Intermediate 2
Unit 1: Building Blocks
Section 1 “Substances & Atoms” Revision
These next few pages will serve as a quick revision of Section 1. It will contain summary notes, key points,
example questions and answers. Though fairly comprehensive, note that when revising for your Prelims and
final exams, do read through the Section 1 booklet and try the home questions and others for yourself.
1.1 Elements
① Atoms in Elements
- Each of the 118 Elements are made from millions of tiny particles called atoms.
- Atoms contain a collection of protons and neutrons in the nucleus and have electrons
whizzing around about.
- The number of protons determines which element an atom belongs to.
- An element’s name can be shortened to a single letter, or sometimes several.
E.g. Lead = Pb Carbon = C and Gold = Au.
② Grouping Elements
- Elements are grouped and displayed in the Periodic Table. This table sorts out elements in a
specific order, based on the number of protons of an atom (its atomic number, Z).
- Because elements that have similar numbers of sub-atomic particles have similar properties,
the periodic table also can help categorise an element.
Metals and Non-metals
Table 1 shows the metallic and non-metallic elements can be divided by the line in bold. There are
some elements called metalloids, shown in green, which can show metallic characteristics even
though they are non-metals (and vice versa).
Solid, Liquid, Gas
Most elements are solid at room temperature but 11 are gases whilst only 2 are liquids.
Synthetic, non-synthetic?
Most elements do exist naturally, but some are man-made. Look in your data book to find out which
ones.
③ Elements in the Air
Air contains a mixture of gases, here are some:
Oxygen – Important for all life. Oxygen atoms are joined in pairs making a diatomic gas – O2
– You can test for O2 by relighting a glowing splint.
– 21% air is O2
Nitrogen – The most abundant gas in the air at 78%
– It also is diatomic, N2
Argon – This gas is monatomic, Ar
– 1% of air is Ar.
1.2 Compounds
① Forming Compounds
A compound is a substance made from different types of atoms which are chemically joined
together, e.g. H2O.
This is different from a mixture, which has different types of atoms together that is not chemically
bound and can be easily separated.
② Names of Compounds
Often compounds will have the suffix “-ide” or “-ate” at the end.
- “-ide” often means there is a metal bounded to a non-metal.
- “-ate” means that the compound has THREE or more elements in it AND has OXYGEN
present
- Note that if a compound has an –OH this is an “-ide” e.g. KOH = Potassium hydroxide
③ Formulae for compounds
Often a compound will have a collection of atoms joined together in definite groups called molecules
A molecule is a particle made of two or more atoms joined together.
E.g. Sulphuric acid, H2SO4 , the numbers mean there are 2 Hydrogen atoms, 1 Sulphur atom and 4
Oxygen atoms.
In a chemical’s names we often get a prefix that relates to the numbers of atoms in that molecule
1
mono
2
di
3
tri
e.g. tungsten hexachloride – WCl6
4
tetra
5
penta
6
hexa
7
hepta
8
octa
or carbon dioxide – CO2
1.3 The Periodic Table
① Arranging by pattern
The periodic table sorts the elements by way of their atomic number, Z. This can also be a measure
of atoms mass. i.e. Hydrogen with 1 proton is lighter than Helium with 2 protons.
② Modern Periodic table
Scientists look for patterns to help us understand things. Atoms with similar chemistry are placed
near each other as chemical trends can be seen periodically
The periodic table matches all the chemical families close together in periods (rows) and
groups (columns).
Some Groups:
Group 1 – Alkali Metals – These are very reactive with water and make alkali solutions
Group 2 & 3 – Transition metals – This large block of elements vary slightly in properties.
Group 7 – Halogens – Diatomic, non-metallic salt forming elements
Group 8 – Noble Gases – Monoatomic, non-reactive gases.
③ Linking Atoms
Generally speaking, the periodic table can be used to work out the valency of an atom, a bit like how
much space it has for other atoms or molecules to bind to. This lets us see how atoms can be linked
together.
Group
Valency
1
1
2
2
3
3
4
4
5
3
6
2
7
1
8
-
1.4 Solutions and formulae
① Soluble or Insoluble?
 If a substance is Soluble, it dissolves well. (like salt in water)
 The solid which dissolves is known at the Solute.
 A liquid which dissolves it is called the Solvent.
 The resulting mixture is the Solution.
 If the solution can dissolve no more solute it is saturated.
② Formulae with Groups
- A specific collection of atoms in a compound can be called a functional group. e.g.
Sulphates, as we found out earlier, the name suggests they contain sulphur and oxygen( “ate”) : SO4
- Functional groups like SO4, CO3 or OH can be said to also have a valency. So we can work out
which elements are likely to join with which functional group.
③ Brackets
Often compound formulas will have brackets in them. These can be for 3 reasons
1. State symbol i.e. whether it is a solid, gas or liquid. e.g. H2O(aq)
2. How many of the same functional groups are present e.g. Ca(OH)2
3. What the charge on an atom is e.g. Fe(III)Cl3 , Iron (III)chloride.
1.5 Atomic Structure
① The Dalton Model
- Atom is a single particle which is tiny, round and solid.
- Useful when thinking of valency diagrams or molecular models
- Best shows how solid, liquids and gases are arranged in space
② Rutherford’s Model
- A central nucleus with electrons in orbits around it.
- Negative electrons
- Positive protons and neutral neutrons in the nucleus.
- Helps explain electrical conduction by showing that the –ve
charge (the electron) can be free to move away from the
atom.
③ Atomic Numbers
- The atomic number of an element, Z, is the number of protons in a nucleus.
- Each Proton has a charge of +1, so the charge on the nucleus is the same as the number of
protons. e.g. Sodium Z = 11, Charge on nucleus = +11.
- To make a neutral atom, we must have the same number of electrons as protons.
e.g. Na has 11 electrons.
1.6 Arranging Electrons
① Target Model
This shows how the electrons in an atom are arranged in separate energy levels.
In the first E level we can have 2 electrons. In the second and third we can have 8.
E.g. this is target model of a phosphorus atom with 15 electrons.
The 2 electrons closest to the nucleus are in the lowest energy level.
② Electron arrangements
- Note that in the periodic table, atoms of each of the elements in a group have the same
number of outer electrons.
- The outer electrons are the ones that are the most important and denote an atom’s
chemistry.
③ Drawing Target Models
- Work out the atomic number and therefore the number of electrons in an atom
- Work out the electronic configuration (remember 2,8,8 etc.)
- Work out which electrons pair in each energy level. (when adding electrons; draw four in
separately then begin to pair up for 5,6,7 and 8.)
1.7 Nuclear Numbers
-
-
Almost all the mass of an atom is in the nucleus. As we touch on earlier, the nucleus contains
protons and neutrons. In terms of mass they can be considered to both be 1 atomic mass
unit (a.m.u.).
Electrons have such mall mass that it can be ignored.
Atoms of the same element (i.e. the same atomic number, Z) can have different masses as
they have a different number of neutrons
These are called isotopes.
The mass number, A, is the number of protons and neutrons.
So A-Z = number of neutrons.