Chapter 4 - Chemistry: The Periodic Table

cramming rn im gonna cry

relative atomic (molecular) mass number

the average mass of an atom (molecule) of an element compared to 1/12 the mass of the carbon-12 isotope, taking into account the relative abundances of the naturally occurring isotopes

*rarely a whole number as it’s the average of all the known isotopes of an element

Robert Boyle

came up with the first accurate definition of an element

element

a substance that cannot be broken down into simpler substances by chemical means

Humphry Davy

discovered & isolated many elements by passing electricity through their compounds to investigate what would happen (electrolysis

discovered K, Na, Mg, Ca, St, Al

electrolysis

passing electricity through a compound

Dobereiner’s Triads

group of 3 elements with similar chemical properties, where the relative atomic mass of the middle element is approximately equal to the average of the other two

the appearance and reaction of the elements in a triad were similar to each other

e.g. L, Na, K

Newland’s Law of Octaves

these are the arrangements of elements (in order of atomic mass) in which the 1st and 8th element have similar properties

so every eighth element had similar properties

only applies to the first 16 elements

Mendeleev’s Periodic Law

when elements are in order of increasing atomic weight, their chemical and physical properties repeated every eighth element

Mendeleev

• came up with the Periodic Law

• he placed the elements in order of atomic mass but reversed Iodine and Tellurium out to accomodate their properties

• left gaps for undiscovered elements and predicted their properties

Moseley

used x-rays to determine the number of protons in the nucleus of each element

Comparison between Mendeleev’s Periodic Table and the Modern (Moseley’s) Periodic Table

Mendeleev:

• arranged in order of increasing atomic mass

• less elements and no noble gases

• gaps left for undiscovered elements

• transition metals mixed in with other elements

• only 63 elements

Modern:

• arranged in order of increasing atomic number

• more elements, including noble gases

• no gaps

• transition metals in their own block

periodicity

a repeating pattern in properties of elements across periods of the periodic table

atomic number (Z)

the number of protons present in the nucleus of an atom of that element

atomic mass number (A)

the number of protons and neutrons in the nucleus of an atom of that element

Group 1 - Alkali Metal

• all elements have 1 electron in their outer (valence) shell

• very reactive and must be stored in oil to prevent reactions with water vapour and oxygen in the air

• low ionisation energies and electronegativity values mean they lose electrons, readily forming ionic compounds

• soft metals and have low densities

• shiny when freshly cut but tarnish rapidly as they react with air to form metal oxides

• react vigorously with water to form metal hydroxides and hydrogen gas

i.e.

metal + oxygen gas → metal oxide

reactive metal + water → metal hydroxide + hydrogen gas

Group 2 - Alkaline Earth Metals

• all elements have 2 electrons in their outer (valence) shell

• very reactive with water and oxygen but not as reactive as Alkali Metals / magnesium reacts slowly with water

• hard

• compounds of these are found in many common rocks

Group 7 - Halogens

• all elements have 7 electrons in their outer (valence) shell

• exist as diatomic molecules

• very reactive non-metals but less reactive as you travel down the group

• low m.p and b.p

• reacts with hydrogen to form compounds that dissolve in water to for acidic solutions

• reacts with alkali metals to form salts

Group 8 - Noble Gases

• inert due to full valence shell

• all noble gases are gases at room temperature

• b.p increases down group

isotope

atoms of the same element that have the same atomic number but different mass numbers due to the different numbers of neutrons present in the nucleus

valency

tells us the number of chemical bonds an atom can form

the octet rule

states that when atoms bond, most atoms tend to want an electron arrangement of 8 electrons in their outermost shell (energy level)

compound

a substance made up of two or more elements chemically combined