c02 atomic structure

LEARNING OUTCOMES

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

Atomic StructureChapter 2

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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).


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 1 0

Electron ≈ 0 1−


__1_1 836

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.


Nucleon number

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.

Atomic StructureChapter2

Atomic symbol The structure of an atom can be written in symbol form as:

Nucleon number

Protonnumber

E.g. What are the atomic particles found in an atom of fluorine, 19

9 F ?ANSWER: 9 protons 9 electrons 10 neutrons (19 – 9 =10 )


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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 = 22nd shell = 83rd shell = 8 ( for the first 20 elements in the Periodic Table )


Electron arrangement The electron arrangement

of an atom is also called its electronic structure.

Na

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: 2.8.1

1st shell

2nd shell

3rd shell


Outer Electrons The electrons in the outermost shell are called the outer

electrons or valence electrons.


Valence electrons

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 1

1H, 21H and 3

1H.

21H is called deuterium, 3

1H is called tritium. Both isotopes are found in sea water and the Sun.

Isotopes of hydrogen


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: 0.75 x 35 + 0.25 x 37 = 35.5

Hence, in the Periodic Table the atomic mass of chlorine is given as 35.5

17Cl.


Ions 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.


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 Mg2+ + 2e-

(Mg loses two electrons to form a positive ion, Mg2+)

Cl + e- Cl- (Cl gains one electron to form a negative ion, Cl-)


Positive and Negative Ions


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.


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

Quick check 1

1. The nucleus of an atom contains ______ and _______.2. A proton has a relative mass of _____ and a relative charge of

________ .3. An electron has a ______ mass and a charge of ______ .4. A neutron has a relative mass of _____ and a charge of _______ .5. An atom of potassium is represented by the symbol 39

19K. How many protons, electrons and neutrons are there in a potassium atom?

6. An atom of chlorine contains 17 protons, 18 neutrons and 17 electrons. Write the atomic symbol for chlorine.

Solution


Quick check 21. What are isotopes? Give an example of an element

with isotopes.2. The table below shows four atoms.

Atom Nucleon number

Proton number

Number of neutrons

Number of electrons

A 14 6

B 15 7

C 15 6

D 16 8

(a) Complete the table.

(b) Which two atoms are isotopes?

(c) Which atom has the greatest mass?

Solution


Quick check 3

2. Complete the following equations: (a) Li Li+ + __e-

(b) Mg Mg2+ + __ (c) Cl + e- ____(d) S + ____ S2-

(e) Al ____ + 3e-

3. Complete the table on the right. 4. 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

1. Calcium is in Period IV, Group 2 of the Periodic Table. Deduce the electronic structure of calcium.


1. The nucleus of an atom contains protons and neutrons.2. A proton has a relative mass of 1 and a relative charge of

+1 .3. An electron has a negligible mass and a charge

of - 1.4. A neutron has a relative mass of 1 and a charge

of 0 .5. 19 protons, 19 electrons and 20 neutrons.

6. 3517Cl

Return


Solution to Quick check 1

1. 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.

2. Atom Nucleon

numberProton number

Number of neutrons

Number of electrons

A 14 6 8 6

B 15 7 8 7

C 15 6 9 6

D 16 8 8 8

(b) Which two atoms are isotopes? A and C

(c) Which atom has the greatest mass? DReturn




1. The electronic structure of calcium is 2.8.8.2

2. (a) Li Li+ + e-

(b) Mg Mg2+ + 2e- (c) Cl + e- Cl-

(d) S + 2e- S2-

(e) Al Al3+ + 3e-

3. Complete the table on the right.

Return


Element No. of protons

No. of electrons

Formula of ion

Na 11 10 Na+

Ca 20 18 Ca2+

Br 35 36 Br -

O 8 10 O2-

4.

(a) boron (b) phosphorus

(c) chloride ion (d) calcium ionReturn



1. http://www.unisanet.unisa.edu.au/08365/h&patoms.htm 2. http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm 3. http://www.chem4kids.com/files/atom_structure.html 4. http://www.colorado.edu/physics/2000/isotopes/

To learn more about Atomic Structure, click on the links below!


ReferencesReferences

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver

c02 atomic structure

Science

mass number

neutronsdefine atomic

total number of protons

nucleon numberproton

atomic numbers

atomic particles

atom of fluorine

sodium atom