Atomic Structure

Mr Tan Kok Kim

Recap: Periodic Table

• Elements in the periodic table - arranged in rows on the basis of increasing atomic mass.

• What are the vertical columns in the periodic table called?

• Ans: Groups

Recap: Periodic Table

• What are the horizontal rows in the periodic table called?

• Ans: Periods

Recap: Periodic Table

• The periodic table is so called because elements with similar properties occur at regular intervals or periods.

• What is the distinctive characteristic of the elements in a group?- They share similar chemical properties.

Mendelev’s periodic table

The modern periodic table

• Click here to see the modern periodic table.

• Click here to learn the symbols of the elements.

The modern periodic table

• The periodic table shown includes the names, symbols and atomic numbers of all the known elements.

• The symbols are a form of shorthand for writing the names of the elements and are recognised worldwide.

• Which state do you think most elements exist in under normal room conditions?

• Ans: Solids

An educated guess

• Mendeleev was so confident that he deliberately left gaps in his periodic table.

• He was able to predict the properties of the unknown elements that would fill the gaps.

• E.g. Mendeleev predicted the existence of germanium which he called eka-silicon. This element was discovered in 1886, fifteen years later!!!

An educated guess

Properties of eka-silicon as predicted by mendelev.

• A grey metal• melting point of about

800 degrees celsius.• Density 5.5 g/cm3

• Relative atomic mass = 73.4

Properties of germanium which was discovered in 1886

• A grey-white metal• melting point of about

958 degrees celsius.• Density 5.47 g/cm3

• Relative atomic mass = 72.6

What is an atom?

• About 400 BC: Greek philosopher Democritus first suggested that all substances consisted of tiny indestructible particles called atoms.

• Since then, scientific experiments have shown that all matter is indeed made up of atoms.

• However, they are very different from what Democritus thought them to be.

The current model of an atom

• All atoms are built from just 3 particles - protons, neutrons and electrons.

• The centre of an atom is called the nucleus and this contains the protons and neutrons.

• The nucleus takes up less than 1 % of the volume of a nucleus.

The current model of an atom

• Proton mass = Neutron mass

• The proton and neutron are each assigned a relative mass of one.

• Protons have a positive charge, but neutrons are neutral.

• More than 99 % of an atom is empty space occupied by moving electrons.

The current model of an atom

• Electrons have a negative charge and move around rapidly.

• Electrons - mass about 2000 times less than that of a proton or a neutron.

• They tend to occupy layers or shells at different distances from the nucleus.

The current model of an atom

• An atom can be described as an electrically neutral entity made up of a positively charged nucleus at its centre with negatively charged electrons moving around the nucleus.

In summary

Particle Position inthe atom

Relativemass

Relativecharge

Proton Nucleus 1 +1

Neutron Nucleus 1 0

Electron Shells 1/2000 -1

Counting sub-atomic particles

• Atomic Number/ proton number = Number of protons present in an atom.

• Mass number/ nucleon number = Sum of the number of protons and neutrons in an atom.

• Number of neutrons = ???

Counting sub-atomic particles

• The figure shown shows how the mass number and atomic number are often shown with the symbol of an element.

197

79 AuMass number at top left of symbol

Atomic number at bottom left of symbol

Test Yourself

35

17 Cl12

6C

• The mass number and the atomic number for chlorine and carbon are shown below. What is the atomic number, mass number, number of protons, electrons and neutrons of each?

Shells of electrons

• An electron shell diagram - simplified model of an atom.

• In these diagrams:– nucleus containing the number of protons and

neutrons.– Electrons arranged in a series of energy levels

called shells.

Filling up in turn

• Each shell contains a limited number of electrons.

• For the 1st 20 elements:

– 1st shell - max of 2 electrons

– 2nd shell - max of 8 electrons

– 3rd shell - 8 electrons

– 4th shell - 8 electrons

The atomic structure of hydrogen and Lithium

Do you know?

• Lead poisoning was a common occurrence in ancient Rome.

• Because the lead used to make water pipes slowly dissolved into the water.

• Acute lead poisoning causes mental disorder and personality changes.

• Some historians attribute the strange behavior of several Roman emperors to lead poisoning.

Do you know?

• ‘Plumber’ and ‘plumbline’ are derived from the latin word plumbum, meaning lead.

• Look up the symbol of lead in the periodic table.

• Where do you think this symbol came from?

Isotopes

• Isotopes are atoms of the same element with same number of protons but different number of neutrons.

• E.g. Chlorine-35 and Chlorine-37

• Do you think both have same chemical properties? Why?

• Ans: Yes because chemical properties are determined by the number of electrons.

Test Yourself

• The figure below shows 2 isotopes of chlorine. What is the number of protons, neutrons and electrons in each case?

35

17 Cl37

17 Cl

Video Clip

Have fun with the Quiz

• Applet Quiz on Atomic Structure, Formulas & Equations(mcq) http://www.chs.edu.sg/~teocc/AQ3/ATOMICSTRUCTURE.html

• Applet Quiz on Atomic Structure, Formulas & Equations(short-ans) formulas & Equations(short-ans question) http://www.chs.edu.sg/~teocc/AQ3/ATOMICSTRUCTUREFF.html