The ATOM

Atoms

the basic building block of all matter the smallest particle of an element that

retains the properties of that element electrically neutral: the number of positive

charges (protons) equals the number of negative charges (electrons)

composed of 3 subatomic particles: protons (p+), electrons (e-) and neutrons (no)

Models of the Atom - History

Democritus a fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called atoms (“atoma” - Greek for indivisible).

John Dalton (1803)- Billiard Ball

John Dalton (1803)

Billiard Ball Model  viewed the atom as a small solid sphere.  He really

got the "ball" rolling for modern chemistry! Each element was composed of the same kind of

atoms. Compounds are composed of atoms in specific

ratios. Chemical reactions are rearrangements of atoms

(mass is conserved).

J. J. Thomson (1897) – Plum Pudding

J. J. Thomson (1897)

Plum Pudding Model proposed that the atom was a sphere of

positive electricity with negative particles imbedded throughout after discovering the electron

a discovery for which he was awarded the Nobel Prize in physics in 1906.

Ernest Rutherford (1908) - Nuclear

Ernest Rutherford (1908)

discovered that the atom is mostly empty space with a dense positively charged nucleus surrounded by negative electrons. 

Rutherford received the Nobel Prize in chemistry in 1908 for his contributions into the structure of the atom. 

Neils Bohr (1913)

Neils Bohr (1913)

proposed that electrons traveled in circular orbits and that only certain orbits were allowed. 

He received the Nobel Prize in physics in 1922 for his theory.

Quantum Mechanics Model

Quantum Mechanics Model

Electron Cloud Model (1920's) an atom consists of a dense nucleus

composed of protons and neutrons surrounded by electrons that exist in different clouds at the various energy levels. 

Erwin Schrodinger and Werner Heisenburg developed probability functions to determine the regions or clouds in which electrons would most likely be found.

A combination of all models…

Drawing Bohr Diagrams

Atomic number:

identifies the element equal to the number of protons in the nucleus since atoms are electrically neutral, # of protons = # of

electrons

 

Mass number: # of protons + # of neutrons # neutrons = atomic mass (rounded off) – atomic # Protons and neutrons account for most of the mass of the atom 

Bohr Diagrams (continued)

an energy level represents a specific value of energy of an electron and corresponds to a general location around the nucleus

the number of occupied energy levels in any atom is normally the same as the period number in which an atom appears

for the first 3 energy levels, the maximum number of electrons that can be present are 2, 8 and 8 in order of increasing energy (increasing distance from the nucleus)

a lower energy level is filled with electrons to its maximum level before the next level is started.

The electrons in the highest (outermost) occupied energy level are called valence electrons. Number of valence electrons is the same as the group number for group A elements (1,2,13-18).

 

Bohr Diagrams (continued)

Draw a circle for the nucleus Put in the number of protons and neutrons

inside the circle. Determine the number of electrons and place

them in energy levels starting closest to the nucleus and filling them in the order of 2, 8, 8.

Stable Atoms

have low chemical reactivity include noble gases, all of which have 8 valence

electrons (except He, which has 2) stability is a function of having a full

complement of valence electrons. Atoms that do not have full electrons energy levels

are unstable and must gain, lose or share electrons to become stable.

other atoms can become more stable by reacting and changing the number of their electrons

Stable Atoms

atoms can follow one of 2 rules: a) Octet rule - atoms attempt to obtain

8 valence electrons - includes most atoms b) Duet rule - atoms attempt to obtain

2 valence electrons - includes H, Li and Be Atoms can achieve a stable octet or duet by

forming ions.

Ions

an atom or groups of atoms that have a positive or negative charge, due to the loss or gain of one or more electrons.

Single atoms form simple ions (monatomic ions); groups of atoms form complex ions (polyatomic ions)

Example:

sodium metal and chlorine gas react to produce NaCl, a very stable and unreactive substance, compared to Na or Cl.

The sodium atom loses 1 electron to the chlorine atom so both of their outer levels are filled. In doing so, the atoms form ions of opposite charge.

Sodium

Sodium

Chlorine

How ionic bonds form:

Begin with atoms of two different elements that do not have 8 electrons in their outer most energy level.

Sodium + chlorine

The sodium atom (Na ) donates 1 electron becoming a positively sodium ion ( Na+).

The chlorine atom (Cl) accepts the donated electron becoming a negatively charged chloride ion (Cl-).

Sodium + chlorine