Atomic Theory

• Atoms are incredibly small!• What we know about them is based on

indirect evidence.

History of atomic theory

• 500 BC Democritus proposed idea of atom-but he had no experimental data to support it.

• 1800 Dalton proposed an atomic theory supported by experimental data.

Dalton’s Atomic theory

• Support for his theory came when he began looking at the masses of elements that combined with eachother.

• He found that elements combine in simple ratios by mass.

Dalton’s atomic theory

• Dalton’s atomic theory– all matter is composed of tiny, indivisible

particles called atoms.– All atoms of the same element are identical.– Atoms can neither be created nor destroyed.– Atoms of two or more different elements can

combine in small whole number ratios to form compounds.

Dalton’s atomic theory

• 2 problems with his theory:– Atoms of the same element have different masses – atoms have subatomic particles (electrons, protons

and neutrons)

So Dalton’s theory is not completely correct.

The Subatomic particles

proton 1 amu +1

neutron 1 amu 0

electron “0amu” -1

MASS Charge

The subatomic particles

• Electron discovered ~1900 by JJ Thomson using his cathode ray tube.

• Electron charge found by American Robert Millikan ~1910. Oil drop experiment

• Proton found using adaptation of cathode ray tube ~1900.

• Neutron discovered in 1932 by Chadwick.

Rutherford’s Gold Foil Experiment

• A consequence of the plum pudding model was that the atom would have a weak positive charge.

• Rutherford tested this idea by firing positively charged particles at gold foil.

• He expected particles to pass through.• His results surprised him.

Results of Gold Foil Experiment

• Atom is mostly empty space.• Protons are found in a dense nucleus in the

center of the atom.

Problems with Rutherford’s model

• Could not explain why electrons would not spiral into the nucleus.

Atoms to Elements

• What makes elements different from one another?– The number of protons, electrons and neutrons

in them.– C has 6 protons, 6 neutrons and 6 electrons,

while H has 1 proton, 0 neutrons and 1electron.

Atomic number

• Atomic number: the number of protons in an atom.– Because atoms are neutral we know that

protons must equal electrons and so atomic number is also equal to the number of electrons.

Atomic number also equals the number of electrons in the neutral atom

How many protons in these atoms? How many electrons?

• H• He• Li• Be• B• F• Ne

Mass number

• Mass number- the total number of neutrons and protons in an atom.

• If we know the mass number and the atomic number we can find the number of neutrons in an atom.

Mass number - atomic number = number of neutrons

How many neutrons? How many electrons

Beryllium 4 9

Neon 10 20

Sodium 11 23

Atomic number Mass number

Notation

Cu65

29

Atomic Number

Mass Number

Another notation

Cu-65

Element Mass number

Isotopes• Isotopes- elements with same number of

protons and electrons, but different numbers of neutrons.– For example:

Cu63

29Cu

65

29

How many protons, neutrons and electrons does each Isotope have?

Isotopes

Cu65

29Cu

63

29

Mass #

protons = 29

electrons = 29

neutrons = 65-29 = 36

Atomic #

protons = 29

electrons = 29

neutrons = 63-29 = 34

FAQ: Average Atomic Masses

• “If protons and neutrons weigh 1amu, then how come the periodic table has atomic masses with decimals? How does this happen?”– for example: the 2 isotopes of copper weigh 65

and 63amu’s. In periodic table it says 63.55amu.

Average atomic masses

• The average atomic mass is an average of the masses of the isotopes of an element.– The average atomic mass of Cu is an average of

the two masses of the isotopes: 65amu and 63amu.

– It’s a weighted average that takes into account the relative abundance of each isotope.

Atoms and ions• Atoms are electrically neutral because they

have equal numbers of protons and electrons.– EXAMPLE: Hydrogen has 1proton and

1 electron. They cancel eachother.

• Atoms can gain or lose electrons when they form compounds.• When atoms lose or gain electrons they becomecharged.• Atoms having a positive or negative charge arecalled ions.

Element Symbol Atomic Mass Protons Neutrons Electrons Number Number

Sodium 11 11 11

Iron Fe 26 56

Sn 28

Pd 106

Nickel 33

Atoms and ions

• Sodium, Na, loses an electron when it forms NaCl.

Na11

23 How many electrons in Na?

Atoms and ions

Na23

11

Sodium has atomic number 11, meaning it has 11 protons and if it is neutral it should have 11 electrons.

11 protons + 11electrons = 0 charge

So how many electrons would Sodium have if it lost an electron?

Atoms and ions

Na23

11

If Sodium lost one electron it would still have 11 protons, but now it would have 10 electrons.

11 protons + 10 electrons = +1 charge

We write the positively charged ion, or cation, as Na+

Atoms and ions

Cl35

17

Chlorine gains an electron when itforms compounds.

How many electrons does chlorine have?

Atoms and ions

Cl35

17

Chlorine has atomic number 17, meaning it has 17 protons and if it is neutral it should have 17 electrons.

17 protons + 17electrons = 0 charge

So how many electrons would Chlorine have if it gained an electron?

If chlorine gained one electron it would still have 17 protons, but now it would have 18 electrons.

17 protons + 18 electrons = -1charge

Atoms and ions

Cl35

17

We write the negatively charged ion,

or anion, as Cl-

Write the symbol and name of the ion formed:

A. Strontium atom loses two electrons

B. Iodine atom gains one electron

C. Magnesium atom loses two electrons

D. Oxygen atom gains two electrons

Complete the table

Elementsymbol

Changein

electrons

Formulaof ion

Nameof ion

Al 3 lost ____ ____

S ____ S 2- ____

_____ ____ Sr2+ ____