Which elements with atomic numbers greater than 40 can be expected to have chemical properties similar to those of the element bromine (Br)? What is the periodicity of these two elements?

Group 17 elements, also known as halogens, are a family of non-metallic elements on the periodic table that include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Each of these elements has unique chemical properties but shares some common characteristics due to their group membership.

For instance, halogens are known for their high reactivity, especially towards alkali and alkaline earth metals, with which they form ionic compounds such as table salt (sodium chloride, NaCl). This reactivity is because they have seven valence electrons, just one electron short of the stable, octet configuration. As a result, they readily gain an electron to achieve a complete outer shell. You can observe a trend in reactivity within the group as well - it typically decreases as you move down from fluorine to astatine.

Another notable feature is their varying states at room temperature: fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. When considering their application in real life, halogens have a vast range of uses from disinfectants to pharmaceuticals.

Periodicity in the periodic table refers to the repeating patterns of chemical properties exhibited by elements as you move across or down the table. These patterns arise due to the arrangement of electrons around an element's nucleus, which is itself organized by increasing atomic number.

The periodic table is divided into rows called periods and columns known as groups or families. Elements within the same group, like the Group 17 halogens, show strong similarities in their chemical properties because they have the same number of valence electrons. They, however, display different physical properties owing to the differences in their atomic sizes and masses.

In addition to group trends, there are period trends that occur as one moves across from left to right within a period. Elements gradually change from metallic to non-metallic in character, and there's an overall increase in electronegativity and ionization energy. Understanding these periodic trends is crucial for predicting the behavior and reactivity of different elements and is a fundamental concept in chemistry which helps explain why elements in the same group exhibit similar chemical characteristics.

The atomic number is a fundamental property of an element and is denoted by the letter Z. It corresponds to the number of protons found in the nucleus of an atom of that element, and it also determines the element’s position on the periodic table. Every element has a unique atomic number, starting with hydrogen, which has an atomic number of 1.

The significance of the atomic number is twofold. Firstly, it determines the identity of an element. For example, carbon atoms always have 6 protons, and no other element can have this same number of protons. Secondly, the atomic number determines the electronic configuration of the atoms, which in turn affects the chemical properties of the element. For instance, as you move across a period, the atomic number increases by one for each subsequent element, which means that one more proton and one more electron are added to the atom. This systematic addition of protons and electrons as you move along the periodic table leads to the periodic trends observed in chemical reactivity and the general properties of the elements.