Atomic Structure

ATOMIC STRUCTURE Particle Charge Mass proton + ve charge 1 neutron No charge 1 electron -ve charge nil

number of electrons = number of protons ATOMIC STRUCTURE He 2 Atomic number the number of protons in an atom 4 Atomic mass the number of protons and neutrons in an atom number of electrons = number of protons

ATOMIC STRUCTURE Electrons are arranged in Energy Levels or Shells around the nucleus of an atom. first shell a maximum of 2 electrons second shell a maximum of 8 electrons third shell a maximum of 8 electrons

1. Electronic Configuration ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1. Electronic Configuration 2. Dot & Cross Diagrams

ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Nitrogen configuration = 2 , 5 7 2 in 1st shell 5 in 2nd shell N 2 + 5 = 7 14

ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; 20 11 8 Na O Ca a) b) c) 16 23 40 2,8,8,2 2,8,1 2,6 17 14 5 Cl Si B d) e) f) 11 35 28 2,8,7 2,8,4 2,3

N DOT & CROSS DIAGRAMS Nitrogen With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; X Nitrogen N 7 X X N X X 14 X X

DOT & CROSS DIAGRAMS O Cl Draw the Dot & Cross diagrams for the following elements; X 8 17 X O Cl a) b) X 35 X 16 X X X X X Cl X X X X X X X O X X X X X X X X X X

Electron Arrangement The maximum number of electrons that can exist in each shell increases with the distance of the shell from the nucleus. The electron limit per shell can be calculated from the following formula, where (n) is the shell number. Electron limit per shell = 2xn2 Physicist call the shell number (n) the principal quantum number (the shell number e.g. K,L,M,N,O,P and Q or 1,2,3,4,5,6 and 7). Every electron in every atom can be precisely identified by the principal quantum number. It has a relationship with the periodic table and the number of shells in an atom in its position on the periodic table. Example: Oxygen (O) has 8 electrons. 8O – 2 K shell – 6 L shell

Periodic table 8O – 2 K shell – 6 L shell The number of the outermost occupied electron shell of an atom is equal to its period in the periodic table. The number of electron in the outermost shell is equal to its group. Oxygen (O) has 8 electrons. 8O – 2 K shell – 6 L shell Means that Oxygen is in the second period and in the sixth group.

Aluminum (13Al) has the following electron configuration: Example Aluminum (13Al) has the following electron configuration: K shell = 2 L shell = 8 M shell = 3

Maximum number of electrons that can occupy each electron shell. Shell Number Shell Symbol Number of Electrons 1 K 2 L 8 3 M 18 4 N 32 5 O 50 6 P 72 7 Q 98

Atomic structure Examples Hydrogen (H) 1 p, 1 e K shell 1 Helium (He) 2 p, 2 e, 2 n K shell 2 Lithium (Li) 3 p, 3 e, 4 n K shell 2, L shell 1

Periodic table All atoms having one electron in the outer shell lie in group I of the periodic table. Atoms with two electrons in the outer shell falls in group II etc. Atoms with eight electrons in its outer shell are in group VIII and are very stable chemically. No outermost shell can contain more than 8 electrons. Atoms associated with this phenomenon are called transitional elements or transitional metals. Transitional elements tends to have high tensile strength, density and melting and boiling point. Due to the d orbital electrons ability to delocalize within the metal lattice. The orderly scheme of atomic progression from smallest to largest atom is interrupted in the fourth period. Instead of adding atom to the next outer shell, electrons are added to an inner shell.

Force that binds the electron to the nucleus The closer an electron is to the nucleus, the more tightly it is bound. The attraction of electron and protons are due to the law of electricity, which states that opposite charges attract each other. The opposing forces cause electron to maintain their distance from the nucleus, and thus electrons travel in a circular or elliptical path. Electrons can exist only in certain shells, which represent different electron binding energies or energy levels. Due to the attractions of charges an electrostatic attraction produces centripetal force. Centripetal force is a center seeking force that just matches the force of motion or velocity.

Electron binding energy K shell is the closes to the nucleus which in turn has a high electron binding energy. While Q shell is the farthest which has low electron binding energy. The complexity of the electron configuration of atoms increases as one progresses through the periodic table to the most complex naturally occurring element. Electron orbital shells are given codes K,L,M,N,O,P and Q to represent the relative binding energies of electrons from closest to the nucleus to farthest from the nucleus. Electron binding energy – is the attachment of an electron to the nucleus (Eb). The closer an electron to the nucleus, the higher its binding energy the more tightly it is bound. The greater the total number of electrons in an atom, the more tightly each is bound.

Ionized Atoms If an atom has an extra electron or has an electron removed, it is ionized. An ionized atom is not electrically neutral but carries a charge equal in magnitude to the difference between the number of electrons and protons. Atoms cannot be ionized by addition or subtraction of protons, since that changes the atom from one element to another. In their normal state, atoms are electrically neutral. The electric charge of atoms is 0. The number of electrons is equal to the number of protons. An alteration in the number of neutron does not ionize an atom because the neutron is electrically neutral. Ionization is when the x-ray photon transfers its energy to an orbital electron and ejects that electron from the orbit of the atom. Ionization of an atom carbon by an x-ray leaves the atom with a net electric charge of +1. The ionized atom and the released electron are called an ion pair.

SUMMARY The Atomic Number of an atom = number of protons in the nucleus. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus. The number of Protons = Number of Electrons. Electrons orbit the nucleus in shells. Each shell can only carry a set number of electrons.