1. Atomic Structure LEARNING OUTCOMES Chapter 2
Describe the structure of atoms of atomic numbers 1 to 20 using illustrations
State the properties of electrons, protons and neutrons
Define atomic number and mass number
Define relative atomic mass
Interpret notations of the form
Define isotopy
List the uses of isotopes
Predict the likelihood of an atom forming an ionic or covalent bond based on atomic structure

2. Atomic Structure Structure of Atoms
Chapter 2
Atomic Structure
Structure of Atoms
An atom is made up of a dense centre called the nucleus, and a cloud of electrons surrounding it.
The nucleus has two types of particles called protons and neutrons.
The protons are positively charged.
The neutrons have no charge (neutral).

3. Relative electric charge
Chapter 2
Atomic Structure
Structure of Atoms
The electrons are negatively charged.
The number of protons is always equal to the number of electrons, so an atom is electrically neutral.
The relative mass and charge of each particle is as shown in the table.
Particle
Relative mass
Relative electric charge
Proton 1 1+
Neutron
Electron
≈ 0 1−
__1_
1 836

4. Proton number and Nucleon number
Chapter 2
Atomic Structure
Proton number and Nucleon number
The proton number of an atom is equal to the number of protons in the atom.
The proton number is also called the atomic number.
The proton number of the atom shown above is 3.

5. Atomic Structure Nucleon number Chapter2
The nucleon number of an atom is equal to the total number of protons and neutrons in the atom.
The nucleon number is also called the mass number since the protons and neutrons give the atom its mass.
The nucleon number for the atom shown is 6.

6. X a b Atomic Structure Atomic symbol
Chapter 2
Atomic Structure
Atomic symbol
The structure of an atom can be written in symbol form as: X a b
Nucleon number
Proton number
E.g. What are the atomic particles found in an atom of fluorine, 199 F ?
ANSWER: 9 protons electrons neutrons (19 – 9 =10 )

7. Atomic Structure Electron arrangement
Chapter 2
Atomic Structure
Electron arrangement
The electrons surround the nucleus like a cloud and move around it in fixed orbits or shells.
Each shell is numbered 1, 2, 3, 4 and so on, going outwards from the nucleus. Each shell can hold a certain maximum number of electrons.
The maximum number of electrons each shell can hold is given by: 1st shell = 2 2nd shell = 8 3rd shell = 8 ( for the first 20 elements in the Periodic Table )

8. Atomic Structure Electron arrangement
Chapter 2
Atomic Structure
Electron arrangement
1st shell
The electron arrangement of an atom is also called its electronic structure. Na
2nd shell
3rd shell
The electronic structure can also be stated as a series of numbers called the electron configuration.
The electronic configuration of the sodium atom shown is:

9. Atomic Structure Outer Electrons Chapter 2
The electrons in the outermost shell are called the outer electrons or valence electrons.
Valence electrons

10. Atomic Structure Isotopes
Chapter 5
Atomic Structure
Isotopes
Isotopes are atoms of the same element which have the same number of protons but different numbers of neutrons.
Isotopes are chemically alike in all aspects, except for their masses. However, some isotopes are radioactive.
Hydrogen has three isotopes 11H, 21H and 31H.
Isotopes of hydrogen
21H is called deuterium, 31H is called tritium. Both isotopes are found in sea water and the Sun.

11. Atomic Structure Isotopes
Chapter 5
Atomic Structure
Isotopes
Chlorine exists as two main isotopes: 3517Cl (75%) and 3717Cl (25%).
The average atomic mass of chlorine is given by the weighted mean of the two isotopes: x x 37 = 35.5
Hence, in the Periodic Table the atomic mass of chlorine is given as Cl.

12. Atomic Structure Ions Chapter 5
In an atom, the number of electrons is equal to the number of protons, so an atom is electrically neutral.
However, atoms can lose or gain electrons to form charged particles called ions.
Atoms form ions so that they can have the stable electronic structure of a noble gas such as helium, neon or argon.
A sodium atom (2.8.1), gives away its 1 outer electron to form a positively charged sodium ion, Na+ (2.8), which has the same electronic structure as neon.

13. Positive and negative Ions
Chapter 5
Atomic Structure
Positive and negative Ions
If an atom gives away its outer electrons, then it has more protons than electrons and becomes a positively charged ion.
If an atom takes in electrons, then it has more electrons than protons, and it becomes a negatively charged ion.
Examples:
Mg  Mg e- (Mg loses two electrons to form a positive ion, Mg2+)
Cl + e-  Cl (Cl gains one electron to form a negative ion, Cl-)

14. Positive and Negative Ions
Chapter 2
Atomic Structure
Positive and Negative Ions

15. Ions of metals and non-metals
Chapter 2
Atomic Structure
Ions of metals and non-metals
Metals e.g. sodium, calcium and aluminium which have less than 4 electrons in their outermost shells, always give away electrons to form positive ions.
Non-metals like chlorine, oxygen and nitrogen which have more than 4 electrons in their outer shell, take in electrons to form negative ions.

16. Molecules of elements and compounds
Chapter 2
Atomic Structure
Molecules of elements and compounds
A molecule is made up of two or more atoms chemically joined together.
Molecules of elements are made up of the same kind of atoms.
Molecules of compounds are made up of two or more different kinds of atoms.
Molecules of elements
Molecules of compounds

17. Atomic Structure Quick check 1 Chapter 2
The nucleus of an atom contains ______ and _______.
A proton has a relative mass of _____ and a relative charge of ________ .
An electron has a ______ mass and a charge of ______ .
A neutron has a relative mass of _____ and a charge of _______ .
An atom of potassium is represented by the symbol 3919K. How many protons, electrons and neutrons are there in a potassium atom?
An atom of chlorine contains 17 protons, 18 neutrons and 17 electrons. Write the atomic symbol for chlorine.
Solution

18. Atomic Structure Quick check 2
Chapter 2
Atomic Structure
Quick check 2
What are isotopes? Give an example of an element with isotopes.
The table below shows four atoms.
Complete the table.
Which two atoms are isotopes?
Which atom has the greatest mass?
Atom
Nucleon number
Proton number
Number of neutrons
Number of electrons A 14 6 B 15 7 C D 16 8
Solution

19. Atomic Structure Quick check 3 Chapter 2
Calcium is in Period IV, Group 2 of the Periodic Table. Deduce the electronic structure of calcium.
2. Complete the following equations: (a) Li  Li __e- (b) Mg  Mg __ (c) Cl + e-  ____ (d) S + ____  S2- (e) Al  ____ + 3e-
3. Complete the table on the right.
Draw the electronic structure of the following: (a) boron,
(b) phosphorus, (c) chloride ion, (d) calcium ion.
Element
No. of protons
No. of electrons
Formula of ion Na 11 10 Ca 20
Ca2+ Br 36
Br - O 8
O2-
Solution

20. Atomic Structure The nucleus of an atom contains protons and neutrons.
Chapter 2
Atomic Structure
Solution to Quick check 1
The nucleus of an atom contains protons and neutrons.
A proton has a relative mass of 1 and a relative charge of +1 .
An electron has a negligible mass and a charge of - 1.
A neutron has a relative mass of 1 and a charge of 0 .
19 protons, 19 electrons and 20 neutrons.
3517Cl
Return

21. Atomic Structure Chapter 2 Solution to Quick check 2
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. An example of an element with isotopes is hydrogen.
Atom
Nucleon number
Proton number
Number of neutrons
Number of electrons A 14 6 8 B 15 7 C 9 D 16
(b) Which two atoms are isotopes? A and C
(c) Which atom has the greatest mass? D
Return

22. Solution to Quick check 3
Chapter 2
Atomic Structure
Solution to Quick check 3
The electronic structure of calcium is
(a) Li  Li e- (b) Mg  Mg e- (c) Cl + e-  Cl- (d) S + 2e-  S2- (e) Al  Al e-
Complete the table on the right.
Element
No. of protons
No. of electrons
Formula of ion Na 11 10
Na+ Ca 20 18
Ca2+ Br 35 36
Br - O 8
O2-
Return

23. Solution to Quick check 3
Chapter 2
Atomic Structure
Solution to Quick check 3 4.
(a) boron
(b) phosphorus ok
Return
(c) chloride ion
(d) calcium ion

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Chapter 2
Atomic Structure
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