The Periodic Table Science 9, 2016

Introduction By the 1780’s chemists wondered why some elements were gas whereas others were metals By the 1860’s the list of known elements had grown to 63 Chemists were searching for a pattern among those elements In 1867, Dmitri Medeleev proposed organizing the elements into a table based on their properties He gathered information on the elements and put it on cards He then sorted the cards into rows and columns

Introduction Cont’d Mendeleev started noticing patterns All the cards that were metals ended up one side All the non-metals ended up on the other side Metalloids were in the middle Most of the elements that were gasses at room temp were grouped together Mendeleev was so confident in his table that he left spaces open for elements that had not been discovered yet In 1886 germanium was discovered and the properties were an exact match to an open space on the table Henry Moseley rearranged the table based on atomic number

Modern Periodic Table Elements are listed from left to right and top to bottom according to their atomic number Moseley proposed that atoms of the same element could have different masses These are called isotopes Periodic Law: description stating that elements arranged in order of atomic mass show a periodic repetition of properties

Atomic Number Atomic number of an element is defined as the number of protons in the nucleus of an element Lowest atomic number is 1 (Hydrogen) Atomic number of Helium is 2 Atomic number of Carbon is 6 The number of protons in the nucleus of an atom, or the atomic number, determines the atom No one knows what the highest possible atomic number is, but of 2010 it was 118

Atomic Mass The average mass of an elements atoms Measured in atomic mass units (amu) Atomic mass of hydrogen is 1.01 amu and the atomic mass of iron is 55.85 amu. What does this mean? Atomic mass usually increases in order of increasing atomic numbers. However, there are some exceptions. Iodine has a lower amu than tellurium

Ion Charge Ion charge is the electric charge that an atom takes on when it loses or gains an electron Elements that can form similar ions are grouped together Example: Elements in the first column on the left side of the periodic table form a 1+ ion

Transition Metals The elements in the middle of the periodic table Include common metals such as iron, copper, zinc, silver, and mercury The ion charge cannot be predicted from atomic theory

Periods and Chemical Families Each horizontal row is called a period The number on the left side of the table identifies each period There are 18 vertical columns and each one represents a different group or chemical family Elements within a family or group share certain or physical properties We discuss alkali metals, halogens, and noble gases

Alkali Metals (Li, Na, K, Rb, Cs) Similarities Silver-grey metals Are malleable and ductile and conduct electricity Have low melting points compared to other metals Soft enough to cut with a knife React easily with water and air Differences Regular increase in density as you move from top to bottom Elements also get softer and easier to cut

Halogens (F, Cl, Br,I) Similarities: Differences: All are non-metals All are reactive All have a distinct colour Differences: The colours grow in intensity as you go down the periodic table Melting points also gradually increase

Noble Gases (He, Ne, Ar, Kr, Xe, Rn) Similarities: All exist naturally as colorless gasses Will glow with bright colors if a current is passed through them None are chemically reactive Differences Density of the gases increases steadily

The Nucleus At the center of the atom Comprised of protons and neutrons The number of protons tells you what atom it is Number of neutrons can vary Contains 99.99% of the mass of an atom

Protons Have a positive charge ALWAYS equal to the atomic number of the element Found in the nucleus

Neutrons Have a neutral charge Equal to: (the atomic mass - number of protons) Found in the nucleus

Electrons Have a negative charge Surround the nucleus in different energy levels Inner most level hold two electrons and the next two levels each hold 8 electrons Can move between energy levels when energy is applied or removed form the atom Number and arrangement in outermost energy level determines chemical properties of atoms and physical properties of atoms and physical properties of elements Often exist in pairs

Steps for Drawing Atoms Determine what atom you are drawing Determine the overall charge of the atom you are drawing Determine the number of electrons in the atom Fill the first energy level with two electrons Fill the second energy level with a maximum of 8 electrons Continue filling energy levels with a maximum of 8 electrons