Unit 3 Periodic Table James Hutchison ROHS

Development of the Periodic Table – By the mid-1800s, about 70 elements were known – Dmitri Mendeleev ordered the elements in columns in order of increasing atomic mass in the very 1 st periodic table In 1913, Henry Moseley determined the atomic number of atoms of elements, and used increasing atomic number to re-order the periodic table. It still stands today.

The Modern Periodic Table – The horizontal rows are known as periods There are 7 periods Properties of elements change as you move across each period – Gives rise to the periodic law – When elements are arranged in increasing atomic number, there is a periodic repetition of their physical and chemical properties – The vertical columns are called groups There are 18 groups, or families Elements in groups have similar physical and chemical properties

Groups are given a number and a letter – The letters, A and B, signify properties – Group A elements are called the representative elements because they exhibit a wide range of physical and chemical properties – Group B are the transition elements

1A 2A3A4A5A6A 7A 8A Elements in the 1A-7A groups are called the representative elements outer s or p filling

The group B are called the transition elements u These are called the inner transition elements, and they belong here

The periodic table consists of 3 main types of elements: metals, non-metals and metalloids Metals – Make-up 80% of all elements – High heat and electrical conductivity – High luster, ductility and malleability – Alkali metals, alkaline earth metals (Gr.A) Alkali Metals are the metals in Group 1A Alkaline Earth Metals are in Group 2A – Transition metals and inner transition metals (Gr.B) Inner transition metals are also called rare-earth metals

Group 1A are the alkali metals (but NOT H) Group 2A are the alkaline earth metals H

Non-metals – Upper right corner of the periodic table – Poor conductors – Low luster, ductility and malleability – Many exist as gases at room temperature – Two Special groups: Halogens & Noble Gases Halogens are in Group 7A Noble Gases called inert gases are in Group 0 or 8A

Group 8A are the noble gases Group 7A is called the halogens

Metalloids – Consists of elements with properties that are intermediate between those of metals and non-metals

Periodic TRENDS Ionization energy – The energy required to overcome the attraction of the nuclear charge and remove an electron from a gaseous atom is the ionization energy – The first ionization energy generally decreases as you move down a group Electronegativity – Electronegativity is the tendency for an atom to attract electrons when they are chemically combined with atoms of another element – Measured in units of Paulings – Electronegativity generally decreases as you move down a group – As you move across a period from left to right, electronegativity of the representative elements increases

The arrows indicate the trend: Ionization energy and Electronegativity INCREASE in these directions

Trends in atomic size – Atomic radius is ½ the distance between the nuclei of two like atoms in a diatomic molecule – Atomic radius indicates relative size (usually measured in picometers) – Atomic size usually increases as you move down a group – Electrons are added to successively higher energy levels – The shielding of the nucleus by electrons increases (shielding effect) – Overcomes the shrinking effect caused by the increasing charge of the nucleus

Trends in ionic size – Atoms of metallic elements have low ionization energies and they form positive ions easily – Atoms of non-metallic elements readily form negative ions – The loss or gain of electrons affects the size of the ions formed – Going from left to right across a period, there is a gradual decrease in the size of positive ions (cations) – Beginning with Group 5A, the negative ions (anions) gradually decrease in size as you continue to move right – The ionic radii of both cations and anions increase as you move down a group

Atomic size and Ionic size increase in these directions:

Electron Affinity – The electron affinity is a measure of the energy change when an electron is added to a neutral atom to form a negative ion.