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Atoms and Elements
Atoms are very small, and are made up of a nucleus that's surrounded by electrons.
The nucleus is in the middle of an atom, and is
home to protons and neutrons. Protons have a
positive charge and neutrons have no charge,
Nucleus
which gives the atom an overall positive charge.
Electrons move around the nucleus and occupy the
shells that surround it. They're negatively charged.
Contains
protons and
Atoms have no overall charge (they're neutral).
neutrons The charge of the electrons is equal but opposite
to the charge of the protons (eg. If E=-1, P=+1). The
number of protons in a nucleus also equals the
Electron Shells / Energy number of electrons in a nucleus.
Levels / Orbits
Carries electrons
around the nucleus…read more

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Mass Number Total number of protons and neutrons
The Periodic Table Atomic Number Number of protons (so number of electrons)
Atoms are represented by symbols, that can be one or two letters. All atoms are
arranged on the periodic table, where atoms of similar properties are grouped
together.
The numbers running along the top of the table
symbolise the 8 different groups of elements.
Elements that are in the same group have the same
number of electrons in their outer shells. They
have similar properties because of this.
Group 1 elements are all metals and all react the
same way. They all react with water to form an
alkaline solution and hydrogen gas.
Group 8 elements (noble gasses) are stable and
unreactive because their outer shells are full.…read more

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In group 1 because only one
electron in the outer shell
Electron Shells
Electrons always occupy shells/energy levels/orbits,
and the lowest energy levels are always filled up first
before the highest energy levels.
A set number of electrons is allowed in each shell:
¬1st Shell ­ 2 electrons
nd Has an electronic
¬2 Shell ­ 8 electrons structure of 2, 8, 8,1
¬3rd Shell ­ 8 electrons
Atoms are content when their outermost shells are full
(like those of noble gasses). Most atoms have outer shells
that aren't full, which is what makes them reactive
In reactions they lose
because they aim to fill their shells and be like noble or gain electrons to
gasses. fill their outermost
shells.…read more

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Compounds
Different elements react and make chemical bonds with other atoms ­ which creates a
compound. To make a bond an atom must give away, take, or share electrons with
another atom. The nuclei of an atom isn't affected by chemical reactions, because only
electrons are exchanged.
Ionic Bonding Covalent Bonding
¬Compounds that are made from a metal ¬Compounds that are made from non-
and a non-metal are made up of ions metals are made up of molecules.
¬The metal atoms lose electrons and form ¬In covalent bonding each atom shares
positive ions and the non-metal atoms gain electrons with another atom to fill up their
electrons and become negative ions outer shell
¬The opposite charges of the ions make
them strongly attracted to each other
Filled their
outermost
shells…read more

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Balancing Equations
In chemical reactions, atoms aren't lost or gained. There must be the same amount of
atoms at the start of a reaction as there are at the end of a reaction. Sometimes
chemical reactions don't show this, and so they need to be balanced.
Worked Example
How to Balance an Equation HSO + NaOH NaSO + HO
¬There has to be the same number of atoms
of each element on both sides of the H=3 S=1 O=5 Na=1 H=2 S=1 O=5 Na=2
equation, so first tally up how many atoms The Hydrogen atoms (H) and the sodium atoms (Na) don't
balance.
of each element there are.
HSO + NaOH NaSO + 2HO
¬Identify the elements that don't balance H=3 S=1 O=5 Na=1 H=4 S=1 O=6 Na=2
¬Balance the equation by putting numbers You can put 2 in front of the HO to try and balance out the
in front of the places its needed number of hydrogen atoms on each side. However, when we do
this we make it so that there are too many H and O atoms on the
right side of the equation.
HSO + 2NaOH NaSO + 2HO
H=4 S=1 O=6 Na=2 H=4 S=1 O=6 Na=2
We can solve this by putting a 2 in front of NaOH, which balances
out the whole equation.…read more

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