Cracking the SAT Chemistry Subject Test

For the SAT Chemistry Subject Test, you have to know certain things about the
elements and other information you see on the periodic table.

Atomic Number

Look at any element on the periodic table. Above every element’s symbol is a
whole number that represents the element’s atomic number. The atomic number
of an element tells you the number of protons in the nucleus of an atom of that
element. The number of protons in the nucleus of an atom gives that atom its
identity. Oxygen, for instance, has the atomic number 8; an atom of oxygen has
8 protons in its nucleus. If we take a proton away from an oxygen atom, it would
have only 7 protons in its nucleus, and it wouldn’t be oxygen anymore; it would
be a nitrogen atom. Any atom with 7 protons in its nucleus is nitrogen.

What if an oxygen atom loses an electron but not a proton? Well, as long as the
atom has 8 protons in it, it’s still oxygen. If it has only 7 electrons, then it’s a
positively charged oxygen ion, and if it has 9 electrons, then it’s a negatively
charged oxygen ion. But as long as the atomic number—the number of protons
in the nucleus—doesn’t change, the element doesn’t change either: It’s still
oxygen.

Atomic  number: number  of  protons in  nucleus;    identity    of  element

Group Number

The group number refers to the number above each column of the periodic table,
such as 1A, 2, 4, etc. Elements in the same group have the same number of
electrons in their outer shells and share many properties. Carbon (C), Silicon
(Si), and Germanium (Ge), for example, are all in the group 4A and have similar
chemical properties.

Mass Number, Isotopes, and Atomic Weight

We said before that the nucleus of an atom is made up of two types of subatomic
particles: protons and neutrons. Protons have a positive charge, and neutrons are
neutral. Another important point is that both protons and neutrons have mass,