lavoiser’s Law of Conservation of Mass and Proust’s Law of Definite Proportions.

Dalton’s Model

The law of Definite Proportions

Models of the Atom

JJ. Thomson – 1900’s- discovers electron (negative charge) and proton (positive charge) proposed “plum pudding” model of atom

Rutherford – Bohr – discover atom is mostly empty space, propose “planetary” model of atom.

Chadwick – discovers Neutron after Rutherford predicts its existence.

Electron Cloud Model – developed in 1926 – proposed that electrons moved about in a region of space around the nucleus, not a single orbit like a planet.

Plum Pudding Model

Discovery of the size of atoms

Geiger- Marsden Experiment

Structure of the gold Foil

Bohr Atom - Planetary Model

Electron Cloud

Region of space which surrounds the nucleus. Describes the likely location of each of the

electrons. Size is approximately 100,000 times as large as the

nucleus. Exact location of the electron at any instant is

impossible to know with certainty. Most probable location can be predicted, but other

possible locations exist with a lower probability. Since the electron moves very fast its position

seems to blur, for this reason it appears cloud-like.

Electron Cloud Model

Energy Levels

Electron cloud is divided into many levels (7) (K, L, M, N, O, P, Q).

Electrons in each level have different amounts of energy (energy levels).

Electrons near the nucleus have low energy, farther out have more energy.

Further subdivided into sublevels (s, p, d, f) Energy levels must fill in a definite order from

inside out. Each sublevel can hold only 2 electrons (pair)

energy levels can hold various numbers of sublevels

Energy levels

K – 2L – 8M – 18N – 32O – 32P – 17Q – 2

Energy levels of a Cesium Atom

Electron orbitals

Smaller ParticlesQuarks and Leptons

Particles of which protons and neutrons are composed.

6 types of quarks (top, bottom, charmed, strange, up, down)

6 types of leptons (tau, muon, electron, tau nutrino, muon nutrino, and electron nutrino)

Quarks always occur in groups, leptons always alone.

Held together by exchanging field generating particles (gluons).

Quarks and Leptons

Quarks are always found in groups. Make up other particles.Leptons are always found alone. Fundamental particles.

Particle Accelerators

Used to discover new particles by speeding up known particles and colliding them at near the speed of light.

The collision fragments are then examined and properties are determined.

The top quark was discovered in 1995 by colliding protons.

Particle Accelerators

Where the new particles are being discovered

Formation of Quarks

Particle Evidences

The tracks of the particles after collision between high energy and high speed particles.

Masses of Atoms

Atomic mass – the average mass of all atoms of a certain element.

Atomic mass unit – defined as 1/12th the mass of a carbon-12 atom.

Mass Number – the number of neutrons and protons in the nucleus.

Isotopes – Atoms of an element with varying amounts of neutrons.

Counting Particles in Atoms

Atomic number (AN) – the number of protons in the nucleus.

Mass number (MN) – total number of neucleons (protons and neutrons).

AN = #P+ #P+ = #e-

#No = MN – AN MN = #P+ + #No

Basic Structural Diagram

Identifying Isotopes

Since isotopes have different numbers of neutrons (even though they have the same number of protons) they have different masses.

Each isotope of an element is identified by the elemental symbol followed by the mass number (ex. Ca-40, C-12, U-238)

Isotopes of Mercury

Isotopes of Hydrogen

Average Atomic Mass

The average mass of a mixture of all the naturally occurring isotopes of an element. Average atomic mass is used on the periodic table of the elements

Determined by multiplying the relative abundance times the mass of each isotope then adding the contribution of each (often called a weighted average)

Boron is 20% B-10 and 80% B-11 so the average atomic mass is:

(.2*10)+(.8*11)=10.8u

Calculating Average Atomic Mass

Hafnium 5% Hf-176 .05*176u = 8.80u 19% Hf-177 .19*177u = 33.63u 27% Hf-178 .27*178u = 48.06u 14% Hf-179 .14*179u = 26.06u 35% Hf-180 .35*180u = 63.00u Total 178.55u

The Periodic Table

The Periodic Table

Dimitri Mendeleev – Russian – in the late 1800’s organized elements into the first periodic table - by mass.

Discovered a pattern in the changing properties of the elements.

Was able to predict the existence of some elements that had not as of yet been discovered and the properties of these elements.

Mendeleev’s Table

Problems with Mendeleev’s Table

Mendeleev found that when the elements were arranged by mass some seemed to be out of order based on their properties. (Ni – Co, Te – I)

He believed that the masses had been incorrectly measured.

When carefully remeasured the new data showed the original masses were correct.

New Table Based on Atomic Number

Henry Moseley – determined a method of determining the atomic number of an element using an x-ray tube.

Used different elements as a target, produced different wavelengths and correlated with the number of protons.

New Periodic Table

Now elements are arranged by the atomic number, properties follow the same pattern.

Table is arranged in order of the number of protons, but columns reflect the arrangement of electrons (electron configuration).

Columns or families have similar chemical properties because they have the same number of electrons in the outer energy level.

How to read the Periodic Table of the Elements

Families or Groups of Elements

Group 1 – alkali metals (1 outer electron) Group 2 – alkali earth metals (2 e-) Group 3-12 – Transitional metals (0-2 e-) Group 15 – nitrogen family (5 e-) Group 16 – chalcogen family (6 e-) Group 17 – halogen family (7 e-) Group 18 – noble gases (8 e-) Lanthanide and Actinide series

Families of elements

All members of a family of elements have the same number of outer electrons.

Number of outer electrons determines the chemical properties of the element since only outer electrons take part in a chemical reaction between atoms.

Lewis electron dot diagrams are used to show the number of outer electrons. Uses the elemental symbol and dots which represent the electrons in the outer energy level.

Lewis Electron Dot Diagrams

Only the electrons in the outermost energy level are shown.

Nepx1 px4

s1

s2px3 px6

px2

px5

New elements

Elements are identified by the number of protons. The number of protons in an atom can only be changed

in a nuclear reaction. Some scientists believe that elements that exist today

may have been created when heavier primordial elements split in the process of Fission.

New heavier elements can be created by bringing atoms together in a nuclear reaction called Fusion.

Elements 110 and 111 have been created, though their half-lives may be as short as 270 microseconds.

Periods of Elements

Horizontal rows on the periodic table are known as periods.

Each member of a period has the same number of energy levels.

Generally the mass increases as you go from left to right across the table.

The number of protons (and e-) increases from left to right.

Other Information from the Periodic Table

The stair-step line divides the elements into metals (to the left except H) nonmetals (to the right) and metalloids (touching the line on two sides except AL) Metals – exist as solids at room temp, shiny, good

conductors of heat and electricity. Nonmetals – most are gases at room temp, dull

and brittle if solid, poor conductors of heat and electricity.

Metalloids – have properties of both metals and nonmetals.

Absorbtion Spectra

As light passes through a gas sample certain wavelengths are absorbed by the electrons causing dark lines. Creates a “fingerprint” of the element.

Emission Spectra

When a gas is engergized by high voltage the electrons absorb energy and then give off energy in the form of specific wavelengths called an emission spectra.

THE END