Structure of an Atom (1) PDF

ATOMIC STRUCTURE VIII THE ATOM: SUBATOMIC PARTICLES Discoveries in the late 19th and the early 20th century proved that atoms are divisible. There are three main particles in an atom: electrons, protons and neutrons. These...

ATOMIC STRUCTURE VIII THE ATOM: SUBATOMIC PARTICLES Discoveries in the late 19th and the early 20th century proved that atoms are divisible. There are three main particles in an atom: electrons, protons and neutrons. These particles are known as sub atomic particles. ROTATION OF ELECTRONS AROUND THE NUCLEUS POSTULATES OF BOHR’S ATOMIC THEORY: Electrons revolve around the nucleus in fixed orbits called energy levels. The energy levels 1, 2, 3… are represented by integer n or as K, L, M, N… Electrons rotate around the nucleus in one or more of the energy levels. ATOMIC NUMBER (Z) MASS NUMBER(A) Atomic number of an element is equal to the Mass number of an element is equal to the total number of protons(p) in the atom of the element. number of protons(p) and neutrons(n) in the atom of Since atoms are electrically neutral, the element. No. of protons(p) = No. of electrons(e) Since electrons carry negligible mass, the mass of an ∴ Atomic number of an element is also equal to atom is almost the mass of the protons and neutrons in the number of electrons in the atom of an element. the nucleus of the atom. Z = p = e A = p + n  STRUCTURE OF AN ATOM An atom consists of two parts: the nucleus and the extra-nuclear portion. The nucleus is present in the centre of the atom and is surrounded by extra-nuclear portion. All the protons and neutrons are present in the nucleus while the electrons are present in the extra-nuclear portion. Of this, Electron is negatively charged, Proton is positively charged and neutrons have zero charge. The electrons present in the extra nuclear portion are not stationary but are continuously revolving around the nucleus at high speed following a circular path. The revolving electrons do not come close to the nucleus or drawn towards the nucleus because their force of attraction towards the nucleus is balanced by the centrifugal force of same magnitude.  HOW TO REPRESENT AN ATOM? ✔ In the picture, suppose, X is the symbol of any element. ✔ Then the mass number (i.e. no of p + n) is written at the upper-left or upper right hand side of the symbol. It is represented by A. So if we compare the two pictures, we find that A = 12. ✔ On the other hand, the atomic number(i.e. no of protons) is written on the lower-right or left hand side of the symbol. It is represented as Z. So, Z = 6 for Carbon. ✔ Similarly, He, N, O and all other elements can be represented. ✔ Now, can you say the values of A and Z for the  ARRANGEMENT OF ELECTRONS IN AN ATOM We have read that electrons revolve around the nucleus in fixed paths called orbits or shells. ❑ The shells are designated as 1, 2, 3, 4, 5…. Or K, L, M, N, O… starting from the innermost shells. ❑ In general, the maximum number of electrons that a shell can have is 2n2, where n is the number of shell. ❑ For example, the first shell (K shell) can have 2 × 12 = 2 electrons, the second shell (L shell) can have 2 × 22 = 8 electrons. The outermost shell of an atom cannot have more than 8 electrons. The arrangement of electrons in shells is called electronic configuration.  ELECTRONIC CONFIGURATION ELECTRONIC CONFIGURATION OF FIRST 20 ELEMENTS REPRESENTATION OF FIRST 20 ELEMENTS  ISOTOPES AND ISOBARS ❑ Isotope: The different atoms of the same element having same atomic number but different mass numbers. The difference in mass number occurs as though the number of protons are same but the number of neutrons varies in these species. Look into the following figure showing the three isotopes of hydrogen Other examples include isotopes of chlorine (35Cl17,37Cl17), Carbon (12C6,13C6, 14C6). As you can see, the atomic number is same for all the three different isotopes implying same number of protons. Also notice, mass number is different which denotes that the number of neutrons is different. Now, find out the number of electrons, protons and neutrons for each of these isotopes. ❑ Isobar: The atoms belonging to the different elements with same mass numbers but different atomic numbers. Look into the following figure: As you can see, the mass numbers for all the three different elements are same but the atomic numbers of all these species are different. Similarly 15N7,15O8 is another example of isobars. ********************************************************************************* ❖ Memorise the atomic number and mass number for the first 20 elements which is given as a chart. ********************************************************************************** ********** THANK YOU