Atomic structure and the periodic table

1. HOW ARE ELECTRONS ARRANGED IN AN ATOM?

Electrons are arranged in energy levels around the nucleus. These electrons are not stationary but continue to move in different energy levels. The arrangement of electrons in the shells or energy levels is governed by certain rules or guidelines.

Each shell or energy level can hold a fixed number of electrons. When a shell is filled, electrons will enter the next shell with the lowest energy. Electrons start to fill up the shells of lowest energy first, that is, the closest one to the nucleus. The first shell (K shell) can hold a maximum of 2 electrons. The next shell (L shell) can hold a maximum of 8 electrons. The third shell (M shell) holds 8 electrons for the first 20 elements, but can hold up to 18 electrons. The electronic configuration of the first 20 elements is shown below. 2. Determine the electronic configuration in the following atoms: 11Na, 15P, 5B 11Na - 2:8:1 15P - 2:8:5 5B - 2:3

3. HOW ARE ELEMENTS ARRANGED IN THE PERIODIC TABLE?

Elements are arranged in the periodic table in order of increasing atomic (proton) number, and in GROUPS based on the number of electrons in the outer shell, and in PERIODS based on the number of shells being filled. Elements with filled shells of 2 or 8 electrons are in group 8 or O.

4. IDENTIFY THE DIFFERENT GROUPS ON THE PERIODIC TABLE.

Elements on the periodic table are divided into mainly metals and non-metals. As you go along a PERIOD, elements change from metals to non-metals. Elements that show both metallic and non-metallic properties are called metalloids. Some groups of the periodic table have special names. Group 1 elements are all metals and have one electron in their valence shell. They are called the alkali metals. Group 2 elements are also metals and have two electrons in their valence shell. They are called the alkaline earth metals.

Group 17 elements are called the halogens or salt producers. They are called salt producers as they are found in many salts. Group 17 elements have seven electrons in their valence shell.

Group 18 elements (commonly called group O or 8) are called the noble gases. The elements in this group all have the maximum number of electrons in their outer or valence shells. Between GROUP 2 and GROUP 13 is the middle block of the periodic table that contains the transition elements (metals). These include elements such as iron, copper and chromium.

5. WHAT ARE THE SIMILARITIES BETWEEN ELEMENTS IN A GROUP AND ELEMENTS IN A PERIOD?

Elements in the same group have similar properties. Elements in the same group also have the same number of electrons in the outer shell. These are called valence electrons and determine how the element (atom) reacts or bonds. For example, sodium (Na) and lithium (Li) are in the same column, GROUP 1. The electronic configurations are as follows: 37Li = 2: 1 and 1123 Na = 2:8:1.

Elements in the same period are filling the same shell, hence have the same number of shells. For example, elements in period 2 are filling the second shell, while elements in period 3 are filling the third shell.

6. WHY ARE NOBLE GASES SO NAMED?

Group 8 elements are called noble gases (inert) as their outer shells are filled and are, therefore, not in need of any electrons. This makes them stable and unreactive. Other elements normally lose or gain electrons in order to have a full shell like the noble gases or to have a noble gas configuration.