The Periodic Table

History of the Periodic Table In the 1860s, scientists had discovered 63 elements Russian scientist Dmitri Mendeleev tried to find a way to organize them. Mendeleev looked for patterns in the elements’ properties: – Mass, density, boiling point, color, hardness, reactivity

History of the Periodic Table Mendeleev arranged the elements into rows in order of increasing mass so that the elements with similar properties were in the same column He created a periodic table of the elements – Periodic means “repeated in a pattern” Several decades later the table was modified so that elements increased in atomic number, not atomic mass

Mendeleev’s Predictions Mendeleev’s periodic table left gaps for elements that had not been discovered. He made predictions of new elements yet to be discovered. – He called them eka- (meaning one) plus the name of the element above it. – Ekasilicon (Es)

Ekasilicon (predictions): – Atomic mass: 72 – High melting point – Density = 5.5 g/cm 3 – Dark gray metal – Density when combined with oxygen: 4.7 g/cm 3 Germanium (actual properties) – Atomic mass: – Melting point: 938°C – Density = g/cm 3 – Gray metal – Density when combined with oxygen: 4.23 g/cm 3 Mendeleev predicted an element he called Ekasilicon in In 1886, Germanium was discovered

Lanthanide and Actinide Series Elements (called the Lanthanide series) and (called the Actinide series) are cut out and placed underneath the periodic table This is done to save space In addition, the La and Ac series elements have a unique electron arrangement

What are those strange U’s? Scientists have recently discovered new elements using particle accelerators These elements are temporarily named after their atomic number in Latin For example, element 112 is Ununbium, which means one- one- two- in Latin The International Union of Pure and Applied Chemistry (IUPAC) officially names elements

Trends in the periodic table Rows in the periodic table are called periods Columns in the periodic table are called groups or families There are 7 periods and 18 groups in the periodic table

Trend #1: Atomic Mass In general, atomic mass increases across a period This is because the number of protons increases as atomic number increases There are a few exceptions – Argon (atomic number 18) to Potassium (atomic number 19) – Cobalt (atomic number 27) to Nickel (atomic number 28)

Trend #2: Metallic properties In general, elements become less metallic across a period Properties of metals include: – Usually solid at room temperature – Malleable (can be hammered into sheets) – Ductile (can be drawn into wires) – Good conductors of heat and electricity at room temperature Most elements in groups 1-13 are metals

Trend #2: Metallic properties Nonmetals are found on the right side of the periodic table Properties of nonmetals include: – Mostly gases at room temperature – Brittle (break apart easily) – Poor conductors of heat and electricity at room temperature Poor conductors are called insulators

Trend #2: Metallic properties Metalloids are found between metals and nonmetals Metalloids have some metallic and some nonmetallic properties Some metalloids can be semi-conductors – Under some conditions they are good conductors, and under other conditions they are insulators – We utilize this in computers “Silicon Valley”

Trend #3: Atomic Radius Atomic radius generally decreases across a period – This is because as you add more protons they have a stronger grip on their electrons, pulling them tightly Atomic radius generally increases down a group – This is because with each new group, a new orbital of electrons is added

Remember the Bohr model: electrons are located in orbitals At the end of each period, the outermost orbital fills up At the start of each new period, a new orbital begins to fill up

Atomic Radius in the periodic table

Trend #4: Reactivity Elements in the same group generally have the same reactive properties This is because elements in the same group have the same number of valence electrons Valence electrons are the electrons in an atom’s outermost orbital

Valence Electrons Each element has a different number of electrons, but all elements have between 1 and 8 electrons in their outermost orbital – These are called its valence electrons Elements are most stable when they have a full 8 valence electrons OR 0 valence electrons Elements react and bond with other elements to try to get 8 valence electrons

So how many valence electrons does each element have?

Electron Dot Diagrams (Lewis structures) We can draw electron dot diagrams, or Lewis structures, to show how many valence electrons each element has 1.Write the element symbol 2.Determine the number of valence electrons 3.Place one dot for each valence electron around the element

H B FNe Electron Dot Diagrams (Lewis structures)

Elements with the same number of valence electrons have similar chemical properties All the elements in a group have the same number of valence electrons Therefore, elements in the same group have similar chemical properties

Alkali Metals Group 1 (minus hydrogen)is known as the alkali metals Alkali metals are very reactive