BASIC CHEMICAL DEFINITIONS
Chemists make a distinction between substances and mixtures.
Substance – pure, containing only one type of particle. A particle could be an element or a compound.
Element – one type of atom
Compound – one type of molecule or crystal
Mixture – contains 2 or more types of particles that have not been chemically
combined
Hetero means
different.
Mixtures fall into two categories based on the distribution of particles:
 Heterogeneous mixture- a mixture in which there is an uneven
Homo means
distribution of materials
o sand in water
same.
o tossed salad
o oil in water
 Homogeneous mixture – a mixture in which all of the particles are evenly distributed
o One example is salt water
o Homogeneous mixtures are commonly referred to as solutions. In a
solution one type of particle spreads out to fill the spaces between
Soluble means able
the particles of a second substance.
to dissolve
o A homogeneous mixture has a solute and a solvent
 Solute – substance that is being dissolved
Insoluble means not
 Solvent – substance that is used to dissolve a second
able to dissolve
substance
 Most common solvents are water and alcohol.
 In salt water salt is the solute and water is the solvent
Matter
Mixture
Substance
1 kind of particle
more than 1 kind
of particle
Element
Compound
Heterogeneous
Homogeneous
Oxygen (O2)
(Water H2O)
(Sand in water)
(Salt water)
HISTORY OF OUR UNDERSTANDING OF ATOMIC STRUCTURE:
GREEK PHILOSOPHERS:
460 – 380 BC DEMOCRITUS:
He came to the conclusion that matter could not be divided up into smaller and smaller pieces forever.
Eventually the smallest possible piece would be obtained, he called this particle the atom. The word
atom comes from the Greek word atomos which means “not to be cut or indivisible”.
384 – 322 BC ARISTOTLE:
All things are made up of 4 elements, earth, air, fire and water. The
proportions of these elements present determines something’s properties.
The followers of Democritus, called the Atomists, debated Aristotle and lost
the debate resulting in the acceptance of Aristotle’s ideas for the next 2000
years.
DEVELOPMENT OF SCIENTIFIC CONCEPTS
1766 – 1844 JOHN DALTON:
Was an English chemist who based on experimentation and a review of the scientific findings of the time
came up with a new concept of the atom, based on 4 atomic laws:
1. All elements are composed of atoms. Atoms are indivisible and indestructible particles. (He
gave credit to Democritus for this idea.)
2. There are as many different kinds of atoms as there are elements. Atoms of the same element
are exactly alike. Atoms of different elements are different.
3. Compounds are formed by the joining of atoms of two or more elements. The smallest particle
of a compound is called a molecule.
4. Chemical changes involve whole atoms or groups of atoms, but never fractions of atoms.
Dalton’s model became the foundation of modern chemistry but eventually modifications were
required.
1897 – JOSEPH J. THOMSON
The first to show atoms were made of smaller particles.
He was responsible for finding the electron (negative
charge).
He described the already known neutral atom to be a
pudding-like positively charged material throughout which
negatively charged electrons were scattered,
like plum-pudding with raisins.
1903 – ERNEST RUTHERFORD:
Ernest Rutherford, English physicist in 1908. While conducting an
experiment involving firing a stream of tiny positively charged
particles at a thin sheet of gold foil, he discovered most of his
positively charged “bullets” passed right through the gold. How could
this be the case if there were a lot of scattered protons? (only a few
bounced back, straight back). He realized the atom’s positive charged
particles were contained in the nucleus. The negatively charged
electrons were scattered outside the nucleus at the atom’s edge.
(Discovered protons)
1913 - NIELS BOHR:
Electrons move in definite orbits around the nucleus, much
like planets circle the sun. These orbits, or energy levels, are
located at certain distances from the nucleus.
Each orbit has a limited number of electrons that can occupy
that orbit. These limits are:
1st orbit = 2 e2nd orbit = 8 e3rd orbit = 18 e4th orbit = 32 e5th orbit = 32 e6th orbit = 18 e7th orbit = 8 eWith a special additional limit that no outer most orbit can
hold more than 8 e-.
1932 – JAMES CHADWICK
Discovered neutrons.
ATOMIC MODEL (BOHR-RUTHERFORD MODEL) AS OF 1932:
QUANTUM THEORY (ELECTRON CLOUD THEORY)
Based on their energy and on probability electrons occupy a cloud-like region about the nucleus in
which they will most likely
ATOMIC STRUCTURE
What are the three main subatomic particles?
99.9% of atomic mass is in nucleus.
Proton, neutron, electron
PROTONS:



positively charged particles in nucleus
The mass of a proton is 1 amu. (atomic mass unit)
# of protons never changes. All atoms of the same element have the same # of protons.
NEUTRONS:



found in the nucleus of the atom
neutral in charge.
slightly more mass than protons but considered to have a mass of 1amu.
ELECTRONS:





a negative charge.
located outside the nucleus
Each level has different amounts of energy
Electrons with lowest energy level are closest to the nucleus.
Mass of an electron is 1/1836 amu
ATOMIC NUMBER:




The number of protons in the nucleus
Determines what the element is.
i.e- an atom of carbon has 6 protons in its nucleus.
Find the atomic numbers on a periodic table. Found on the bottom left corner
ISOTOPES:


atoms of the same element with same number of protons but different numbers of neutrons
The atomic number is same for isotopes of an element.
ATOMIC MASS:




the sum of the protons and neutrons in nucleus.
average mass of all the isotopes of that element
it is usually not a whole number, because it is an ave.
For example, the atomic mass of carbon is 12.011. Found in upper left corner, usually a decimal
Isotopes & Symbols
TWO DEFINITIONS
1. Atoms whose nuclei contain the same # of protons, but a different # of neutrons
2. Atoms with the same atomic number, but different mass numbers.
Example:
 Hydrogen – 1
 Hydrogen – 2
 Hydrogen – 3
1 p+ 0 no Atomic Number = 1, Mass Number = 1
1 p+ 1 no Atomic Number = 1, Mass Number = 2
1 p+ 2 no Atomic Number = 1, Mass Number = 3
All are isotopes of Hydrogen. The number following the name is the Mass Number for the isotope.
Differences in mass have little effect on an atom’s chemical and physical properties. However, different
isotopes will have very different nuclear properties.
Example:
 Uranium – 235 when hit by a neutron splits to form Krypton, Barium and 3 neutrons while
releasing large amounts of energy.
 Uranium – 238 when hit by a neutron absorbs the neutron to become Uranium – 239.
ISOTOPIC SYMBOLIsotopic symbols are a shorthand way to tell the mass number and atomic number of one atom of an
element.
Mass #
Chemical Symbol
Atomic #
Basic Format:
17
O
8
Example 1:
Example 2:
#p+ = 23, #n0 = 28
Atomic # = 23, Mass # = 51, #e- = 23, Element is Vanadium
51
V
Isotopic Symbol is
23
Step to Drawing Atoms:
EXAMPLE 1: 21𝐻
1.
Draw a circle to represent the nucleus
2.
Enter the number of protons and neutrons into the nucleus
3.
Draw a circle to represent each orbit needed around the nucleus (the number of
orbits needed is equal to the number of the row the element being drawn is in on
the Periodic Table)
3. Enter the electrons on each orbit starting from the orbit closest to the nucleus and
working out from there. Be careful to obey Bohr’s limits while adding electrons. (2,
8, 18, Max. 8 in the outer)
EXAMPLE 2:
29
SI
14
Step 1:
Finished model:
Step 2:
Step 3:
or