Chapter 2: Atoms, Molecules, and Ions
2.1 – Conservation of Mass and the Law of Definite Proportions
- Robert Boyle: The first to define an element as a substance which could not be chemically
broken down into anything simpler.
- Joseph Priestley: Prepared and isolated oxygen gas by heating mercury(II) oxide.
- Antoine Lavoisier: Discovered that the products of combustion are equal to the mass of the
o Law of Conservation of Mass: Mass is neither created nor destroyed in chemical
- Joseph Proust:
o Law of Definite Proportions: Different samples of a pure chemical substance always
contain the same proportion of elements by mass.
o Elements combine in specific proportions and not in random proportions.
2.2 – Dalton’s Atomic Theory and the Law of Multiple Proportions
- John Dalton proposed a new theory of matter:
o Elements are made up of tiny particles called atoms.
o Each element is characterized by the mass of its atoms. Atoms of the same elements
have the same mass, but those of different elements have different masses.
o Chemical combination of elements to make different substances occurs when atoms
join together in small whole-number ratios. Fractional parts of atoms are not involved in
o Chemical reactions only rearrange the way that atoms are combined; the atoms
themselves don’t change. Atoms are chemically indestructible.
- Law of Multiple Proportions: Elements can combine in different ways to form different
substances, whose mass ratios are small, whole-number multiples of each other.
o Eg) Comparison of N:O ratios in NO and NO2
2.3 – The Structure of Atoms: Electrons
- Cathode-ray tubes:
o Sealed glass vessel with no air.
o Two thin pieces of metal called electrodes are sealed inside.
o When a high voltage is applied across the electrode, an electric current is sent from the
negatively charged cathode to the positively charged anode.
o If there is still air or gas in the tube, the current can be seen as a cathode ray. o If there is a hole at the end of the anode, a bright spot of light can be seen when the
electric current hits.
- J.J. Thomson proposed that cathode rays must consist of negatively charged particles known as
o This was because the beam was produced at the negative electrode.
o All metals must have electrons since they are used to make many electrodes.
- Thomson believed that the deflection of an electron beam by a nearby magnetic or electric field
depended on three factors:
o The strength of the field: the greater the strength, the greater the deflection.
o The size of the charge on the electron: greater size means greater interaction with the
fields, which results in greater deflection.
o The electron mass: the lighter the electron, the more easily it can be deflected.
- Thomson could determine the ratio of the electrons charge to its mass (1.758820 x 10 C/g).
- R.A. Millikan determined the mass of an electron (m = 9.109382 x 10 ).8
2.4 – The Structure of Atoms: Protons and Neutrons
- Matter is neutral overall, therefore if it could give of negatively charged particles, there must be
positively charged particles as well.
- Rutherford directed positive alpha particles toward a thin gold foil, and found that most went
through, a small amount was deflected at an angle, and a few bounced back toward the source.
o The atom is mostly empty space.
o Must have a concentrated central core, known as the nucleus.
The nucleus contains most of the mass and positive charges.
o Particles were reflected only if it encountered a large positive mass.
- It was soon discovered that the nucleus was composed of both protons and neutrons.
o Protons: positively charged particle with a mass of 1.672622 x 10 g
o Neutrons: carry no charge, and have a mass of 1.674927 x 10 g24
The number of neutrons is not related to the number of protons.
2.5 – Atomic Number
- Elements are different because of the number of protons they are made up of.
o The atomic number is the number of protons of the element, as well as the number of
- Mass number: the sum of the protons and neutrons in an atom.
- Atoms may have different masses depending on the number of neutrons that they have.
- Isotopes: atoms with the same atomic number but a different mass number.
o Mass number is the left superscript, and the atomic number is the left subscript.
o The number of neutrons can be found by subtracting the number of protons from the
o Most isotopes have similar behaviours since neutrons have very little effect on the
atom’s chemical properties. 2.6 – Atomic Mass
- Atomic mass unit (amu) is the unit used in biological work because the values are often much to
small to be convenient.
o 1 amu = 1.660539 x 10
o The atomic mass of an atom is in atomic mass units.
- Atomic mass: the weighted average of the isotopic masses of all its naturally occurring isotopes.
o Atomic mass = (Mass of isotope A)(Abundance of isotope A) + (Mass of isotope
B)(Abundance of isotope B)…etc
2.7 – Compounds and Mixtures
- Matter is classified as either a pure substance or a mixture.
- A pure substance can be an element or a chemical compound.
o Chemical compound: pure substance formed when two or more elements combine to
create a ne