Chemistry eText

Standard 1a Preknowledge

1a) Students know how to relate the position of an element in the periodic table to its atomic number and atomic mass.

CALIFORNIA FRAMEWORKS SUMMARY: An atom consists of a nucleus made of protons and neutrons that is orbited by electrons. The number of protons, not electrons or neutrons, determines the unique properties of an element. This number of protons is called the element’s atomic number. Elements are arranged on the periodic table in order of increasing atomic number. Historically, elements were ordered by atomic mass, but now scientists know that this order would lead to misplaced elements (e.g., tellurium and iodine) because differences in the number of neutrons for isotopes of the same element affect the atomic mass but do not change the identity of the element.

Atomic Number and Mass Number

	Where found	Charge	Mass Number
Protons	nucleus	+1	1
Neutrons	nucleus	0	1
Electrons	electron cloud	-1	0

Definitions

Atom: the smallest particle of an element that retains the properties of that element
Atomic number: the number of protons in the nucleus of an atom
Mass number: the total number of protons and neutrons in the nucleus of an atom
Isotopes: Atoms of the same element that differ in mass number (differing numbers of neutrons)

Atoms are neutral, meaning that they carry no net charge. In order for this to be true, the number of protons and the number of electrons must be the same. If they are not, you have a charged particle called an ION.

The only two particles that have any significant mass in an atom are the protons and the neutrons. Because of this, the mass number of an atom is the sum of the protons and neutrons in that atom.

In summary:

Atomic number = Protons

Protons = Electrons (in atoms, not ions)

Mass number = Protons + Neutrons

Isotopes

Isotopes are atoms of the same element having different masses due to different numbers of neutrons. Because they are atoms of the same element, they all must have the same number of protons. In order to be neutral, the protons and electrons must balance each other, but the neutral particle, the neutron, can vary in number.

The simplest set of isotopes to examine are those of hydrogen. Isotopes are denoted by their name and mass number, though some isotopes were given individual names. Hydrogen - 1 was first called "protium," hydrogen - 2 was called "deuterium" and hydrogen - 3 was called "tritium."

Isotope	Atomic #	Mass #	Protons	Neutrons	Electrons
hydrogen - 1	1	1	1	0	1
hydrogen - 2	1	2	1	1	1
hydrogen - 3	1	3	1	2	1

Think you understand it? Then go ahead and try this review activity!

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California Standards Chemistry

Standard 1a Preknowledge

Atomic Number and Mass Number

Isotopes