Models of Matter A model is a tentative description
of a system or theory that accounts for all of its known properties
Models are invented (usually) to interpret the invisible and relate experimental results
The Four Elements
An Ancient Greek model of the atom was based on logic
Matter is composed of four elements: air, fire, earth and water
An atom is a single indivisible particle
John Dalton’s Atomic Theory (1803)
Based on experimentation Experimental data showed: Elements combine in the same percent
by mass each time the same compound is formed
Atoms of different elements combine in fixed proportions
Dalton’s Theory
1. All matter is made of small spherical particles called atoms
2. Each element is made of its own type of atom
3. Atoms of different elements have different properties
4. Atoms of different substances can combine in constant ratios to form new substances
5. Atoms cannot be created or destroyed during chemical or physical changes
6. Atoms can neither be subdivided nor changed to one another
Dalton’s Theory Continued…
Law of Constant Composition: Atoms of different elements can be
chemically combined in a fixed whole number ratio to form compounds. Example: H2O
Law of Conservation of Mass: The mass of the reactants in a chemical
reaction is equal to the mass of the products
Thomson’s Atomic Theory (1897)
Used a cathode ray tube to energize electrons
Experimental data showed: Charged particles originated from the
gas inside the tube Charged particles are deflected away
from a negatively charged object, but towards a positively charged object.
Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive end.positive end.
Thomson’s ExperimentThomson’s Experiment
Voltage source
+-
Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field, he found that By adding an electric field, he found that the moving pieces were negative the moving pieces were negative
+
-
Thomson’s Model
Found the electron. Couldn’t find
positive (for a while).
Said the atom was like plum pudding.
A bunch of positive stuff, with the electrons able to be removed.
Atoms are composed of smaller particles. These particles are the same for all
different types of atoms. These particles are negatively charged
and are called electrons. Electrons are embedded throughout the
uniform sphere of positive charge to make up a neutral atom.
Matter is naturally neutral except when it gains or loses electrons
“Plum Pudding Model”
Rutherford’s Gold Foil Experiment (1911)
Rutherford used very small positive particles (alpha particles) and a fluorescent screen detector to see the particles
Since the mass is evenly distributed in gold atoms alpha particles should go straight through.
Gold Foil Experiment
Experimental data showed: Alpha particles (+ve) were shot at
a thin sheet of gold Most alpha particles passed
through unaffected Some alpha particles were
deflected at large angles. Some are even reflected back.
Atom is mostly empty space Small dense positive region at the
center (a nucleus?) Alpha particles are deflected by this
region if they get close enough. Electrons surround but do not fill
the rest of the atom
Rutherford’s Theory…
Modern View
The atom is mostly empty space.
Nucleus contains protons and neutrons.
Electron cloud- region where you might find an electron.
Symbolic Representation of Sub Atomic Particles
Name Symbol Charge Mass Location
proton p+ +1 1 a.m.u. nucleus
neutron n 0 1 a.m.u. nucleus
electron e- -1 1/1837 a.m.u.
energy levels around nucleus
Sub-atomic Particles
Z - atomic number = number of protons determines type of atom.
A - mass number = number of protons + neutrons.
Number of protons = number of electrons if neutral.
Number of neutrons = atomic mass – atomic number
IONS Charged atoms formed by a neutral
atom to attain a stable structure on the last shell (octet or doublet)
Two types of ions: Positive ions (cations) are formed
when an atom loses electrons Negative ions (anions) are formed
when an atom gains electrons
Ionic symbols
Cation Anion
X+N X-N
X = Symbol X = Symbol(+) = losing (-)= gainingN= # of electrons lost N= # of electrons
gained
Subatomic particles for ions
Cations Anions
protons p+ = Z p+ = Z
electrons e- = Z - N e- = Z + N
neutrons n = A - Z n = A - Z