Science 9
Chapter 4 Notes: The Periodic Table
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Dmitri Mendeleev constructed the first periodic table in the 1860s. He
arranged the known elements in order by increasing atomic mass, and
grouped them in the same columns according to similar properties. A
positive feature of his table was that it had gaps or spaces, where he
predicted undiscovered elements. However, a negative feature was that
in some cases, he had to have elements out of order by mass – e.g. Co
and Ni, which he couldn’t explain.
The modern periodic table (constructed after Henry Moseley
discovered atomic number by protons in 1913) arranges the elements in
order by atomic number (number of protons), and groups them into
columns by similar electron configuration (e’s in outer level).
The rows correspond to the number of energy levels in an atom –
elements in the same row have the same number of energy levels.
The groups are based on the electron arrangement (i.e. number of e’s in
outer level) – elements in the same group have the same e- arrangement.
The table has 2 general regions – metals and nonmetals, separated by a
staircase or zig-zag line, based on how the elements react.
Metals, Nonmetals, and Metalloids
metals - left of staircase line, not including those touching the line,
except Al (it's a metal)
- hard (except Hg), shiny, good conductors
- lose electrons to form +ve ions
nonmetals - right of staircase line, not including those touching the line
- most are gases, one liquid; solids are brittle, dull, poor conductors
- gain electrons to form -ve ions
metalloids - the 8 elements touching the staircase line, except Al (it's a metal)
- have weak properties of both metals and nonmetals
Groups Names (columns) - some of the groups have special names:
group 1 - alkali metals; the most reactive metals
group 17 - halogens; the most reactive nonmetals
group 18 - noble gases; unreactive
Science 9
Chapter 4 Notes: The Periodic Table
Page 2 of 2
Predicting Reactivity
• metal reactivity - the reactivity of metals increases as you move down
a group – the metals farther down have their outer e’s farther from the
nucleus, so they are held less tightly and are easier to remove (more
reactive as losers of e’s).
• nonmetal reactivity - the reactivity of nonmetals decreases as you
move down a group – the nonmetals farther down have their outer e’s
farther from the nucleus, so there is a weaker attraction to gain more
e’s.
• e.g. Mg and Ca are both in group 2, so they both want to lose 2 e’s to
become stable. Mg’s 2 e’s are in the 3rd level, while Ca’s 2 e’s are in
the 4th level. The 2 e’s in the 4th level are held less tightly, so Ca is
more reactive.
• Copy the following elements, and then rewrite them in order of
increasing reactivity (least reactive first, most reactive last):
K, Li, Na
Li, Na, K
Ba, Be, Sr
Be, Sr, Ba
Cl, I, Br
I, Br, Cl
S, Te, O
Te, S, O