# Atomic Structure Atomic Structure Name

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```					                                       Atomic Structure

Name         Mass      Charge               Location
called        Protons        1         +1                  Nucleus
“nucleons”     Neutrons        1          0                  Nucleus
Electrons    1/1837       -1        Orbit nucleus in outer shells

The number of protons equals the atomic number. This is the number which defines the
element (i.e. all elements have different atomic numbers). The atomic number can be
written as a subscript (below the line) in front of the symbol (e.g. 6C).

In a neutral atom, the number of electrons equals the number of protons.

The mass number is equal to the number of nucleons. The mass number can be written as a
superscript before the symbol (e.g. 12C).

Isotopes are when a single element has atoms with the same number of protons, but
different number of neutrons (and therefore mass number).
Example:     Chlorine comes in two different forms. One form has a mass number of 35
and the other has a mass number of 37:
35
Cl         -             17 protons       18 neutrons
37
Cl         -             17 protons       20 neutrons

Relative atomic mass (Ar) – this is the mass of an “average” atom of an element divided by
one atom of 1H. The Ar takes into account the relative abundance of the naturally occurring
isotopes of the element….ok, so basically, you find the average of the masses of the
different isotopes! The mass number is always an integer (that’s a “whole number” for all
you non-mathematicians) BUT the Ar does not have to be.

Example:
What is the Ar of chlorine given that there are two isotopes: 35Cl (which is 75% abundant) and 37Cl
(which is 25% abundant)?

To do this, we need to look at the abundance to work out the mass of an “average atom” of
Cl. As 35Cl is 75% abundant, we multiply 35 by 75% (i.e. 0.75) and as 37Cl is 25%
abundant, we multiply 37 by 25% (i.e. 0.25). The two numbers are then added together and
divided by the mass of one atom of 1H (i.e. 1 – so this step is pointless…):

(35 × 0.75) + (37 × 0.25) = 35.5

(If you didn’t understand that…don’t worry, here’s an easier way…just for you…simply
remember this formula:
the sum of: mass × abundance (for all the isotopes)
The Periodic Table
There are approximately 100 elements. Everything is made up of one or more elements. The
periodic table shows elements in increasing atomic number.

Electrons are accommodated in shells. The maximum in the first shell is 2, the maximum in
the second shell is 8 and the maximum in the third shell is also 8. Electrons always fill
lower energy shells (i.e. closer to the nucleus) first.

1st electron shell (can hold up             ..          2nd electron shell (can hold up
to 2 electrons)                                         to 8 electrons)
.
.
.                 .
.
.
..      nucleus – with protons and neutrons

The first 20 elements:
Atomic
Symbol                  Element    Shell 1     Shell 2       Shell 3        Shell 4
Number
1          H                  Hydrogen        1
2          He                   Helium        2
3          Li                  Lithium        2             1
4          Be                  Berillium      2             2
5          B                     Boron        2             3
6          C                    Carbon        2             4
7          N                   Nitrogen       2             5
8          O                   Oxygen         2             6
9          F                   Fluorine       2             7
10          Ne                    Neon         2             8
11          Na                   Sodium        2             8             1
12          Mg                Magnesium        2             8             2
13          Al                 Aluminium       2             8             3
14          Si                   Silicon       2             8             4
15          P                 Phosphorous      2             8             5
16          S                   Sulphur        2             8             6
17          Cl                  Chlorine       2             8             7
18          Ar                   Argon         2             8             8
19          K                  Potassium       2             8             8            1
20          Ca                  Calcium        2             8             8            2
Other important elements include:
Atomic                                                        Atomic
Symbol Element                                                      Symbol       Element
number                                                        number
26           Fe        Iron                                  47                  Ag        Silver
29           Cu      Copper                                  53                   I        Iodine
30           Zn       Zinc                                   56                  Ba        Barium
35           Br      Bromine                                 82                  Pb         Lead
In the periodic table, elements are arranged in rows named periods when a single electron
shell is filling. We start a new period when we start filling the next electron shell.

At the top of each column or group, there is a number showing how many electrons are in
the outer shell (e.g. group 7 has 7 electrons in the outer shell).

Element
An element is a pure substance which cannot be broken down any simpler by chemical
means. Atoms of the same element have the same atomic number. A sample of an element
may be made up of atoms or molecules.

Compound
Compounds are pure substances containing two or more elements chemically bonded
together in fixed proportions by mass.
Example:       H2O
2g Hydrogen,       16g oxygen
1g Hydrogen,       8g oxygen

The properties of a compound are different to those of its constituent element. The chemical
bonds in compounds are either ionic or covalent (we will look at this a bit later…fun fun
fun!). Compounds can be either ions or molecules.

Mixtures
Vary in composition and can be physically separated.

Mixtures and Compounds – the difference!

Iron and sulphur mixed together looks like yellow powder with grey bits in it…(a mixture)
You can separate then by:
-      using a magnet (the smart way of getting a needle from a haystack!)
-      dissolving the sulphur in a solvent (e.g. toluene) and filtering when hot. Then
let the solution cool and the sulphur crystallises.

If the mixture is heated, an exothermic reaction (a reaction that gives out heat) takes place.
There are 3 things to prove that a reaction has taken place:
1.     It continues to glow red hot even when you stop heating it
2.     The product is a brittle grey solid, which does not look like the mixture
3.     The mixture reacts with dilute hydrochloric acid to produce hydrogen whereas
the compound (after heating) reacts with hydrochloric acid to make foul
smelling hydrogen sulphide.

Iron(s) +     Sulphur (l)   →      Iron (II) Sulphide(s)
Fe(s) +         S(l)       →              FeS(s)

Atoms of different elements can combine to form compounds by the formation of new
chemical bonds.
Magnesium(s) +     Oxygen(g)    →    Magnesium Oxide(s)
2Mg(s)   +        O2 (g)    →        2MgO(s)
Ions
Ions are formed from atoms by the gain or loss of electrons. The nucleus is left unaffected.
The maximum that can be lost or gained is 3. Generally we form an ion which has a noble
gas configuration (i.e. all full outer shells).
Positive ions are called cations.
Negative ions are called anions.
Atoms of elements in group 1 LOSE one electron to form a 1+ ion of noble gas
configuration:
Na → Na+ + 1e– (electron)
2, 8, 1        2, 8 (same electron configuration as neon)

Atoms of elements in group 2 LOSE two electrons to form a 2+ ion of noble gas
configuration:
Ca → Ca2+ + 2e– (electrons)
2, 8, 8, 2     2, 8 (same electron configuration as argon)

Atoms of elements in group 3 LOSE three electrons to form a 3+ ion of noble gas
configuration:
Al → Al3+ + 3e– (electrons)
2, 8, 3        2, 8 (same electron configuration as neon)

Atoms of elements in group 7 GAIN one electron to form a 1– ion of noble gas
configuration:
(electron) e– + Cl → Cl–
2, 8, 7               2, 8, 8 (same electron configuration as argon)

Atoms of elements in group 6 GAIN two electrons to form a 2– ion of noble gas
configuration:
(electrons) 2e– + O → O2–
2, 6                  2, 8 (same electron configuration as neon)
Formulae of Ionic compounds
The ones in red / green need to be learnt, the rest can be worked out by looking at their
position in the periodic table.
2–                     1–                     1+                     2+                           3+
Mg2+
O2– (oxygen)           F– (fluoride)          Li+ (lithium)                                Al3+ (aluminium III)
(magnesium)
S2– (sulphide)         Cl– (chloride)        Na+ (sodium)           Ca2+ (calcium)
Br– (bromide)         K+ (potassium)          Ba2+ (barium)
I– (iodide)           Ag+ (silver)            Zn2+ (zinc)
H+ (hydrogen)            Pb2+ (lead)
Cu+ (copper I)         Cu2+ (copper II)
Fe2+ (iron II)              Fe3+ (iron III)
SO42– (sulphate)      OH– (hydroxide)            NH4+
CO32– (carbonate)       NO3– (nitrate)         (ammonium)               Radical Ions (2 or more atoms
HCO3–                                     covalently bonded together carrying a
SO32– (sulphite)                                                      charge. they behave as a single unit)
(hydrogencarbonate)
CH3CO2– (ethanoate)

Both copper and iron can form ions of more than one charge. We have to write the charge in
roman numerals in the name to distinguish between the compounds.
Cu2O =        Copper (I) oxide
CuO =         Copper (II) oxide

FeCl2 =        Iron (II) chloride
FeCl3 =        Iron (III) chloride

Learning how to write formulae is VERY important…lots of chemistry is about formulae...
Basically, all you have to do is take the symbol of each element and put it together making
sure that the charges cancel out and that you put the positive ion first.
So…three steps:
1.      In name and formulae, the positive ion goes first
2.      The total charge must equal 0
3.      If you have to multiply a radical ion, put it in brackets

Examples:
Sodium Chloride (Na+ and Cl–)         →      NaCl

Calcium Chloride (Ca2+ and Cl–) →            CaCl2      (notice that 2 Cls are needed to cancel the 2+ of the Ca)

Calcium Nitrate (Ca2+ and NO3–) →            Ca(NO3)2

Try some for yourself (just to see if you really understood it and aren’t just pretending to):

1.     Sodium sulphate                                   5.      Sodium nitrate
2.     Aluminium chloride                                6.      Ammonium chloride
3.     Calcium sulphide                                  7.      Ammonium sulphide
4.     Aluminium nitrate                                 8.      Aluminium sulphide
A GCSE Chemistry word-fill worksheet on
"ATOMIC STRUCTURE, ISOTOPES and ELECTRON STRUCTURE"

15   2      2    2.7    2.8.5 21 21 22 26 26 2nd 3 30 4th 5 5th 7 8 8 9                atomic
atomic electrons electrons electrons group iron isotopes
mass       mass neutrons neutrons nucleus number period shell shells three

Q1(a) Atoms are made of               fundamental particles called protons
(+),              (0) and         (-). (b) The centre of the atom is called
the       . (c) It consists of protons and                and contains most
of the mass of the atom.

Q2(a) The number of protons in the atom is called the
number. It also equals the number of             in a neutral
atom. (b) The                  is the sum of the protons and neutrons in
the nucleus.

Q3 An atom of mass number 43, and atomic number 21, consists of
protons,        neutrons and      electrons.

Q4 (a) In the symbol for an atom of          shown on the left,
the atom consists of       protons,        neutrons and
electrons. (b) Atoms of the same element with a different
number due to different numbers of neutrons are called
.

Q5(a) Electrons are arranged around the nucleus in energy levels or
. (b) The 1st shell can have a maximum of        electrons, the
shell a maximum of         electrons and the 3rd shell a maximum
of          electrons. (c) The 19th and 20th electrons go in the
shell. (d) The electron arrangement, showing the number of
electrons in each      can be written out in shorthand eg 2.8.6 for
the 1st, 2nd and 3rd shells respectively.
Q6(a) Apart from hydrogen and helium in period 1 of the Periodic
Table the last number in the arrangement is equal to the
number in the Periodic Table. (b) The number of shells used
containing electrons is equal to the           number. (c) The number
of electrons in an atom is equal to the           or proton number.

Q7(a) Phosphorus has an atomic number of 15 so the atoms have
protons or 15            . (b) The electron arrangement will be
. (c) In the Periodic Table it will be a member of Group
and it will be the       element along Period    .

Q8(a) Fluorine has an atomic number of            so the atoms have 9
protons or electrons. (b) Its electron arrangement is          and it
belongs to Group            on Period    .

Ionic Equations – Questions

(A) Translate into symbols and balance each equation:

1.  calcium carbonate breaks down on heating to give calcium oxide and carbon dioxide.

2.  sulphur burns in oxygen to form sulphur dioxide gas.

3.  heating magnesium and sulphur powders together produces solid magnesium sulphide.

4.  sodium hydroxide and hydrochloric acid react to form sodium chloride and water.

5.  hydrogen gas and oxygen gas burn together to form water.

6.  zinc metal and copper sulphate solution react to produce copper and zinc sulphate solution.

(B) Balance these equations and add state symbols:
1. CuO + HNO3        to     Cu(NO3)2 + H2O

2. Al + Cl2   to    AlCl3

3. N2 + H2   to     NH3

4. Na + H2O         to       NaOH + H2

5. K2CO3 + H2SO4   to        K2SO4 + H2O + CO2

6. Fe + H2SO4       to       FeSO4 + H2

7. CH4 + O2         to   CO2 + H2O

8. Pb(NO3)2 + NaCl  to    PbCl2 + NaNO3

(C) Balance these equations:

1. MnO2 + HCl       to       MnCl2 + Cl2 + H2O

2. HBr + H2SO4      to       SO2 + Br2 + H2O

3. HI + H2SO4       to       I2 + H2S + H2O

4. KMnO4 + HCl      to       KCl + MnCL2 + Cl2 + H2O

5. FeSO4 + KMnO4 + H2SO4 to                    (Fe)2 (SO4)3+K2SO4+MnSO4+H2O

A.   .
1.   CaCO3(s)                   CaO(s) + CO2(g)
2.   S(s) + O2(g)               SO2(g)
3.   Mg(s) + S(s)               MgS(s)
4.   NaOH(aq) + HCl(aq)         NaCl(aq) + H2O(l)
5.   2H2(g) + O2(g)             2H2O(l)
6.   Zn(s) + CuSO4(aq)          ZnSO4(aq) + Cu(s)

B.
1.   CuO(s) + 2HNO3(aq)         Cu(NO3)2(aq) + H2O(l)
2.   2Al(s) + 3Cl2(g)           2AlCl3(s)
3.   N2(g) + 3H2(g)             2NH3(g)
4.   2Na(s) + 2H2O(l)           2NaOH(aq) + H2(g)
5.   K2CO3(s) + H2SO4(aq)       K2SO4(aq) + H2O(l) + CO2(g)
6.   Fe(s) + H2SO4(aq)          FeSO4(aq) + H2(g)
7.   CH4(g) + 2O2(g)            CO2(g) + 2H2O(g)
8.   Pb(NO3)2(aq) + 2NaCl(aq)   PbCl2(s) + 2NaNO3(aq)

C.
1. MnO2 + 4HCl                  MnCl2 + Cl2 + 2H2O
2. 2HBr + H2SO4                 SO2 + Br2 + 2H2O
3. 8HI + H2SO4                  4I2 + H2S + 4H2O
4. 2KMnO4 + 16HCl               2KCl + 2MnCl2 + 5Cl2 + 8H2O
10FeSO4 + 2KMnO4 +           5(Fe)2(SO4)3 + K2SO4 + 2MnSO4 +
5.
8H2SO4                       8H2O

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