Atomic Structure

Element : Aluminium (Al) What is an atom ? An Atom of Aluminium Atoms of Aluminium

What is an atom ? smallest particle of any substance average diameter is approx m (analogy : atoms needed to cover a full stop) each atom is made up of even smaller sub-atomic particles atoms differ from each other due to the different combinations of these sub-atomic particles

Atoms of different elements differ in size and mass.

Trends in

Discovery of structure of atom Source : CD-ROM Chemistry Concepts and Applications by Glencoe

Structure of an Atom electrons protons and neutrons nucleus shell

Structure of Helium atom Source : Intro to Chemistry by Cyber Ed Inc. 2 protons 2 neutrons 2 electrons

Sub-atomic particles particle protonneutron electron charge +1 zero mass 1 1 1/1850 location In nucleus In nucleus In shells around nucleus

Mass number; Atomic Number Mass number (or nucleon number)-the sum of the number of protons and neutrons in an atom Nucleon = protons + neutrons Atomic number (or proton number) -the number of protons in an atom The atomic number is fixed for each atom. An atom is identified by its atomic number. Different elements have different atomic number.

Proton and Mass Numbers The data are available in the Periodic Table : E.g. Na Mass number Proton number

Proton (Atomic) Number The proton (atomic) number of an element is the number of protons in its atom. E.g. Since the proton number of sodium is 11, it means that every sodium atom has 11 protons. Conversely, any atom with 11 protons is a sodium atom.

Mass (Nucleon) Number The mass (nucleon) number refers to the total number of protons and neutrons in an atom. Hence, number of neutrons in an atom = mass number - proton number

Derive the number of protons, electrons and neutrons in a sodium atom Example 11 Na 23 Mass number = 23Atomic number = 11 Number of protons = 11 Number of electrons = 11 Number of neutrons = = 12

A short Quiz…….  Malaysium (Ma), an imaginary element contains 111 protons and 141 neutrons. How would you represent the atom of Malaysium in symbolic form? Atomic number = Number of protons = 111 Mass number = Number of protons + Number of neutrons = = 252 Ma Hence

Isotopes Definition : Isotopes are atoms of the same element with the same number of protons but different number of neutrons; Isotopes have the same atomic number but different mass numbers.

Isotopes Isotopes have the same atomic number but different mass numbers = 1224 – 11 = number of protons number of neutrons number of electrons Isotopes Na Na

Arrangement of electrons in the atom Electrons are arranged in energy levels (called electron shells) around the nucleus. The nearer the shell to the nucleus, the lower the energy level. Each energy level can hold a certain number of electrons. For the first 20 elements in the Periodic Table, 1st shell : 2 electrons 2nd shell : 8 electrons 3rd shell : 8 electrons

To work out the arrangement of electrons (i) Find out the proton number of the atom or element ; then for a neutral atom, the no. of electrons = the proton number (i) Main rule -- electrons always go into the shell nearest to the nucleus,if there is room. Once the shell is filled up, the electrons go into the next available shell and so on.

Eg. electronic structure of Al

Valence electrons Outermost shell OR Valence shell - shell outermost from the nucleus Valence electrons (Outermost electrons) electrons in the outermost shell determines the chemical properties of an element

Electron Arrangement & the Periodic Table The position of an element in the Periodic Table is determined by its proton number. All elements in the same group have the same number of valence electrons, which is the same as the Group number. All elements in the same period have the same number of electron shells.

Electron Arrangement & the Periodic Table Elements in : (a) the same Group  same number of outermost / valency electrons (b) the same Period  same number of electron shells

group number simplified electronic structure IIIIIIIVVVIVII VIII full electronic structure the Periodic Table proton number name of element symbol electronic structures and the Periodic Table