Understanding Atoms, Molecules, and Ions: Structure and Properties

Atoms, Molecules, and Ions

Atoms: Electrically balanced particles consisting of one positively charged center called the nucleus, surrounded by a cloud of negatively charged particles called electrons, which exactly balance the nuclear charge.

Molecules: Electrically balanced particles with more than one positive center (nuclei). Molecules are sets of bonded atoms that act as a unit.

Ions: Atoms (or groups of atoms) with unbalanced charges.

The number of protons determines the number of electrons necessary to balance the atom's charge. Neutrons are the cementing material in the nucleus. The force between protons and neutrons is called the strong nuclear interaction. Number of protons = ATOMIC NUMBER of the atom (Z)

The atomic number is also given the more descriptive name of proton number.

The number of protons determines the number of electrons necessary to balance the atom's charge.

Number of protons + Number of neutrons = MASS NUMBER of the atom (A)

The mass number is also called the nucleon number. How many protons and neutrons does this atom have?

The atomic number counts the number of protons (9); the mass number counts protons + neutrons (19). If there are 9 protons, there must be 10 neutrons for the total to add up to 19.

Atoms are electrically neutral, and the positive charge of the protons is balanced by the negative charge of the electrons. It follows that in an atom:

Number of electrons = Number of protons

So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have 8 electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must also have 17 electrons.

The nucleus is at the center of the atom and contains the protons and neutrons. Protons and neutrons are collectively known as nucleons.

Virtually all the mass of the atom is concentrated in the nucleus because the electrons weigh so little. An element is formed by all the nuclei that have the same number of protons. There are 92 different stable natural elements: the lightest element (Z = 1) is hydrogen, and the heaviest (Z = 92) is uranium.

Atoms are electrically neutral, and the positive charge of the protons is balanced by the negative charge of the electrons. It follows that in an atom:

Number of electrons = Number of protons

So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have 8 electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must also have 17 electrons.