ATOMIC STRUCTURE

Atom is the smallest particle of an element that can take part in a chemical reaction. An atom has all properties of a given element.

Atoms can combine together to form a molecule.

A molecule is the a particle that consists of two or more atoms which are chemically combined together.

Generally, molecules can be categorized into:

• a molecule of an element
• a molecule of a compound.

A molecule of a compound must contain more than one type of atom; for example a molecule of water (H₂O ) is made up of two atoms of hydrogen element and one atom of oxygen.

Both molecule and atom are neutral, that is, they have no charges.

Molecules usually exist on their own. Atoms may or may not exist on their own.

Atomic Theory

The word "atom" derived from Greek word which means "indivisible". Democritus was a Greek philosopher who was the first person to use the term atom (atomos meaning indivisible).

He developed his theory which he called Theory of the Universe.

Short history on the development of theories of matter

Ancient Greek philosophers developed a theory of matter 2000 years ago.

Democritus and Aristotle lived in different generations, though Democritus preceded the generation of Aristotle, proposed their different views on theory of matter.

• Democritus believed that matter was made up of tiny, indivisible particles called atomos.
• Aristotle, however, believed that matter was continuous and could be infinitely divided.
• Aristotle's view was accepted for almost 200 years.

In 1803, John Dalton proposed a different theory of matter based on scientific observations and logical laws. John Dalton was the first to use the Democritus theory into modern atomic theory.

Dalton's Atomic Theory of Matter

1. All matter is made up of tiny indivisible particles called atoms.
2. Atoms cannot be created or destroyed.
3. Atoms of same element are identical and they differ from those of other elements.
4. Atoms may combine to form a molecule, but the combination must involve small whole numbers.

The Weakness of Dalton’s Atomic theory

1. It did not explain why atoms combine with each other.
2. Discovery of isotopes indicated that all atoms of the same element are not perfectly identical.
3. With the discovery of sub-atomic particles, atom is no longer considered as indivisible.
4. With the existence of complex compounds and polymers atoms are no longer combine in small number ratios. .

Modern Atomic Theory of Matter

1. An atom is made of even smaller sub-atomic particles called protons, neutrons and electrons.
2. Atoms can be created or destroyed through nuclear reactions.
3. Atoms of the same element can have different masses. Such atoms are called isotopes.
4. Atoms of different elements may combine in many different ratios to form complex compounds. Examples are hydrocarbons.

Subatomic Particles

• Protons
• Neutrons
• Electrons

Electron

Proton

Neutron

The collective name for neutrons and protons is known as nucleons.

An atom is neutral because the number of electrons equals to the number of protons in an atom.

Arrangement of Electrons in an Atom

• Always the electrons move around the nucleus of an atom in the special path which is known as shell or energy level
• Each shell can hold only a certain number of electrons.
• The number of electrons each shell can hold is determined by the formula 2n² where n = the number of shell starting from that nearest to the nucleus (the position of a shell from the nucleus). For example,

It is, however, important to note that for the first 20 elements, the third shell or energy level can only hold a maximum of 8 electrons.

The arrangement of electrons around the nucleus of an atom is known as electronic configuration.

The electronic configuration of an element can be represented simply by numbers separated by commas (,) or dots (.) or ratio symbol sign (:).

Electrons fill up shells from the inner shell to the outer shell. This is because the innermost shell is of lowest energy.

ATOMIC NUMBER, MASS NUMBER AND ISOTOPES

1. Atomic number (Z)

This is the number of protons in the nucleus of an atom.
→It is also the number of electrons in a neutral atom.
→The official symbol for atomic number is Z.
→It is written as a subscript in front of the symbol for the atom.
For example an atom of Calcium which has atomic number of 20, it can be written as ₂₀Ca.


2. Mass number (A)

This is the sum of the numbers of protons and neutrons in the nucleus of an atom.
→Its official symbol is A.
→It is written as superscript in front of the symbol for the atom.
For example, calcium has a mass number of 40, therefore it can be written as ⁴⁰Ca.
→The combination of atomic number and mass number for a particular atom can be written as _Z^AX. This way of writing is known as NUCLIDE NOTATION.
→Therefore the nuclide notation of calcium is ₂₀⁴⁰Ca


The relationships among atomic number, mass number and sub-atomic particles.

• Atomic number (Z)= number of protons(p)=number of electrons in a neutral atom.
• Mass number (A) =Number of protons (p) + Number of neutrons (N).
• Number of neutrons (N) = Mass numbers (A) -Atomic number (Z).
N=A-Z.

3. Isotopes

1. the same number of protons, electrons and atomic number.
2. the same chemical properties but may have slightly different physical properties.

Why isotopes have the same chemical properties?
Because they have the same arrangement and number of electrons.
3. different mass numbers and neutron numbers.

1. Hydrogen has three isotopes
Protium ¹₁H or hydrogen-1
Deuterium ²₁H or hydrogen-2
Tritium ³₁H or hydrogen-3.

2. Chlorine has two isotopes
Chlorine-35
Chlorine -37

3. Carbon has three isotopes
Carbon-12
Carbon-13
Carbon -14

Relative Atomic mass

Example.

Ar =∑(Relative Abundance × mass number)

   = (75×35)+(25×37)
            100
   = 35.5
Therefore, 
the relative atomic mass of chlorine is 35.5
The relative atomic mass is always a decimal 
number between the mass numbers of the isotopes.
The isotope with the higher abundance has
a greater influence on the relative atomic mass.  

Solution

Ar = (x/100 × 35) + (25/100 × y)

Substituting the given values:
35.5 = (x/100 × 35) + (25/100 × y)

Rearranging the equation:
35.5 = 0.35x + 0.25y

Since the abundances of the two isotopes must
add up to 100%, we can say:
x + 25 = 100

Rearranging this equation:
x = 75

Now, we can substitute the value of x into 
the first equation:
35.5 = 0.35(75) + 0.25y
35.5 = 26.25 + 0.25y
9.25 = 0.25y
y = 37

Therefore, the values of x and y are:
x = 75% (the abundance of the 35 isotope)
y = 37 (the mass of the other isotope)

Example 5:Given the following information about the isotopes of copper:
Cu-63 has an abundance of W in a sample.
Cu-65 has an abundance of 30.83% in a sample.
The relative atomic mass of copper is 63.546.
Calculate the value of W.
Answer: 69.05%.

Multiple Choice Questions
(i)The smallest part of an element that can exist is called:

    A. Molecule
    B. Atom
    C. Compound
    D. Mixture
(ii)Which of these particles has a positive charge?

    A. Electron
    B. Neutron
    C. Proton
    D. All of the above
(iii)The part of an atom where protons and neutrons are found is called:

    A. Electron shell
    B. Nucleus
    C. Orbit
    D. Energy level
(iv)Which particle in an atom has almost no mass?

    A. Proton
    B. Neutron
    C. Electron
    D. All have equal mass
(v)Atoms of the same element with different numbers of neutrons are called:

    A. Isotopes
    B. Ions
    C. Molecules
    D. Compounds
(vi)Electrons are found:

    A. In the nucleus
    B. Orbiting the nucleus
    C. Between protons and neutrons
    D. Nowhere in an atom
(vii)The atomic number of an element tells you the number of:

    A. Protons
    B. Neutrons
    C. Electrons
    D. All of the above
(viii)Which of these is NOT a part of an atom?

    A. Proton
    B. Neutron
    C. Electron
    D. Molecule
(ix)The mass of an atom mostly comes from:
    A. Protons and neutrons
    B. Electrons
    C. The empty space in the atom
    D. The number of electrons
(x)Atoms combine to form:

    A. Elements
    B. molecules
    C. Mixtures
    D. Isotopes
    
    
    
Answers
    B. Atom
C. Proton
B. Nucleus
C. Electron
A. Isotopes
B. Orbiting the nucleus
A. Protons
D. Molecule
A. Protons and neutrons
B. Molecules

Fill in the Blanks:
 
Basic Structure
    (i)An atom is the smallest unit of an _______________ that retains its chemical properties.
    (ii)The center of an atom is called the _______________.
    (iii)The nucleus contains _______________ and _______________.
   (iv) Particles with a negative charge that orbit the nucleus are called _______________.
   (v) The arrangement of _______________ in shells determines an atom's chemical properties.

Components and Properties

    (i)The number of _______________ in an atom determines its atomic number.
    (ii)Atoms with the same number of protons but different numbers of neutrons are called _______________.
    (iii)The mass of an atom is primarily determined by the _______________ and _______________.

Why is the relative atomic mass of an element often a decimal number?

The relative atomic mass is a weighted average of the masses of different isotopes. Since the isotopes have different masses and abundances, the average is usually a decimal number.

Question:How does the relative atomic mass of an element change if the abundance of its isotopes changes?

If the abundance of isotopes changes, the relative atomic mass will also change. A higher abundance of a heavier isotope will increase the relative atomic mass.

Question:Can the relative atomic mass of an element be a whole number? If so, under what conditions?

Yes, the relative atomic mass can be a whole number if the element has only one naturally occurring isotope or if the abundances of its isotopes are such that their weighted average is a whole number.

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