1) Atoms are made up of protons, neutrons, and electrons. Protons and neutrons are in the nucleus, while electrons orbit the nucleus in shells. 2) Isotopes are atoms of the same element that have different numbers of neutrons, giving them different mass numbers but the same chemical properties. 3) Electrons fill the lowest available shell around the nucleus. The arrangement of electrons is shown in an element's electronic structure.

1. Structure of Atoms

2. contents
You will learn about:
• Structure of atoms
• Isotopes
• Electron arrangement in atoms
• The Periodic Table
• Ions

3. structure of atoms
composition
 three types of particles in an atom: proton, neutron and
electron
Particle Relative Mass Relative Electric Charge
proton 1 +1
neutron 1 0
electron 1/2000 ≈ 0 -1
 consists of a nucleus surrounded by electrons in shells
 nucleus in turn contains protons and neutrons

4. structure of atoms
composition
 structure of an atom:
nucleus
(protons + neutrons)
- electrons in shells
around nucleus
+ -
structure of an atom
 number of protons is the proton number (atomic number)
 number of electrons = number of protons ⇒ atom is
electrically neutral
 total number of protons and neutrons is the nucleon
number (mass number)

5. structure of atoms
simplified structure
 simplified structure of a typical atom:
- electron
+ proton
neutron
6 -
proton number 6+ nucleon number
=6 6 = 6 + 6 = 12
structure of a carbon atom

6. structure of atoms
symbol form
 written in symbol form
nucleon
Cl
number 35 symbol of
proton 17 the element
number
 symbol tells us that the atom
o is of element chlorine
o has proton number 17 (so it contains 17 protons)
o has 17 electrons (number of protons = number of electrons)
o has nucleon number 35 (so number of protons + number of
neutrons = 35)
o it must contain 35 – 17 = 18 neutrons

7. isotopes
isotopes
 are atoms of the same element with different numbers
of neutrons
 same proton number but different nucleon numbers
(mass numbers)
Isotopes
23
11Na 24
11 Na
number of protons 11 11
number of electrons 11 11
number of neutrons 23 - 11 = 12 24 – 11 = 13

8. isotopes
isotopes
 have same chemical properties and form compounds with
the same formula because they have the same number and
arrangement of electrons
 have small differences in physical properties
 can be radioactive or non-radioactive

9. electron arrangement in atoms
electrons
 electrons are arranged around the nucleus of the atom in
shells
 atomic nucleus (positive charge) attracts the electrons
(negative charge) by an electrostatic force
 each shell can hold a maximum number of electrons
Maximum Number
Shell Number
of Electrons
1 2
2 and above 8
 electrons always go into the shell nearest to the nucleus, if
there is room; otherwise they go to the next available shell

10. electron arrangement in atoms
electronic structure & outer electrons
 is the arrangement of electrons in atoms
 three ways of showing the electronic structure of a silicon
atom:
valence
valency shell
- electrons -
- -
- -
-
-
- + -
-
- - + - 2.8.4
- -
- -
full electronic outer electronic electronic
structure structure structure

11. electron arrangement in atoms
electronic structure & outer electrons
 electronic structures of the first 20 elements:
Number of Electrons in Simplified
Proton
Element Symbol 1st 2nd 3rd 4th Electronic
Number
shell shell shell shell Structure
hydrogen H 1 1 1
helium He 2 2 2
lithium Li 3 2 1 2.1
beryllium Be 4 2 2 2.2
boron B 5 2 3 2.3
carbon C 6 2 4 2.4
nitrogen N 7 2 5 2.5
oxygen O 8 2 6 2.6
fluorine F 9 2 7 2.7
neon Ne 10 2 8 2.8

12. electron arrangement in atoms
electronic structure & outer electrons
 electronic structures of the first 20 elements:
Number of Electrons in Simplified
Proton
Element Symbol 1st 2nd 3rd 4th Electronic
Number
shell shell shell shell Structure
sodium Na 11 2 8 1 2.8.1
magnesium Mg 12 2 8 2 2.8.2
aluminium Al 13 2 8 3 2.8.3
silicon Si 14 2 8 4 2.8.4
phosphorus O 15 2 8 5 2.8.5
sulphur S 16 2 8 6 2.8.6
chlorine Cl 17 2 8 7 2.8.7
argon Ar 18 2 8 8 2.8.8
potassium K 19 2 8 8 1 2.8.8.1
calcium Ca 20 2 8 8 2 2.8.8.2

13. the Periodic Table
the Periodic Table
 arrangement of elements in a table
 periods show the elements arranged horizontally in order of
proton number (atomic number)
 across each period, the number of outer shell electrons
increases from 1 to 8
 groups show the elements with the same number of outer
electrons arranged in vertical columns

14. the Periodic Table
the Periodic Table
VIII
group number I II III IV V VI VII
proton number
name of element
symbol
full electronic
structure
simplified
electronic
structure

15. ions
ions
 are charged particles; can be either positively or
negatively charged
Negative Ions Positive Ions
movement of atom takes in atom loses
electrons electron(s) electron(s)
type of atoms non-metallic metallic
 atoms form ions by gaining or losing electrons to obtain
the electronic structure of a noble gas

16. ions
ions
 a negative ion is formed by gaining electrons to an atom
Cl atom Cl- ion O atom O2- ion
No of protons 17 17 8 8
Number of electrons 17 18 8 10
 a positive ion is formed by losing electrons to an atom
Na atom Na+ ion Mg atom Mg2+ ion
No of protons 11 11 12 12
Number of electrons 11 10 12 10

17. furthest from
nucleus is members have
negative outer same number of
charge in shells
ge
d shell electrons in
an
ar r
have are electron
group
arrangement is
electronic vertical
Electrons column is
Positive structure
ions Periodic
positive Table
charge
contain elements in
lose electrons
to become have order of
contain is the sum of
Proton
Atoms Protons
gain electrons
number
to become contain
nucleus
Negative contain contain
ions is the sum of
Nucleon
Neutrons
number
have different are
numbers of
which give out
radiation are
Isotopes radioactive neutral