CHAPTER 2 -ATOMS
3 CLASSIFICATIONS OF MATTER:
1) ELEMENT – Pure substances that cannot be broken down
any further by chemical means.
(composed of only 1 type of atom)
i.e. copper, mercury
Periodic Table – produced by Dimitri Mendeleeve (1869) elements arranged in order by atomic number
Period: row in periodic table
 118 known elements
 88 naturally occurring elements
 99% of earth and our body composed of only 10
elements (oxygen O2 most abundant)
 4 most abundant elements necessary for life: H O N C
Chemical Symbol – abbreviation for name of element
i.e. C = carbon
Ne = neon
Na = sodium
Element Family or Group (column): similar properties
i.e. alkali metals (reactive), halogens (colored), noble gases
(non-reactive)
Element Classification: metals, non-metals, metalloids
Metals: shiny, ductile, malleable, conductors of heat/ electricity
Non-metals: dull, poor conductors of heat & electricity
Metalloids (semiconductors): properties of metals & non-metals
Monotomic, Diatomic, Polyatomic Elements
 Most are monatomic (Na, Pb, Ca)
 7 are Diatomic (natural state is a diatomic molecule)
H2, O2, N2, Cl2, Br2, I2, F2
 Polyatomic
(P4, S8)
2) COMPOUND – Pure substances made up of 2 or more
elements in a fixed ratio.
(composed of one type of molecule)
i.e. salt, water, caffeine
3) MIXTURE – Not pure substances (composed of 2 or more
different types of atoms or molecules in variable proportions)
A) Homogeneous – uniform composition; i.e. air, salt water
B) Heterogeneous – compositions varies in 2 or more regions
i.e. oil & water
Methods to Separate Mixtures: (filtration, chromatography,
distillation, etc.)
DIVISIONS OF MATTER
MATTER
(has mass and occupies space)
PURE SUBSTANCE
(composed of only one kind of matter)
ELEMENTS
(composed of only one kind of atom)
COMPOUND
(composed of only one kind of molecule)
MIXTURE
(composed of several kinds of matter)
HOMOGENEOUS
(same composition)
HETEROGENEOUS
(composition varies)
ATOMS
ATOM – the smallest particle of an element that shows the
chemical behavior of that element.
Atomic Theory - History
470 BC – Democritus, Greek Philosopher
“Everything is made of small particles, he called atoms.”
1805 – John Dalton, English Chemist
1. All matter made up of tiny indivisible particles called atoms.
2. All atoms of the same element are identical to each other.
3. Atoms of different elements are different.
4. Atoms combine to form molecules. A molecule is a tightly
bound group of atoms that functions as a unit.
1780 - Lavoisier, French Chemist
Law of Conservation of Matter - Matter can neither be created
or destroyed.
CO +
PbO
CO2 +
Pb
1780 - Proust, French Chemist
 “Water is water wherever you go.”
 89% O and 11% H for water
Law of Constant Composition - Any compound is always made
up of elements in the same proportion by weight.
COMPOSITION OF ATOM
(Atomic Structure)
3 Basic Particles:
Protons, Electrons &Neutrons
Protons (p) – positive charge (+1); 1 amu; located in
nucleus
Electrons (e-) – negative charge (-1); 1/1835 amu; located
in cloud outside nucleus
Neutrons (n) – neutral; 1 amu ; located in nucleus
Atoms are neutral, so # of protons = # of electrons
MASS NUMBER
The number of protons plus neutrons
(p + n)
What is the mass number of an atom with 12 protons, 12
electrons, and 13 neutrons?
Ans =
Electrons are not included in the mass number.
ATOMIC NUMBER
 The number of protons (p)
 Determines the identity of an element
What is the atomic number for helium? How many
protons? How many electrons?
Ans.:
What element has 12 electrons, 12 protons and 13
neutrons?
Ans.:
IONS – Atoms or group of atoms that have lost or gained
electrons
 Have unequal number of protons and electrons
 Have charges (+ or -)
i.e. Potassium (K) has: 19 protons
19 electrons
if lose one electron, 18 electrons, 19 protons, charge is
+1
write as K+
What is the symbol for an ion with 10 electron and 12
protons?
Ans.:
ISOTOPES
 Atoms of the same element with the same # of protons,
but different # of neutrons (different mass numbers)
i.e C-11, C-12, C-13, C-14
 Most elements have isotopes
i.e. Cobalt-60 (for cancer radiation treatment)
How many neutrons does Co-60 have?
Ans.
ATOMIC WEIGHT
The weighted average of the masses of naturally occurring
isotopes
Example:
What is the exact atomic weight? given the following:
Isotope
% Abundance
Exact Atomic Mass
C-12
C-13
98.89%
1.11%
12.00 amu
13.00 amu
Calculation:
C-12:
98.89/100.00 x 12.00 amu =
C-13:
1.11/100.00 x 13.00 amu =
_____amu
+
_____amu
=
amu
ELECTRONIC STRUCTURE OF ATOMS
I)
SHELL (ENERGY LEVEL)
A) SUBSHELL (located in shell)
1) ORBITAL (in subshell)
SHELL (ENERGY LEVEL)
 7 Energy (E) levels surround nucleus
“like layers of an onion”
 max # of e- per E level: 2n2; where n = E level
ORBITAL
 Orbitals are electron clouds
 Each orbital can hold up to 2 electrons
 Orbitals are named with # (energy level: 1-7) and letter
(shape: s, p, d, f) s = sphere, p = dumbbell
i.e 1s, 2s, 2p, 3s, 3p, 4s
SUBSHELL
 All the orbitals having the same energy level and the
same letter
i.e. for energy level = 2
orbital
2px 2py 2pz
2s
subshell
shell (energy level)
Electron Configuration – tells us where the electrons are
located for specific elements
Rules for Writing Electronic Configurations:
1. Lowest energy first (AUFBAU). (1s, 2s, 2p, 3s, 3p, 4s)
2. Only 2 electrons per orbital (Pauli Exclusion).
Opposite spins
Lithium: ____ electrons
Orbital Box Diagram:
1s
2s
Li electron config.:
____________
3. Don’t pair them until you have to (Hund’s Rule).
Nitrogen: ___ electrons
Orbital Box Diagram:
1s
2s 2px 2py 2pz
N electron config.: _____________
Write orbital box diagrams and electron configs. for:
Element #of e-
Orbital Box Diagram
He
Na
K
Noble Gas Electron Configuration:
He
Na
K
[He]
e- config.
Valence Electrons – electrons in the outer most shell (the
one with the highest shell #)
Core Electrons – electrons in the inner shells
i.e. Phosphorus (P) electron configuration
1s22s22p63s23p3
Core e-
valence e-
Lewis Dot Structures – show valence electrons as dots
and element symbol represents nucleus and core electrons
i.e.
..
.P.
.
Group # corresponds to # of valence electrons
Group
No.
Lewis
Structure
1A
2A
3A
.
.
Be.
.
Li
.
B.
4A
.
.
C.
.
5A
6A
7A
8A
.
N:
.
..
O:
..
:F:
..
:Ne:
.
.
..
.
.
Write the Lewis dot Structure for silicon.