Mileposts on the road to the atom - cod.edu · The number of protons in the nucleus of an atom determines the atomic number (Z) and indicates the element's identity. For a neutral

To Understand The Atom Is

To Understand Chemistry

Learning Objectives

Describe the three particles in the atom

Define atomic number and mass number

Describe isotopes

Write symbols for elements

Determine the numbers of particles in any

atom from the element symbol

Determine average atomic mass from isotope

distribution

Identify important groups in periodic table

Atoms are not the

smallest thing

Growing evidence for the divisibility

of the indivisible

Electrostatics and electricity

Static electricity was observed by

Thales (300 BC). Some

“charged” objects repel and

others attract

The voltaic cell (Volta, 18th

century) generated electrical

current from chemical reactions

Mechanical electrical generation

was achieved in 1825

The point: Atoms are neutral. If

indivisible, where do electrical

charges come from?

Faraday’s prescience

“Although we know nothing of what an atom

is, we cannot resist forming some idea of a

small particle; and though we are in equal

ignorance of electricity, there is an immensity

of facts which justify us in believing that the

atoms of matter are associated with electrical

powers to which they owe their most striking

qualities, and amongst them their chemical

affinity.”

Ray of hope

1858

Discovery of cathode rays by Julius Plucker

Application of a large voltage across an evacuated

tube causes a current to flow. The current flow is

accompanied by radiation from the excited gas

molecules

How does the neutral and indivisible atom create a

charge?

Cathode rays are negatively charged particles

1897.J. Thomson demonstrates that cathode rays consist of negatively charged

particles. The first sighting of the electron: a particle much smaller than an atom.

The “oil drop” experiment gave a

measurement of electron charge

The Thomson model of the atom

"I regard the atom as containing a large number of smaller bodies

which I will call corpuscles, these corpuscles are equal to each

other.... In the normal atom, this assemblage of corpuscles

forms a system which is electrically neutral. Though the

individual corpuscles behave

like negative ions, yet when they are

assembled in a neutral atom the negative

effect is balanced by something which

causes the space through which the corpuscles are spread to act as

if it had a charge of positive electricity equal in amount to the

sum of the negative charges of the corpuscles…”

X-rays and atoms’ invisible rays

1895

Wilhelm Conrad

Roentgen discovers X-

rays while doing

experiments with

cathode rays

Radioactivity: the search for

invisible rays

1896

Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles.

1903

Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible.

1909 Ernest Rutherford’s gold foil

experiment

The atom’s inner secrets exposed by its own

offspring

Hierarchy of discovery

The path to knowledge occurs in steps, each

depending on a previous advance

The battery and electricity: Volta 1799

Cathode rays: Plucker 1858

X-rays: Roentgen 1895

Radioactivity:Becquerel 1896

The nucleus: Rutherford 1909

The nucleus

Tiny

Incredibly dense – contains all the mass

of the atom

Positively charged

Contains protons (charged) and neutrons

(neutral) – not discovered until much

later

Summary of Atom Pictures Dalton: Indivisible atom

Thomson: Electrons

Rutherford: Nucleus

Atoms are mostly nothing

Comparison of subatomic particles

Particle Mass Charge

grams amu coulombs e

Electron 9.1094 x 10-28 5.486 x 10-4 -1.6022 x 10-19 -1

Proton 1.6726 x 10-24 1.0073 +1.6022 x 10-19 +1

Neutron 1.6749 x 10-24 1.0087 0 0

Atoms are neutral:

# electrons = # protons

The number of protons in the nucleus of an atom

determines the atomic number (Z) and indicates the

element's identity. For a neutral atom, the atomic

number also describes the number of electrons around

the nucleus.

Variations on the number of neutrons in the

nucleus give rise to different isotopes of the

same element.1

1 H 2

1 H 3

1 H

Element notation: Atomic number

and mass number

13

6C

Mass number:

number of protons

+ neutrons

Atomic number:

number of protons

Element symbol

Counting particles:

Number of electrons = number of protons = 6

Number of neutrons = mass number – atomic number (13 –

6 = 7)

Atomic mass

Atoms are very small and so normal units of

mass are inconvenient for describing atomic

mass

Atomic Mass Unit

Mass of one atom of carbon-12 = 12 amu

1 amu = 1.660 x 10-24 g

Similarity of atomic mass and mass

number

Almost all mass is

concentrated in the

nucleons

Means mass

number (integer)

and atomic mass are

very similar

Isotopes cause disparity

Carbon has two isotopes: C-12 and C-13

Atomic mass C-12 = 12 amu; C-13 = 13.0034

amu

Atomic mass C is weighted average of the

masses of the two isotopes (98.89 % C-12 +

1.11 % C-13

= 12.011 amu

What of the electrons?

We now understand the atom to contain a tiny

positively charged massive nucleus surrounded

by a comparatively vast empty space

containing the electrons

When atoms combine the electrons must

interact

We need to understand the arrangement of

electrons in the atom

Rudiments of elements All of matter is made from elements, in combination with other elements or

alone.

There are some ninety naturally occurring – have “always” been.

More than twenty have been created artificially

Element names and symbols

Aluminum Al Chlorine Cl Manganese Mn Copper (cuprium) Cu

Argon Ar Fluorine F Nitrogen N Iron (ferrum) Fe

Barium Ba Helium He Oxygen O Lead (plumbum) Pb

Boron B Hydrogen H Phosphorous P Mercury

(hydrargyrum)

Hg

Bromine Br Iodine I Silicon Si Potassium (karium) K

Calcium Ca Lithium Li Sulfur S Silver (argentum) Ag

Carbon C Magnesium Mg Zinc Zn Sodium (natrium) Na

Elements organized – a triumph of

chemistry The periodic table of the elements is organized into 18 groups and 7

periods. Elements are represented by one- or two-letter symbols and are

arranged according to atomic number.

History of discovery

The Periodic Table: Groups and PeriodsGroups: columns of

elements

Periods: rows of

elements

Elements in groups have similar

properties One of the eight main groups of elements, Group 1A

is known as the alkali metals group. Elements in this group are highly reactive and form alkaline (basic) solutions with water.

All groups have characteristic

properties One of the eight main groups of elements, Group 8A is known as the noble

gas group. Elements in this family have such a low reactivity that they were formerly known as the inert gases.

Although their chemical properties are very uninteresting, their importance to understanding chemistry cannot be understated

Group 7A – the halogens

Reactive nonmetals

Fluorine and chlorine

are gases

Bromine liquid

Iodine solid