Download Atomic Structure Notes

Elements, Atoms, and Ions
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small particles
called atoms.
2. All atoms of a given element are identical, having the
same size, mass and chemical properties.
3. The atoms of one element are different from the atoms
of all other elements.
4. Atoms of one element can combine with atoms of other
elements to form compounds. A given compound
always has the same relative numbers and types of
atoms.
5. Atoms are not created or destroyed in chemical
reactions. A chemical reaction simply changes the way
the atoms are grouped together.
2
Each water
molecule has 2
hydrogen
atoms (2 g)
and 1 oxygen
atom (16 g).
The mass ratio
of oxygen to
hydrogen is
always 8:1.
16 X
+
8Y
8 X2Y
J.J. Thomson, measured charge/mass of e(1906 Nobel Prize in Physics)
Measured mass of e(1923 Nobel Prize in Physics)
e- charge = -1.60 x 10-19 C
Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
Rutherford Experiment (1902)
(Uranium compound)
Shown to be incorrect by Rutherford’s experiments
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
Elements, Atoms,
Molecules, Ions and
Isotopes
Rutherford’s Model of
the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
What are atoms?
• Atoms are the smallest particle of an
element that retains the chemical identity of
that element.
• Atoms are composed of protons, neutrons,
and electrons.
– Even the subatomic particles are made up of
particles themselves.
Subatomic Particles
Mass
(g)
Particle
-
-28
Charge
(units)
-19
-1
1.67 x 10-24 +1.6 x 10-19
+1
Electron (e ) 9.1 x 10
Proton (p+)
Charge
(Coulombs)
Neutron (n) 1.67 x 10-24
-1.6 x 10
0
0
mass p = mass n = 1840 x mass e-
Units of Mass
• Particles’ masses measured in grams are
extremely small.
• For convenience, atomic mass units are
used to express masses instead.
• 1 AMU = 1/12 the mass of a Carbon-12
atom
– Scientists massed a sample of carbon-12,
determined the mass of a single carbon-12
atom, and divided it by 12 to find the mass of
each proton and neutron.
Subatomic Particles
Particle
Mass Charge
Location
proton
1 AMU
+
in the nucleus
neutron
1 AMU
none
in the nucleus
-
surrounding the
nucleus
electron
0 AMU
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
U
(T)
Isotopes are atoms of the same element (X) with
different numbers of neutrons in their nuclei
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 116
6 protons, 5 (11 - 6) neutrons, 6 electrons
C?
A molecule is an aggregate of two or more atoms in a
definite arrangement held together by chemical bonds
H2
H2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
Do You Understand Ions?
+
27
3
How many protons and electrons are in 13 Al ?
13 protons, 10 (13 – 3) electrons
2- ?
Se
How many protons and electrons are in 78
34
34 protons, 36 (34 + 2) electrons