Atoms: Building Blocks of Matter

Physical Science

Chapter 4

An Atomic Model of Matter

• Scientists have tried to determine the composition of matter for 2400 years using various methods

o Indirect evidence

• Evidence gathered without actually seeing or touching the object

oModel

• A model is a representation of evidence & ideas that explains facts or a concept

The Greek Model • Democritus devised a model with the

following ideas:

oMatter could not forever be divided into smaller pieces.

o The smallest piece possible was an atom

oAtoms were all made of the same material but different sizes and shapes

oAtoms are infinite in number, constantly moving and joining together

Dalton’s Model

• John Dalton, an English chemist, conducted many experiments and came up with the following theory:

oAll elements are composed of atoms.

oAtoms are indivisible & indestructible

oAtoms of the same element are alike

oAtoms of different elements are different

oCmpnds are formed by the joining of elemental atoms

Thomson’s Model • J.J. Thomson conducted experiments (passing

electric current thru gas, causing negatively

charged particles to escape).

• …..led him to believe atoms were made of

smaller particles.

• Thomson’s Model:

o The atom is made up of pudding like

positively charged material in which

negatively charged electrons are

scattered like plums in a pudding (or

chips in a cookie).

Rutherford’s Model • Ernest Rutherford experimented with shooting

positively charged rays through gold foil.

• Some of the positive charges bounced off the gold foil rather than passing through it.

• He developed a new theory on the structure of the atom.

• Rutherford’s theory stated: o The atom has a

small, dense positively charged center.

o Negative electrons scattered around nucleus on edges of atom.

The Bohr Model • Niels Bohr, a Danish

scientist, proposed & tested that negatively

charged e- were held by attraction to the + nucleus.

• Bohr’s Model stated:

oElectrons moved in orbits around the nucleus.

o These orbits were at certain distances from the nucleus depending upon the energy of the electrons.

The Wave Model • Today’s atomic model is

based on wave mechanics.

• The wave model states:

o Electrons do not travel around the nucleus in

a specific path, but can be found in an area

called the electron cloud.

oAn atom has a small positively charged

nucleus surrounded by a large region in

which enough electrons are located to

make the atom neutral.

Structure of an Atom An atom is made up of 3 subatomic particles: protons, neutrons, & electrons which have

consistent structure in all atoms. • Protons: positively charged particles in the nucleus

• Neutrons: neutrally charged particles in the nucleus

• Electrons: negatively charged particles surrounding the nucleus

• Neutral atoms have equal #s of protons & electrons, so that positive and negative charges balance.

Subatomic particle masses • The mass of both protons & neutrons is

one amu (atomic mass unit)

• Electrons have almost no mass.

Atomic Number

• The difference between elements is the number of protons they have in their nucleus.

• The number of protons in the nucleus is called the atomic number.

• The number of protons in an element always stays the same, however the number of neutrons can vary.

Mass Number and Atomic Mass

• All atoms have a mass number.

• The mass number is equal to the number of

protons + the number of neutrons.

• Any sample of an element that occurs in

nature contains a mixtures of isotopes.

• Therefore, the atomic mass is an average of

the atomic masses of all the existing isotopes

of a particular element.

• An element with a different number of

neutrons (but same proton #) is called an

isotope.

Electron Cloud • The electron cloud is the area around the

nucleus in which the electrons can be found.

• Electrons whirl around the nucleus billions of time in a second, however their movement is not random.

• Electrons are locked into an orbit a specific distance from the nucleus depending on how much energy they have.

Energy Levels • Distance an electron is

from the nucleus is called

it’s energy level.

• Each energy level can

hold a specific number of

electrons.

• The arrangement of the electrons in the

energy levels accounts for how an atom

bonds with other atoms.

• Levels farther from the nucleus have

more energy

Arranging the Elements

• Russian chemist Dmitri Mendeleev was the first scientist to arrange the elements in a logical manner.

• He classified the elements according to chemical and physical properties & then put them in order of increasing atomic mass.

• This led to a table of elements that increased by atomic mass and were arranged in columns of similar characteristics.

Predictions

• Mendeleev found that there were holes in his

periodic table of elements.

• Mendeleev predicted that elements would be

discovered that would fit into the holes in his

table of elements.

• Within his lifetime, three of the elements that

Mendeleev predicted were discovered.

The Modern Periodic Table

• Mendeleev’s table had some problems.

• Some elements did not fit the

characteristics of the columns where

they were located.

• Mendeleev’s table was refined by Henry

Moseley after the discovery of atomic particles.

• Moseley arranged the elements according

to atomic number (# of protons) instead of atomic mass (protons + neutrons).

Periodic Law • Moseley’s table was based on periodic law,

which states, “the physical and chemical properties of the elements are a function of their atomic number.”

Periodic Table Design • The periodic table is a

classification system that organizes

the elements in a logical, usable

and meaningful way.

• The periodic table is designed so

that it is easy to predict an

element’s physical and chemical

properties.

Columns of the Periodic Table

• The columns in the periodic table

represent groups or families of

elements.

• Families of elements have similar

chemical and physical properties.

• BECAUSE each atom of these

elements have the same number of

electrons in the outer shell (valence).

Rows in the Periodic Table • Each horizontal row in the periodic table

represents a period.

• Elements in a period do not have similar properties.

• Elements in a period always increase by one proton or one electron from left to right.

• There are seven periods of elements.

• The rare earth elements are pulled out of their rows, however and are listed below the table.

Element Key • Important information about the

elements is given in each square

of the periodic table including: oAtomic number

oChemical Symbol

oName

oAtomic Mass

• Practice using the Periodic Table

Forces within the Atom not in ch 4, but impt

• The atom is held together by four forces.

• The four forces account for the behavior of the subatomic particles.

• The four forces are:

oElectromagnetic Force

oStrong Force

oWeak Force

oGravity

Electromagnetic Force • The electromagnetic force can either attract or

repel particles.

• Particles with the same charge are repelled.

• Particles with different charges are attracted.

• Electrons are kept orbiting the nucleus cause they

are attracted to positively charged protons.

• Protons in nucleus, however, are repelled by

each other.

Strong Force • The strong force glues protons (and

neutrons) together to form the nucleus.

• The strong force only works when

protons are close together.

• The strong force is the strongest of the

four forces.

Weak Force • The weak force is responsible for the

process of radioactive decay.

• The weak force allows a neutron in the

nucleus to change into 1 proton and 1

electron.

• Radioactive decay is what powers the

sun.

Gravity

• Gravity is the force of attraction

between all objects that have mass.

• The effect of gravity on the nucleus is

not fully understood

and is the weakest

of the four forces.