electrons are at specific distances from the nucleus
mendeleev ordered his table in order of atmic mass (not strictly)- in some places he changed order based on atomic weights and group properties.
Mendeleev left gaps to make sure elements with similar properties were in the same group, indicated existence of undiscovered elements (able to predict their properties).
newlands ordered his table in order of atomic mass, he realised similar properties occured every eighth element (law of octaves) but it broke down after calcium.
why is the periodic table periodic?
similar properties occur at regular intervals, elements with similar properties are found in the same group/column.
knowledge of isotopes confirms Mendeleev was correct to not order elements strictly using atomic weights as they have different weights of the same element.
when electrons, protons and neutrons were discovered, elements were ordered in atomic number.
alkali metals:
all react with chlorine to form a white precipitate
all react with oxygen to create an oxide
reactivity increases going down the groups
due to single electron in outer shell
lithium reaction with oxygen: burns with a strong red flame and produces a white solid.
lithium reaction with chlorine: white precipitate produced and settles on sides of container
sodium reaction with oxygen: strong orange flame and produces white solid
sodium reaction with water: fizzes rapidly, melts into a ball and quickly disappears
sodium reaction with chlorine: burns with bright yellow flame, clouds of white precipitate produced
potassium reaction with oxygen: large pieces produce lilac flame, smaller ones make a solid immediately.
potassium reaction with water: ignites with sparks and lilac flame, floats, melts, disappears very quickly
potassium reaction with chlorine: more vigorous than with sodium
noble gases (group 0)
full outer shells
unreactive and do not easily form molecules because of full shell
boiling points of noble gases increase going down the group
halogens (group 7):
7 outer electrons so similar reaction to noble gases
exist in molecules of pairs
form ionic compounds with metals
form covalent compounds with nonmetals
melting and boiling point decrease as you go down the group
reactivity decreases as go down
reactivity of halogens decrease as you go down the group as when the number of shells increase down the group, it is more difficult to attract electrons from other atoms (weaker electrostatic forces).
in halogens, a more reactive halogen can displace a less reactive one in an aqueous solution of its salt.
relative charges:
proton: +1
neutron: 0
electron: -1
atoms have a radius of 0.1nm and the radius of a nucleus is less than 1/10,000 of the atom
relative mass:
proton: 1
neutron: 1
electron: very small
what is the mass number of an element?
the sum of protons and neutrons
what is the atomic number of an element?
number of protons
group 1 properties:
soft
low density
first 3 less dense than water
very reactive with similar properties
transition element properties compared to group 1:
harder and stronger
higher melting points and higher densities
less reactive, so react less vigorously with oxygen or water
transition metals form coloured compounds and are useful as catalysts