ATOMS, MOLECULES, AND IONS
Some elements exist as molecular substances. That is, hydrogen normally exists as H
molecules, not single hydrogen atoms.
The same is true for N
A compound will always contain the same numbers (and types) of atoms. A given amount of
hydrogen will react only with a specific amount of oxygen. Any excess oxygen will remain
The halogens have a high affinity for electrons, and one important way they react is to form
anions of the type X
. The alkali metals tend to give up electrons easily and in most of their
compounds exist as M
: These two very reactive groups are only one electron
away (in the periodic table) from the least reactive family of elements, the noble gases.
Law of conservation of mass: Mass is neither created nor destroyed. The total mass before a
chemical reaction always equals the total mass after a chemical reaction.
Law of definite proportion: A given compound always contains exactly the same proportion
of elements by mass.
For example, water is always 1 g H for every 8 g oxygen.
Law of multiple proportions: When two elements form a series of compounds, the ratios of
the mass of the second element that combine with 1 g of the first element always can be
reduced to small whole numbers: For CO
and CO discussed in Section 2.2, the mass ratios of
oxygen that react with 1 g carbon in each compound are in a 2 : 1 ratio.
The smaller parts are electrons and the nucleus. The nucleus is broken down into protons
and neutrons, which can be broken down into quarks. For our purpose, electrons,
protons are the key smaller parts of an atom.
All atoms of hydrogen have 1 proton in the nucleus. Different isotopes of hydrogen have
0, 1, or 2 neutrons in the nucleus. Because we are talking about atoms, this implies a
neutral charge, which dictates 1 electron present for all hydrogen atoms. If charged ions
were included, then different ions/atoms of H could have different numbers of electrons.
Hydrogen atoms always have 1 proton in the nucleus, and helium atoms always have 2
protons in the nucleus. The number of neutrons can be the same for a hydrogen atom and
a helium atom. Tritium (
He both have 2 neutrons. Assuming neutral atoms, then
the number of electrons will be 1 for hydrogen and 2 for helium.
d. Water (H
O) is always 1 g hydrogen for every 8 g of O present, whereas H
is always 1
g hydrogen for every 16 g of O present. These are distinctly different compounds, each
with its own unique relative number and types of atoms present.