Chapter 2:Atoms, Molecules and Ions
2.1 – TheAtomic Theory
• In 500 BC, the Greek philosopher Democritus expressed his belief that all matter is
composed of small, indivisible particles (which he called atomos).
• In 1808, John Dalton generated an atomic theory which defined what we call atoms. The
main ideas of his atomic theory are summarized below:
1. Elements are composed of tiny, indivisible particles called atoms.
2. All atoms of a given element are identical (same size, mass, and chemical properties).
However, the atoms of one element are different from atoms of other elements.
3. Compounds are composed of atoms of more than one element. In any compound, the ratio
of the number of atoms of any two elements will be in whole numbers or fractions.
4. Achemical reaction involves the separation, combination and rearrangement of atoms. It
does not involve the creation or destruction of any atoms.
• Rule 3 supports the law of definite proportions and law of multiple proportions.
o Law of Definite Proportions → Different samples of a compound contain the
elements that the compound is composed of in the same proportions by mass.
o Also, in a compound, the ratios of the atoms of the elements that make up the
compound must be the same.
Every molecule in carbon dioxide must have a 2:1 ratio of O atoms to C
atoms.Also each molecule must be 73% O and 27% C by mass.
o Law of Multiple Proportions → When two elements, A and B combine to form
more than one compound, in which the mass of elementAremains constant, the
ratio of the masses of element B in the compounds will be in whole numbers.
The ratio of O in CO to the O in CO2is 1:2.
• Rule 4 support the law of conservation of mass.
o Law of Conservation of Mass → Matter can neither be created nor destroyed.
In a chemical reaction, the atoms get rearranged to form new compounds.
No atoms are created or destroyed. 2.2 – The Structure of theAtom
• Dalton proposed that atoms were indivisible. However, it was later discovered that atoms
consisted of three subatomic particles: protons, neutrons, and electrons.
• The cathode ray tube experiment, performed by JJ Thomson led to the discovery of the
• When two metal plates are connected to a high voltage source, the negatively charged
plate, called the cathode emits an invisible ray called the cathode ray.
• The cathode ray is drawn towards the positively charged plate called the anode and
passes through a hole in it.
• The cathode ray passes through the anode and then to the end of the tube. Here, it strikes
the fluorescent screen (specially coated surface) and produces a bright light.
• When a magnet (magnetic field) and two electrically charged plates (electric field) are
added to the tube, the cathode ray experienced a magnetic force and an electric force.
o When the magnetic field is on and the electric field is off, the ray deflected toA.
o When the magnetic field is off and the electric field is on, the ray deflected to C.
o When the magnetic and electric fields both were on, the forces exerted by both
fields on the ray got cancelled and the ray continued straight to B.
• JJ Thomson said that since the cathode ray got attracted to the positively charged plate
and repelled the negatively charged plate, the cathode ray consisted of negatively charged
particles he called electrons.
Any moving charged body behaves like a magnet and can interact with external magnetic and
• When a magnet was brought close to the cathode ray tube, the cathode rays deflected
either up or down, depending on the polarity of the magnet. Reversing the polarity caused
the cathode rays to deflect in the opposite direction
• JJ Thomson then used the cathode ray tube to determine the charge to mass ratio of an
electron. He calculated it to be -1.76 × 10 C/g.
2HjM • Between 1908 and 1917, Robert Millikan performed an experiment that helped him to
determine the charge of an electron.
o In his experiment, an atomizer sprayed a fine
mist of oil into the upper chamber.
o Then, some of these oil droplets moved
through a hole in the upper, positive plate and
down into the chamber below. Millikan used
the terminal velocity of the oil droplets to
determine their mass.
o X-rays were used to ionize gas particles in the
second chamber. Electrons lost by the gas
particles combined with oil droplets, giving
them a negative charge.
o When the voltage across the plates increased, the oil drops started to attract to the
positive plate and began to slow down.
o The voltage was adjusted in such a way that the F aed F cougteracted each other,
causing the oil droplets to remain floating in the second chamber.
When this happened, he used the formula qε = mg and solved for the
charge, q. He noticed that all the charges were multiples of -1.6022 × 10 -19
C and concluded that -1.6022 × 10 C is the charge of an electron.
Then he used this value and the charge to mass ratio determined by
Thomson to calculate the mass of an electron, 9.10 × 10 -2g.
• In 1895, Wilhelm Röntgen noticed that cathode rays emit highly energetic radiation
which caused the photographic plates in the cathode ray tube experiment to darken.
• Since the rays of this radiation could not be deflected by a magnet, they were not
composed of charged particles.
o Röntgen called these rays X-rays.
• Antoine Becquerel noticed that thickly wrapped photographic plates started to darken
when exposed to a certain Uranium compound. o Similar to the X-rays that Röntgen discovered, the rays from the Uranium
compound were highly energetic and could not be deflected by a magnet.
However, unlike X-rays, they were emitted spontaneously.
• Any element that spontaneously emits radiation is said to be radioactive.
• Three types of rays can be produced from the radiation emitted by radioactive substances.
o Alpha rays consist of positively charged α particles. Therefore, they are deflected
towards the negatively charged plate.
o Beta rays consist of β particles (electrons). They are deflected towards the
positively charged plate.
o Gamma rays are composed of γ particles, which are electrically neutral. They do
not get deflected towards any of the electrical plates as they are NOT affected by
The Proton and the Nucleus
• In the early 1900s, Thomson’s plum pudding model of the atom was
o In the plum pudding model of the atom, the electrons are
embedded in a uniform positively charged sphere.
o The positive charge of the sphere is enough to cancel the
negative charge of all the electrons.
• In 1910, Rutherford performed a series of experiments that helped him in discovering the
structure of an atom.
o He directed alpha particles at various metal foils (i.e. gold foil). Rutherford
predicted that most of the alpha particles would be slightly deflected as they pass
through the foil since an atom’s positive charge is evenly distributed throughout
the atom in Thomson’s model.
o However, most of the alpha particles went through the foil without getting
deflected, or with a very slight deflection.Avery small number of particles had a
huge deflection or bounced back. • Rutherford proposed a new model of the atom that explained the observations of his
o Most of an atom is empty space.
This explains why many alpha particles were not deflected or only slightly
o All the positive charge and most of the mass are in the centre of the atom called
the nucleus. The nucleus only occupies 1/10 of the atom’s volume.
The few alpha particles that bounced back or had a huge deflection were
directed at the nucleus. The great repulsion force between the alpha
particle and the nucleus caused the particles to deflect greatly or bounce
o The nucleus takes up 1/10 of the volume of an atom.
• One major problem in Rutherford’s model of the atom was that it could not explain why
the ratio of the masses in helium and hydrogen is 4:1.
o Since a hydrogen atom has 1 proton and a helium atom has two protons, the ratio
should be 2:1.
o Rutherford said that the difference in ratio is because the nucleus probably
consists of a third subatomic particle.
• James Chadwick discovered the third subatomic particle, the neutron.
o When he bombarded a thin sheet of beryllium with α particles, a high energy
radiation of electrically neutral particles was
o Chadwick called these neutral particles neutrons
and said that they exist in the nucleus. Neutrons
have a mass slightly greater than protons.
• The three main subatomic particles are protons,
neutrons, and electrons. Protons and neutrons are
located in the nucleus, whereas electrons are located in
orbitals at various distances from the nucleus.
2.3 –Atomic Number, Mass Number, and Isotopes • Atomic Number (Z) = Number of Protons
• The number of protons in an atom determines the element the atom is from.
o Ex:Any atom that contains 9 protons is a fluorine atom
• In a neutral atom, Number of Protons = Number of Electrons
• Mass Number (A) = Number of Protons + Number of Neutrons
Isotopes:Atoms of the same element that have different numbers of neutrons (same atomic
number, different mass number).
o Ex: Hydrogen (1p, 0n), Deuterium (1p, 1n), and Tritium (1p, 2n)
• All isotopes except for the ones
for H are identified by their
o Ex: carbon-12, carbon-13, carbon-14
• Since the numbers of protons and electrons in an atom determine its chemical properties,
all isotopes have the same chemical properties (only the number of neutrons vary
o This means that all isotopes are equally reactive and can form the same
2.4 – The Periodic Table
Periodic Table:Achart in which elements with similar chemical and physical properties are
• In the periodic table, elements are arranged in terms of atomic number.
• Horizontal rows are called periods. There are 7 periods in the periodic table.
• Vertical columns are called groups or families. There are 18 groups in the periodic table.
• The elements in the periodic table can be divided into 3 categories – metals, non-metals,
o Ametal is a good conductor of heat and electricity. The majority of the elements