The Atom

The Building Block of Matter

400 BC Democritus (Greek Philosopher)Atomos = “indivisible”…like a brick is used to build a building; an

atom is used to build all matter…

First Atomic Theory

1808 John Dalton (English Chemist) Using experiments formulates the first

atomic theory1. All elements are composed of indivisible

particles called atoms2. Atoms of the same elements are exactly the

same3. Atoms of different elements are different4. Two or more elements combine to form

compounds

Subatomic Particles

The idea that the atoms was not a solid sphere, but a composition of small particles which came to be known as subatomic particles

Julius Plucker, William Crooks & J J Thomson

mid-1800’s

J J Thomson

1897 Discovers the electron

Ernest Rutherford (English Physicist)

Gold Foil Experiments1908 Discovers that the

atom is mostly empty space

Discovers a hard, dense center of the atom called the NUCLEUS

The Gold Foil Experiment

The atom is mostly empty space

Most of the particles passed straight through to gold foil.

First important discovery by Rutherford

The NUCLEUS is Discovered

1911 Rutherford proves that the particles that do not pass through are hitting a small, dense center of the atom called the nucleus.

Rutherford later goes on to discover the proton

Electron Energy Levels

1913 Niels Bohr (Danish physicist) discovers the electrons travel around the outside of the atom on different energy levels

The Bohr Model = solar system model

The Neutron is Found

1932 James Chadwick proves the existence of the neutron.

Only 4 years later the neutron is used to split the first atom

Why was the neutron’s existence more difficult to prove?

What evidence hinted of its existence?

The Subatomic Particles

Particle Location Mass (AMU)

Charge

Electron Electron Cloud

0 negative

Proton Nucleus 1 positive

Neutron Nucleus 1 neutral

Finding the Number of Subatomic

Particles in an Atom

Atomic Number = the number of protons in the nucleus of an atom

Identifies each elementHydrogen is 1 and it has 1 proton in its

nucleus (only hydrogen has 1 proton)Helium is 2 and it has 2 protons in its

nucleus (only helium has 2 protons)

Electrons

IF the atom is neutral (same number positive charges as negative charges) then the number of electrons will be the same as protons.

ION = a charged atom (unequal number of protons and electrons)

+1 charge = the atom LOST one electron -1 charge = the atom GAINED one electron+2 charge = LOST 2 electrons -2 charge = GAINED 2 electrons and so on

Example of Ions

Sodium will lose one electron when it bondsNa+1 or Na+

Sodium’s atomic number is 11, so it has 11 protons.

The +1 charge means Na+1 has 10 electrons

Another Example

Chlorine will gain an electron when it bondsCl-1 or Cl-

Chlorine’s atomic number is 17, so it has 17 protons

The -1 charge means Cl-1 has 18 electrons

Mass Number

The two subatomic particles that have mass?Proton and neutron, both found in the nucleusMass Number = # of Protons + # of Neutrons

Mass # - Atomic # = # of neutrons

Practice Problems

Symbol Atomic Number

Mass Number

Charge Protons Neutrons Electrons

Mg 12 24 0 12 12 12

K 19 39 0 19 20 19

Na+1 11 23 +1 11 12 10

F-1 9 19 -1 9 10 10

Mg+2 12 24 +2 12 12 10

ISOTOPES

Atoms of the same element with a different number of neutrons.

Ex: carbon-12 & carbon-14 Carbon-12

1. 6 protons2. 6 neutrons3. 6 electrons

Carbon-141. 6 protons2. 8 neutrons3. 6 electrons

Isotopes of Hydrogen

Hydrogen-1 (protium) 0 neutronsHydrogen-2 (deuterium) 1 neutronHydrogen-3 (tritium) 2 neutrons

The ELECTRON CLOUD

The electron cloud is separated by different energy levels.

Electrons with less energy travel on the levels closest to the nucleus.

Energy levels are divided into sub-levelsSub-levels are divided into orbitalsAll of these levels, sub-levels & orbitals can

overlap.

VALENCE ELECTRONS

Valence electrons = the outermost electrons

The electrons involved in bondingDetermines the chemical properties of an

atom

More about Electrons

Maximum # of electrons on each energy level

Level 1 = 2 electronsLevel 2 = 8 electronsLevel 3 = 18 electronsLevel 4 = 32 electrons

the Stable 8

Due to the overlapping levels, sub-levels and orbitals the maximum number of outer most electrons is EIGHT.

An atom is becomes stable when it outer-most energy levels is full of electrons

This explains the chemical bonding properties of the atoms

Atoms will do one of three things in order to fill their outer most energy levels.

1. gain electrons2. lose electrons3. share electrons

Concepts Coming Up Next

The number of valence electrons an atom has determines the group it becomes to on the PERIODIC TABLE.

Groups or families of elements on the periodic table has similar chemical properties.

Valence electrons determine how atoms will bond with other atoms in order to complete their outer-most energy level.

The Stable 8 CHEMICAL BONDING

Ionic bonding (gaining/losing of electrons) Covalent bonding (sharing of electrons)

Concepts Coming Up Next