Unit 8: The Model of Atoms and Molecules

Question 1

What is an atom?

Answer 1

• An atom is the smallest unit of an element, having all the characteristics of that element. It is the smallest particle of an element.
• Everything around us is made up of atoms.
• An atom has the chemical properties of the element it is made of.
• Each element is made up of its own type of atom.
• Atoms of the same element have the same size and mass.
• Atoms of different elements are of different size and mass.
• An atom has protons, electrons and neutrons.
• It has a neutral net charge (electrically neutral).
• Atoms of elements differ from one another by the difference in the number of protons.
• An atom is electrically neutral and is made up of a positively charge nucleus (protons and neutrons) with negatively charged electrons moving around the nucleus.
• An atom has equal numbers of protons and electrons, that is, the number of positive charges is equal to the number of negative charges.
• Because of this, an atom is said to be electrically neutral.

Question 2

Proton

Type of sub-atomic Particle
Symbol
Number of sub-atomic Particle
Position of sub-atomic Particle
Relative charge of sub-atomic Particle
Relative mass of sub-atomic Particle

Answer 2

Proton
p
1
In the nucleus
1+
1

Question 3

Neutron

Type of sub-atomic Particle
Symbol
Number of sub-atomic Particle
Position of sub-atomic Particle
Relative charge of sub-atomic Particle
Relative mass of sub-atomic Particle

Answer 3

n
1
In the nucleus
0
1

Question 4

Electron

Type of sub-atomic Particle
Symbol
Number of sub-atomic Particle
Position of sub-atomic Particle
Relative charge of sub-atomic Particle
Relative mass of sub-atomic Particle

Answer 4

e-
1
Surrounding the nucleus (valence shell)
1-
0
(1/1840)

*Note: the mass of an electron is too insignificant to be counted into the mass of an atom. Relative charge is with comparison to one another (ratio).

Question 5

What must happen for an atom to be electrically neutral?

Answer 5

• For an atom to be electrically neutral, the number of protons must be the same as the number of electrons.

Question 6

Net charge of positively charged, electrically neutral and negatively charged?

Answer 6

• Positively charged = net change >0
• Electrically neutral = net charge = 0
• Negatively charged = net charge <0
(enrichment: when an atom is no longer neutral (i.e. net charge is not 0), it is an ion instead.)

Question 7

Proton (Atomic) number (6 pts)

Answer 7

• The number of protons in an atom is called the proton number.
• Proton number is also known as the Atomic Number.
• The number of electrons = number of protons (which means that proton number = no. of electrons)
• The proton number will never change but the number of neutrons and electrons may change!
• Each element in the Periodic Table has its own proton number.

• If the number of protons changes, then the element will change into another element.

Question 8

Nucleon (mass) number (ard 8 pts)

Answer 8

• Nucleon number is the number of protons and neutrons in the nucleus of an atom.
• Nucleon number is also called the Mass Number.
• Nucleon (Mass) number = number of protons + number of neutrons
• Number of neutrons = Mass number – Proton number
• The nucleus would be positively charged because the nucleus contains protons and neutrons. Protons are positively charged while neutrons are electrically neutral.
• To change the type of element an atom is, you can add/remove a proton.
• To change the charge of an atom without changing the element, you can add an electron.
• To change the mass of an atom, you can add a neutron.

Question 9

The Atomic model (7 pts)

Answer 9

• The centre of an atom is called the nucleus which contains the protons and neutrons. Protons, which are positively charged, are found only in the nucleus. Thus, the nucleus is also positively charged.
• The electrons, which move rapidly around the nucleus, are negatively charged. These electrons are arranged in electron shells.

-The electron shell nearest to the nucleus is numbered 1, the second nearest is numbered 2 and so on.

• The number of protons in an atom is called the proton number.
• Proton number is also known as the atomic number.
• Nucleon number is the number of protons and neutrons and the nucleus of an atom.
• Nucleon number is also called the mass number.

Question 10

How to find the number of protons?

Answer 10

look at the atomic number/ proton number
or mass number - number of neutrons

Question 11

How to find the number of electrons?

Answer 11

no. of electrons = no. of protons

look at the atomic number/ proton number
or mass number - number of neutrons

Question 12

How to find the number of neutrons?

Answer 12

mass number - number of protons

Question 13

Nucleon (mass) number ( around 9 points)

Answer 13

• Nucleon number is the number of protons and neutrons in the nucleus of an atom.
• Nucleon number is also called the Mass Number.
• Nucleon (Mass) number = number of protons + number of neutrons
• Number of neutrons = Mass number – Proton number
*The bigger number would always be the mass number!

• The nucleus would be positively charged because the nucleus contains protons and neutrons. Protons are positively charged while neutrons are electrically neutral.
• To change the type of element an atom is, you can add/remove a proton.
• To change the charge of an atom without changing the element, you can add an electron.
• To change the mass of an atom, you can add a neutron.

Question 14

What can you do to change the mass of an atom?

Answer 14

• To change the mass of an atom, you can add a neutron.

Question 15

What can you do to change the charge of an atom without changing the element?

Answer 15

• To change the charge of an atom without changing the element, you can add an electron.

Question 16

What can you do to change the type of element an atom is?

Answer 16

• To change the type of element an atom is, you can add/remove a proton.

Question 17

Why is the nucleus positively charged?

Answer 17

• The nucleus would be positively charged because the nucleus contains protons and neutrons. Protons are positively charged while neutrons are electrically neutral.

Question 18

How to find mass number?

Answer 18

bigger number would always be mass number

no of protons + no of neutrons

Question 19

How many electrons can each electron shell hold (until element no. 20)

Answer 19

• Each electron shell can hold a certain maximum number of electrons.
• 1st electron shell - 2 electrons
• 2nd electron shell - 8 electrons
• 3rd electron shell - 8 electrons (1st 20 elements only)
• Enrichment: For elements after calcium in the 4th period, their third electron shell can hold up to 18 electrons.
• Electronic configuration of potassium is 2.8.8.1 (=19 electrons)

• Maximum number of electrons in nth electron shell = 2n²
• When
n =1, e- = 2
n = 2, e- = 8
n = 3, e- = 18 (but is 8 for 1st 20 elements only)
n = 4, e- = 32

Question 20

Valence shell (6 points)

Answer 20

• The electron shell which is farthest from the nucleus and occupied by electrons is called the valence shell (outermost electron shell).
• The electrons in the valence shell are known as valence electrons.
• In a chemical reaction, only these valence electrons are involved in chemical bonding between atoms.
- Electrons not in the valence shell will not undergo chemical bonding!
• Atoms with a complete (fully occupied) valence shell are stable. They will not undergo chemical reaction under normal conditions.
- E.g. noble gases. (18th group)
• Often, only the valence electrons are drawn in the electronic structure. This is called the outer electronic structure.

Question 21

What are the electrons in the valence shell called?

Answer 21

valence electrons

Question 22

which electrons are involved in a chemical reaction?

Answer 22

• In a chemical reaction, only these valence electrons are involved in chemical bonding between atoms.
- Electrons not in the valence shell will not undergo chemical bonding!

Question 23

how do you know if an atom is stable?

Answer 23

• Atoms with a complete (fully occupied) valence shell are stable. They will not undergo chemical reaction under normal conditions.
- E.g. noble gases. (18th group)

Question 24

what are the factors affecting stability of an atom?

Answer 24

Factors affecting stability:
1. number of neutrons
2. electronic configuration (no. of valence electrons)
(anything that ends with 8 is stable incl. helium and/or when the valence shell is completely filled up = stable)

Question 25

The Periodic Table (around 7 points)

Answer 25

• The Periodic Table is an arrangement of the elements in the order of increasing proton number.
• The position of an element in the Periodic Table is related to proton number and electronic structure.
• The character of elements changes from metallic to non-metallic from left to right across a period of the Periodic Table.
• Elements in the same group have the same number of valence electrons.
• Elements of the same Group in the Periodic Table share similar chemical properties.
• Noble gases have filled outermost electron shell thus they are unreactive (but are stable).
• In the Periodic Table, elements are arranged in order of increasing proton (atomic) number, and are classified according to Groups and Periods.
• Elements in the same group has the same number of valence electrons (except for noble gases).
• Elements in the same group have similar chemical properties and will undergo the same type of chemical reactions.

Question 26

Group (3 points)

Answer 26

• A vertical row of elements in the Periodic Table
• Elements in the same group has the same number of valence electrons (except for noble gases).
• Elements in the same group have similar chemical properties and will undergo the same type of chemical reactions.
- E.g. noble gases (group 18) have filled outermost electron shell thus they are unreactive.

Question 27

Period (1 point)

Answer 27

• Across the period, the properties of elements change from metallic to non-metallic.