History of the Periodic Table

Arranged the elements in order of increasing atomic weight and similar chemical properties.

Mendeleev

The reason that there were gaps in Mendeleev's table.

Undiscovered elements

When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically

Mendeleev's Periodic Law

What was missing from Mendeleev's table

Noble gases
Transition elements not in a separate block

Discovered atomic number.
Arranged elements in order of increasing atomic number and similar chemical properties.

Mosely

When the elements are arranged in order of increasing atomic number, certain sets of properties recur periodically

Modern Periodic Law

Arranged elements in triads.

Dobereiner

Group of elements with similar chemical properties in which the middle element has an atomic weight equal to the average of the other two.

Triad

Arranged elements in order of increasing atomic weight where the 1st and 8th have similar chemical properties.

Newlands

The problem with Newlands' octaves

Nobel gases not included. No gaps.

Number of protons in the nucleus of an atom.

Atomic number

Number of protons and neutrons in the nucleus of an atom.

Mass number

The number of electrons in a nucleus.

Atomic number

The reason Triads did not work as a theory.

They did not follow a Mathematical Law.

Ancient Greeks

• 4 elements: earth, air, fire, water

Robert Boyle

•From waterford
•Discovered elements cant be broken down
•Discovered compounds are a combination of elements which can be broken down

Humphrey Davy

•Devolped techniques for breaking compounds into mixtures
•Electrolyzed a sample of potassium hydroxide and isolated potassium which burst into lilac flames
•Isolated more elements

Moseley

• 1913 used xrays to discover a positive charge in the atomic nucleus of each element
• He called this charge the atomic number
• Defined an element as a substance whose atoms all have the same atomic number

What is the periodic table?

• It is a list of all known chemical elements arranged by increasing atomic number and groups of elements according to recurring properties.

Dobereiner’s Triads

• groups of three elements
• similar chemical properties
• atomic weight of the second is midway between that of the first and the third
• example: Li, Na, K
• restricted to a small number of elements

Newlands’ Octaves

• when in order of atomic weight, properties repeat every eight elements
• worked for first 17 elements
• forced all known elements - Cu & Ag with Li, Na, K
• principle: periodic reoccurrence of properties when elements are arranged in order of
increasing atomic weight.

Newlands’ Octaves

• when in order of atomic weight, properties repeat every eight elements
• worked for first 17 elements
• forced all known elements - Cu & Ag with Li, Na, K
• principle: periodic reoccurrence of properties when elements are arranged in order of
increasing atomic weight.

Moseley 2

• discovery of characteristic positive charge / atomic number
• table in order of increasing atomic number

Compare Mendeleev’s and the modern periodic table

The Alkali Metals

• increasing reactivity going down the group
• soft metals - can be cut with a knife
• low densities
• shiny but tarnish quickly

The Alkaline Earth Metals

• increasing reactivity going down the group
• harder than the alkali metals
• less reactive than their corresponding alkali metals
• react with water but less vigorously than their corresponding alkali metals
• calcium + water ⟶ calcium hydroxide + hydrogen

The Halogens

nonmetals
• decreasing reactivity going down the group
• low melting/boiling points
• react with hydrogen to form acidic soln.s
• hydrogen + chlorine
• react vigorously with alkali metals forming white salts
• sodium + chlorine ⟶ sodium chloride

The Noble Gases

• all gases at room temp.
• boiling point and density increase going down the group
• least reactive

Mass number

the sum of the number of neutrons and protons in an atomic nucleus

Atomic number

The number of protons

isotope

Atoms with the same number of protons but with different numbers of neutrons

Relative atomic mass

The RAM of an element is the average mass of an atom of the element compared to one twelfth the mass of an atom of the carbon-12 isotope

Aufbau principle

When building up the electron configuration of an atom in its ground state, the electrons occupy the lowest avaliable energy levels.