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Figure 2.4 Common Families and Groups of the Periodic Table.
The same pattern is true of other vertical groups on the periodic table. Group 2 is called the alkaline earth metals. Once again these elements have similar properties to each other. alkaline earth metals include Beryllium, Magnesium, Calcium, Barium, Strontium and Radium and are soft, silver metals that are less metallic in character than the Group 1 alkali metals. Although many characteristics are common throughout the group, the heavier metals such as Ca, Sr, Ba, and Ra are almost as reactive as the Group 1 alkali metals. They get their name because early “scientists” found that all of the alkaline earth metals were found in the earth’s crust.
The transition metals are the larger block of elements shown in purple on Figure 2.4 extending from Groups 3-12 (also known as the group B elements). Transition elements differ from the main group elements (group A elements) in that they tend to be hard and have high densities. They have high melting points and boiling points and can show various oxidation states when forming chemical bonds (this will be discussed further in chapter 3). They often form colored compounds that are highly stable and they can serve as good catalysts. A catalyst is an agent that helps to speed up a chemical reaction without itself being changed in the process. 
Group 17 elements are also called halogens. This group contains very reactive nonmetals. The halogens are an interesting group. Halogens are members of Group 17, which is also referred to as 7A. It is the only group in the Periodic Table that contains all of the states of matter at room temperature. Fluorine, F2 and chlorine, Cl2 are gases, while Bromine, Br2, is a liquid and iodine, I2, and astatine, At2, are both solids. Another interesting feature about Group 17 is that it houses four (4) of the seven (7) diatomic elements. Diatomic elements only exist in nature as a pair of atoms of the same element that are bonded together. The seven diatomic elements are H2, N2, O2, F2, Cl2, Br2, and I2. Notice that the latter four are Group 17 elements. The word halogen comes from the Greek meaning salt forming. French chemists discovered that the majority of halogen ions will form salts when combined with metals.
The noble gases are in group 18. The two most significant properties of noble gases is that they are extremely unreactive, rarely forming compounds, and that they all exist as gases at room temperature. We will learn the reason for their unreactivity when we discuss how compounds form in chapters 3 and 4. The first person to isolate a noble gas was Henry Cavendish, who isolated argon in the late 1700s. The noble gases were actually considered inert gases until the 1960s when a compound was formed between xenon and fluorine which changed the way chemists viewed the “inert” gases. In the English language, inert means to be lifeless or motionless; in the chemical world, inert means does not react. Later, the name “noble gas” replaced “inert gas” for the name of Group 18. The elements in this group are also gases at room temperature.
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2.5 Defining the Atom

Basic Atomic Structure – electrons, neutrons, and protons

The modern atomic theory, proposed about 1803 by the English chemist John Dalton, is a fundamental concept that states that all elements are composed of atoms. An atom is the smallest part of an element that maintains the identity of that element. Individual atoms are extremely small; even the largest atom has an approximate diameter of only 5.4 × 10−10 m. With that size, it takes over 18 million of these atoms, lined up side by side, to equal the width of your little finger (about 1 cm).
Most elements in their pure form exist as individual atoms. For example, a macroscopic chunk of iron metal is composed, microscopically, of individual iron atoms. Some elements, however, exist as groups of atoms called molecules. Several important elements exist as two-atom combinations and are called diatomic molecules. In representing a diatomic molecule, we use the symbol of the element and include the subscript 2 to indicate that two atoms of that element are joined together. The elements that exist as diatomic molecules are hydrogen (H2), oxygen (O2), nitrogen (N2), fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2).
Atoms are made up of extremely small subatomic particles called protons, neutrons, and electrons. Protons are positively charged particles with a relative mass of 1.672622×10-24g, which form part of the core nucleus of an atom. The other part of the atomic nucleus is made up of neutrons, electrically neutral particles with a relative mass almost identical to a proton (1.674927×10-24g). Electrons are extremely small (9.109328×10-28g) negatively charged particles that form an electron cloud, which orbits the nucleus.Table 2.3 summarizes some of the general properties of subatomic particles.

Experiment have shown that protons and neutrons are concentrated in a central region of each atom called the nucleus (plural, nuclei). Electrons are outside the nucleus and orbit about it because they are attracted to the positive charge in the nucleus. Figures 2.5 and 2.6 depict the structure of an atom.