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Unit 3
Atomic Structure and The Periodic Table
Democritus – 4th century B.C. Greek philosopher who first proposed that matter is composed of indivisible, indestructible particles
called ___________________.
The first atomic model was not proposed until 2000 years later! In the 1700’s, John Dalton performed experiments that related
chemical changes to the individual atom. He developed the following:
Dalton’s Atomic Theory:
1. All elements are composed of ____________________________________________________.
2. Atoms of the same element are __________________. The atoms of any one element are different from those of any other
element.
3. Atoms of different elements can physically mix together or can chemically combine with one another in simple
_____________________________________ to form compounds.
4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element never change into atoms of
other elements during chemical reactions.
An atom is defined as the smallest particle of an element that __________________________________.
Structure of the atom
We now know that atoms are not indivisible. They can be broken down into even smaller, more fundamental particles.
The 3 major subatomic particles:
____________________ are negatively charged subatomic particles.
________________________, an English physicist discovered them in 1897 by doing experiments involving passing an electric
current through a tube containing gases at low pressure. The tube had metal electrodes (cathode and anode) at each end. (see Fig. 5.4
& 5.5).
Thomson found that a beam, called a _________________________, traveled from the cathode (-) to the anode (+), and that this beam
was attracted by a positive charge and repelled by a negative charge. Since he knew opposite charges attract and like charges repel, he
proposed that this cathode ray was a stream of tiny negatively charged particles moving at high speed. He named them electrons.
He experimented with different gases and different metals for the electrodes. He always achieved the same results. This told him that
atoms of all substances contained ______________. He measured the degree of deflection of the cathode ray by charged plates. This
allowed him to determine the ____________________ of the electron (1.76 x 108 C/g).
Further studies by Robert Millikan, an American scientist, led to the determination of the properties of the electron in 1916.
His experiment (Millikan’s Oil Drop Experiment) involved measuring the charge on tiny oil drops. The charge of each drop was
always a multiple of 1.60 x 10-19 C. This was the charge of an individual electron. He used this charge, along with Thomson’s
charge/mass ratio, to determine the mass of an electron.
1.6 x 10 -19 C
x
1g
1.76 x 108 C
= 9.11 x 10-28 g
.
An electron has a charge of ______________ and a mass of ________________ the mass of a hydrogen atom.
Protons: Scientists knew that atoms are neutral (have no charge). So, if there are electrons with negative charge, there must also be
particles with a positive charge. This led to the discovery of the proton.
Protons have a _________ charge and are __________ times more massive than the electron.
Neutrons: Chadwick discovered them in 1932.
Neutrons have no charge, but have essentially the same mass as a proton.
(Refer to Table 5.1, p. 111, for a summary of the properties of electrons, protons and neutrons.)
Once subatomic particles were discovered, Dalton’s model of the atom had to be modified.
Atoms were ______________________________.
The second model of the atom (Thomson’s “Plum Pudding Model”) proposed that _______________ were evenly distributed
throughout an atom filled uniformly with positively charged material.
In 1911, an English scientist, Ernest Rutherford tested this model with his gold foil experiment, Fig. 5.6, p. 111.
He shot a beam of massive alpha particles (He2+) at a very thin sheet of gold foil. He expected the alpha particles to pass easily
through the foil, with little deflection. He was shocked to see that even though most passed through without deflection, a small
fraction was deflected at large angles and some even bounced straight backwards! To explain this, he modified Thomson’s atomic
model.
Rutherford’s model of the atom stated that the atom is mostly __________________________ with all the ____________________
charge and almost all of the _____________ concentrated in a small region, which he called the ________________.
The tiny nucleus is composed of ________________ and __________________.
Atoms of one element differ from atoms of another element because they have different numbers of ____________________________________.
How small is the nucleus? If the atom were the size of a football stadium, the nucleus would be about the size of a marble …10,000
times smaller!
5.3 Atomic number – the number of _________________ in the nucleus of an atom.
The atomic number ___________________ an element.
Ex. What is the atomic number of aluminum?
Which element has 26 protons?
In a neutral atom, the number of protons (p +) equals the number of electrons (e-).
Ex.
How many p+ does a neon atom have?
How many e- does a neon atom have?
An element with atomic number = 6 has how many e-?
Mass Number – The total number of ______________ and ___________________ in an atom.
Ex. oxygen-18 has a mass number of 18.
How many protons, neutrons and electrons does it have?
Isotopes – Atoms that have the same number of _____________ but different numbers of _________________.
When writing chemical symbols for isotopes, the mass number is written as a superscript and the atomic number is written as a
subscript before the element symbol. Ex.
14 C
6
Ex. Hydrogen has three isotopes:
Name:
hydrogen-1
Percent: 99.985%
hydrogen-2
(deuterium)
0.015%
hydrogen-3
(tritium)
negligible
Symbol:
Atomic #:
Mass #:
p+:
e -:
no:
Atomic Mass is given in units called _____________________________________________.
Atomic Mass Unit(u) – One atomic mass unit is defined as 1/12 the mass of a ____________________ atom.
A carbon-12 atom has ______ p+ and _______ n0, and it’s mass is set at 12 u. An atomic mass unit can also be called a
_____________________.
But, the atomic masses on the periodic table are not whole numbers mainly because most elements occur in nature as a mixture of
isotopes. The masses are ________________________________________ reflecting the __________________________ of each
isotope.
Atomic mass =
*when doing weighted averages, change percents to decimal form: ex. Use .55 instead of 55%
Ex. Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu
(59.69%) and 80.92 (40.31%).
Another reason that masses of atoms are not whole numbers is called mass defect. When protons and neutrons come together to form
the nucleus, some of their mass is changed into energy (_______________) to hold the nucleus together. This “lost” mass is called _______________. The energy can be calculated by adding together the masses of the protons, neutrons, and electrons and
subtracting the actual mass of the atom. The missing mass goes into Einstein’s special relativity formula, _____________.
The Periodic Table:
A Russian chemist, ____________________________________ (1834-1907), constructed the first periodic table by listing the
elements in order of increasing atomic _____________ and arranging them according to similarities in their properties. He was able
to predict the physical and chemical properties of missing elements.
In 1913__________________________________, a British physicist, perfected the periodic table by arranging the elements in order
of increasing _____________________________ instead of mass. This is the arrangement of our modern periodic table.
Period – a _____________________________________ on the periodic table. There are 7 periods.
Periodic Law – When the elements are arranged in order of increasing atomic number, there is a periodic repetition of their
________________________________________________________________.
_________________________ – a vertical column on the periodic table. Elements in the same
________________________________ have similar properties. Each group is identified by a number and a letter A or B. Some
versions of the periodic table use numbers 1-18 rather than having A and B.
Representative Elements - _______________________________________
Alkali metals – __________________
Alkaline earth metals – _________________
Halogens – _________________
Noble (or inert) gases – _______________
Group B elements:
Transition metals – Group B elements in the ________ of the periodic table
Inner transition metals (or rare earth elements) – the _____________________ of the periodic table
Classification of elements:
1. Metals - found on left-hand side of periodic table
2.
Nonmetals -found on the right-hand side of the periodic table
3.
Metalloids (or semi-metals) – along the _________________________________.
*properties intermediate between metals and nonmetals
More Subatomic Particles
I.
Leptons (elementary particles)
A.
Electron
B.
Mu-meson (muon) –more massive than electrons
C.
tau-meson (tau) –more massive than electrons
D.
3 types of neutrinos – almost massless
II.
Hadrons (made of quarks)
A.
Mesons of many types
-composed of a quark and an antiquark
B.
Baryons
-composed of 3 quarks of different colors
1. protons (2 up quarks + 1 down quark)
2. neutrons (1 up quark and 2 down quarks)
Every particle has an antiparticle. The antiparticle of the electron is the positron (e+)
Gluons –hold quarks together
6 “flavors” or quarks
1. up
3. top (truth)
5. strange
2. down
4. bottom (beauty) 6. charm
“Colors” of quarks (charge)
+2/3 up, top, charm
-1/3 down, bottom, strange