Atomic Theory Atoms are incredibly small! What we know about them is based on indirect evidence.
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Transcript of Atomic Theory Atoms are incredibly small! What we know about them is based on indirect evidence.
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+ ____