How are groups and periods arranged in the periodic table?

The periodic table is a table where elements having similar chemical and physical properties are grouped together. 

The periodic table consists of horizontal rows and vertical columns.

Elements are arranged in order of increasing atomic number, in horizontal rows of such a length that elements with similar chemical properties fall directly beneath one another in vertical groups.

The horizontal rows in the table are known as periods. 

The first period consists of the two elements hydrogen (H) and helium (He). The second period starts with lithium (Li) and ends with neon (Ne).

The vertical columns are referred to as groups. Arabic and Roman numerals have been used in combination with the letters A and B to name groups (1A, 2A, .....).

According to the International Union of Pure and Applied Chemistry (IUPAC), groups are numbered in the periodic table from the left from 1 to 18.

The groups in the periodic table are classified into sections, groups 1, 2, 14, 15, 16, 17, and 18 are called the main-group elements. Elements in the first group are called alkali metals. Elements in the second group are called alkaline earth metals.

The elements in the centre from periods 4 to 6 (groups from 3 to group 12) are called transition metals. The first transition element falls in the middle of period 4 where 3d is subsequently filled from Scandium (group 3) to Zinc (group 12).

There are two rows that are written below the periodic table the lanthanides (elements from 58 to 71) where 4f sublevel is progressively filled. The second row, the actinides (elements from 89 to 103) where 5f sublevel is filled progressively. Both lanthanides and actinides are called inner transition metals. The two rows are separated from the periodic table to shorten periodic table. The actual position of lanthanides should be between barium and hafnium and actinides position is between radium and rutherfordium,

Elements in each column or groups have similar chemical properties and react in the same way. For example, lithium (Li), sodium (Na), and potassium (K) in Group 1 all react vigorously with water to produce hydrogen gas. Helium (He), neon (Ne), and argon (Ar) in Group 18 are nonreactive.

Elements in each row follow periodic trends, the physical and chemical properties change in a predictable ways. For example, the atomic size decreases as across periods from the left to the right.