Fundamental Concepts of Atomic Structure and Elements

Planetary Model of the Atom

An atom has a very small central region, the nucleus, made up of protons and neutrons. The nucleus is surrounded by a cloud of orbiting electrons. The number of electrons in an atom is equal to the number of protons in its nucleus (atoms have no overall electric charge).

Atomic Number (Z)

The atomic number (Z) of an atom is the number of protons it contains. All the atoms of a particular element have the same atomic number. The atoms of different elements have different numbers of protons.

Mass Number (A)

The mass number (A) of an atom is the total number of protons and neutrons it contains. The full chemical symbol for an element shows its mass number at the top and its atomic number at the bottom.

Ions

• Ion: When an atom loses or gains electrons.
• Cation: A positively charged ion.
• Anion: A negatively charged ion.

Isotopes

In 1913, J.J. Thomson discovered two types of neon atoms differing in mass (from the Greek word meaning "same places"). Shortly after, it was found that most elements consisted of mixtures of isotopes in constant proportions. Isotopes of a given element have the same number of protons but a different number of neutrons.

Electronic Configuration

Niels Bohr had the idea that electrons were arranged in layers around the nucleus according to their energy:

• The first energy level holds 2 electrons.
• The second level holds 8 electrons.
• The third level holds 18 electrons.

The electronic configuration of an atom shows how the electrons are arranged in levels around the nucleus.

The Periodic Table

The modern periodic table displays all the elements in horizontal rows called PERIODS and vertical columns called GROUPS. The elements are in order of their atomic numbers. There are three general classes of elements distinguished by their physical properties: metals, non-metals, and metalloids. The position of an element in the periodic table is directly related to its atomic structure.

Periodic Table Relationships

1. All the elements in a group have the same number of electrons in their outer shell. These are called the Valence electrons and they dictate how an element reacts.
2. The group number is the same as the number of outer electrons, except for Group 0, where the atoms have full shells.
3. The period number shows how many electron shells there are.