The Material World


The Material World




Atoms & Elements

(pp. 4-36)

1 What is an atom?

Humans have been wondering about the nature of matter for thousands of years. There were two ancient Greek philosophers who had varying ideas about matter.

  • ______(384 - 322 BCE) believed that matter was ______divisible.
  • ______(430 – 370 BCE) believed that matter was made of tiny ______particles called ______.

Until the 19th century, ______theory was the most widely accepted since there was no way to test the theories experimentally.

An ______is the smallest particle of matter. It ______be divided chemically.

______fit together like building blocks to form all the ______in our environment.

Because atoms are too ______to be examined directly, scientists have devised various ______to represent them. These ______have been adapted over time and continue to ______today.

Let’s look at the evolution of the model of the atom from the early 1800s to today.


The first scientist to formulate a theory on the nature of matter based on ______results was ______(1766- 1844), an English researcher and teacher.

Dalton’s atomic model is based on the following four (4) principles:

  1. Matter is composed of extremely ______and ______particles called ______.
  2. All the atoms of a single element are ______( ______mass, ______size, ______chemical properties).
  1. The atoms of one element are ______from those of another element.
  1. Atoms of different elements can combine, in ______proportions, to form compounds.

Example: Carbon and oxygen can combine in a ratio of 1:2 forming


  • Chemical reactions result in the formation of ______substances. However, during a chemical reaction, atoms cannot be ______, ______or ______. Example: ______+ ______ ______


The second half of the 19th century and the beginning of the 20th century was a period of unprecedented scientific progress, especially in the study of the nature of matter.

The development of gas discharge tubes paved the way to the discovery of the electron.

A gas discharge tube consists of a gas-filled tube with two metal terminals, or poles (a cathode and an anode) placed at opposite ends.

Fluorescent lights and neon signs are examples of ______.

Figure 1.5 on p. 9 shows a gas discharge tube with a different configuration. It’s called a ______tube. The negative pole, the ______, emitted unknown type of rays which were named ______.

Until the appearance of LCD (______) and plasma technologies, ______tubes were used to make television and computer screens.


In 1897, an English physicist by the name of ______(1856-1940) experimented with ______.

He wanted to determine if the rays were particles of matter or light energy and if they were particles of matter, were they positive, negative or neutral.

  1. To determine if cathode rays were particles of matter or light energy, he placed a small ______inside the tube and found that the cathode rays caused it to ______.

 Only ______can move matter, therefore he concluded that cathode rays must be made up of particles of ______.

  1. To determine if these particles were electrically charged or neutral, he exposed the cathode rays to a ______field.

 Since charged particles are deflected by a magnetic field and the cathode ray was deflected when exposed to a magnetic field, he concluded that cathode rays are electrically ______.

  1. To determine if these particles were electrically positively or negatively charged, he exposed the cathode rays to an ______field.

 Since the cathode rays were attracted to the ______charged pole of the electric field and ______attract, he concluded that cathode rays are ______charged.

Thomson determined that the cathode rays had to be ______charged particles of ______that was part of an ______, but could easily ______itself from the rest of the ______.

Thomson modified Dalton’s model to include the ______.

 The ______is one of the particles that make up an atom. It is ______charged.

Thomson never tested his model experimentally, so he never proved if it was correct or not. But Rutherford did!


The study of cathode ray tubes led to other important scientific discoveries:

  • ______by Wilhelm Conrad Röntgen in 1895
  • ______by Henri Becquerel in 1896.


In 1911, a New Zealand physicist called Ernest ______(1871-1937) became interested in the effect of radioactivity on matter.

Scientists discovered that radioactive substances could emit ______types of radiation:

  1. ______(α): ______charged. Weakest radiation – it is blocked by a sheet of paper.
  1. ______(β): ______charged. Moderate radiation – it is blocked by 3 mm thick sheet of metal.
  1. ______(γ): electrically ______. Strongest radiation – it is blocked by 1 m thick block of lead or concrete.

Rutherford wanted to test Thomson’s model to determine how the electrons were actually distributed in atoms. He decided to bombard a sheet of gold foil with a stream of alpha particles.

If Thomson’s model was correct, then most of the alpha particles would pass through ______because the foil was so thin.


Observation / Conclusion
Most of the alpha particles passed straight through the gold foil ______being deflected. /
  • An atom is mostly ______.

Some alpha particles are ______deflected or ______back. /
  • An atom contains a very ______and very small______.
  • The nucleus of an atom is ______charged.

Rutherford also hypothesized that the ______is made up of ______charged particles which he called ______.

Since an atom is electrically ______, there must an equal number of ______and ______.


Unfortunately, Rutherford’s beehive model did not fully explain atomic structure. Since opposite charges attract, wouldn’t the electrons collapse onto the atom?

In 1913, Danish physicist Niels ______published an improved version of Rutherford’s atomic model.

To understand Bohr’s contribution, we must recall that white light can be separated into all the colours in the visible part of the ______spectrum. (ROYGBIV)

When chemicals are “energized” using ______or ______, they also emit light, but only at certain ______within the visible part of the spectrum.

Bohr studied the emission ______of different elements, focusing particularly on ______.

To explain the presence of specific colours, Bohr modified Rutherford’s atomic model as follows:

  • Electrons are not randomly distributed but occupy specific areas of the atom, which he called ______due to their resemblance to planetary orbits in the solar system.
  • Each ______corresponds to a level of ______. While it stays in its original orbit, the electron does not lose ______. It is this ______that keeps it in the same orbit and prevents it from crashing into the nucleus.
  • When an electron receives energy (when ______or stimulated by an ______discharge) it becomes “______” and can ______to an orbit farther from the nucleus (to a higher energy level).
  • The electron returns rapidly to its original orbit, ______the absorbed energy in the form of ______.


The Rutherford-Bohr model was not 100% accepted since it did not provide an answer to the question: “Why does the nucleus not explode, given that it made up only of ______charged particles?”


In 1932, James Chadwick, a British physicist, found the answer to this question when he discovered a new particle in the ______of the atom. The ______role is to hold the protons together.

Characteristics of Atomic Particles

Particle / Symbol / Electrical Charge / Mass (g) / Mass (u)*
  • Unit of measurement for the relative atomic mass is the atomic mass unit ( or amu). One atomic mass unit is the equivalent of about 1.66 x 10-24 g.

2 The periodic classification of the elements

The existence of more than _____ different elements led 19th century scientists to look for a way to classify the elements according to certain patterns of their ______.

The most widely used system was devised in 1869 by ______

______, a Russian chemist.

(Until they are assigned official name, elements 112 to 116 have been given names and symbols that describe their atomic numbers in Latin.)


All elements can be classified into three categories:

  2. Good ______of heat and ______.
  3. Ductile and ______(easily shaped into ______or flattened into ______)
  4. ______at room temperature (except ______(l))
  5. Many react with ______to produce hydrogen gas.
  6. Located to the ______of the staircase in the periodic table.
  • Poor ______of heat and ______.
  • Many are ______at room temperature. The solids are ______and can easily be reduced to ______.
  • Located to the ______of the staircase in the periodic table (except ______which is at the top of the ______column – above group1A)
  1. METALLOIDS (Semi-metals)
  • _____ elements that have properties of both ______and ______- ______.
  • Used to make ______- important materials for transistors, integrated circuits and lasers.
  • Located along ______sides of the staircase in the periodic table.


 A ______corresponds to a column of the periodic table. The elements of a particular ______have similar ______properties because they all have the same number of ______electrons.

In the space below, draw the Bohr-Rutherford diagrams for Be, Mg, and Ca.

Notice that they all have _____ electrons in their outer shells. And, they are all in group ____ in the periodic table! Check it out! (Cool huh?)

The electrons in the outermost shell of an atom are called ______electrons. They are important because they are more frequently involved in the atoms’ ______.

The group number = number of ______electrons

 So, if nitrogen (N) is in group VA (5A), that means it has ______valence electrons! Yeah!


Alkali Metals

  • Examples: ______, ______and ______
  • All the elements in the first ______(group 1A) - except ______
  • ______(low melting points)
  • ______reactive (must be stored in oil)
  • Not found free in nature. Exist in the form of ______(ex. NaCl, KBr, etc.)

Alkaline Earth Metals

  • Examples: ______and ______
  • All elements in the ______group (Group IIA)
  • Highly ______and ______.
  • Burn ______.
  • Not found free in nature – form many ______found in ______or earth. (Ex. ______, CaCO3 (limestone))


  • Examples: ______and ______
  • All elements in group ______(second to last column)
  • Non- metals that react ______to form ______(salts)
  • Powerful ______

Noble Gases

  • Also called ______gases or ______gases
  • Examples: ______and ______
  • All elements in group ______(last column)
  • Very ______, therefore react ______with other elements
  • Are found in their ______state in nature


 A ______corresponds to a row of the periodic table. All the elements in a period have the same number of electron ______(occupied ______levels).

In the space below, draw the Bohr-Rutherford diagrams for Na, Mg, Al, Si, P, S, Cl and Ar.


 The periodicity of properties is the ______of patterns in properties from one ______to another.

Electrical ______, density and ______are a few examples of properties that can be predicted for each of the elements in the periodic table.

Refer to the handout “TRENDS IN THE PERIODIC TABLE” for a detailed explanation of this topic.


 The ATOMIC NUMBER represents the number of ______in the nucleus of an atom. It distinguishes one element from another.

  • The atomic number is usually found at the ______of each square in the periodic table.
  • The symbol for atomic number is ______.
  • In the periodic table, elements are arranged in ______order according to their ______numbers.
  • Since atoms are ______, the number of ______equals the number of ______.


  • The relative atomic mass is the ______of an ______of an element.
  • The atomic mass is usually found at the ______of each square in the periodic table.
  • The unit of measurement for relative atomic mass is the ______( or amu)
  • One atomic mass unit (1) equals one ______of the mass of a ______atom – about the same as the mass of one ______or ______.

 The relative atomic mass is the ______of an atom measured by comparison with the reference element, ______.


  • The mass number is a whole number indicating the sum of the numbers of ______and ______in an atom.
  • The symbol for mass number is ______.
  • It is found by rounding the relative atomic mass to the nearest ______.


 An isotope is an atom of an element with the same number of ______as another of the same element but with a different number of ______.

All atoms of an element have the same number of ______but not necessarily the same ______.

Example: There are three isotopes (forms) of hydrogen.

Isotope name / Symbol / Mass number / # p+ / # e- / # no
Hydrogen – 1
Hydrogen - 2
(______) / H
Hydrogen – 3
  • The isotopes of an element are distinguished/identified by their ______number.
  • They all have the same number of ______and ______, but a different number of ______.
  • The atomic masses that appear in the periodic table are average masses based on the relative abundance of the different isotopes of the elements as they occur in nature. That’s why the relative atomic mass of lithium is 6.94 and not 7.00.

3 Representing atoms

Atoms can be represented in different ways. In this section, four representations are described:

  1. Lewis Notation (Lewis Dot Diagram)
  2. Bohr-Rutherford Model
  3. Simplified atomic model
  4. “Ball-and-stick” model

As a general rule, electrons seek to fill the shells ______to/from the nucleus before occupying a ______shell.

 Please refer to the table on page 8 of this note-taking worksheet for the maximum number of electrons allowed in each energy level (shell).


 Lewis notation (a.k.a. electron ______notation) is a simplified representation of the atom, in which only the ______are illustrated.

______electrons are placed one by one around the ______of the atom, like the ______points on a compass. When these ______positions are filled, the electrons are ______to form ______.

 A Lewis dot diagram of an atom can have NO MORE than ______dots around it!

Complete the table below.

Group # / IA / IIA / IIIA / IVA / VA / VIA / VIIA / VIIIA
Lewis dot diagram / Li / Be / B / C / N / O / F / Ne


To represent an atom according to the Bohr-Rutherford model, three facts about the element must be known:

  1. The ______= the number of electron ______in the atom
  2. The ______= the number of ______electrons (electrons in the ______shell)
  3. The atomic ______= the total number of ______and ______in the atom

Oxygen is the second period, therefore it has ______shells. / Oxygen is in group 6A, therefore it has _____ valence electrons / The atomic number of oxygen is 8, therefore it has ______e- and ______p+


This representation clearly shows the number of ______and ______in an atom, as well as the number of ______in each ______.

Before you draw simplified atomic model of atom you must first determine the following:

  • The ______number
  • The number of ______(p+)
  • The number of ______(e-)
  • The average atomic mass in order to determine the number of ______(no)

Example: Magnesium (Mg)

Atomic # = ______

# p+ = ______

# e- = ______

Atomic mass rounded to the nearest whole number = ______

# no = ______( ______- ______)


In this model, the atom is depicted as a ______, and its bonds with other atoms are shown as ______. The size of the ball is proportional to the number of electron ______in the atom.

Below is a ball and stick model of a water molecule, H2O. (

4 The concept of the mole (p. 30 – 31) (EST ONLY)

Since atoms and molecules are extremely ______they can not be counted one by one. Sometimes, however, chemists and scientists need to know the number of ______or ______in a sample. To overcome this problem, the concept of a ______was developed.

 A ______is a ______equal to the number of atoms in exactly 12 g of ______. Its symbol is ______.

1.5 Molar Mass

earlier in this chapter you learned that an atom of carbon-12 has a mass of exactly 12  or amu (See page 15). Now we are saying that a mole of ______has a mass of exactly 12 ______. This means that the mass of a ______of carbon is numerically equal to its relative atomic mass. BUT the molar mass is expressed in ______rather than in atomic mass units ().

 The MOLAR MASS of a substance is the ______of one ______of that substance. (TIP: Atomic molar mass or relative atomic mass can be found at the bottom of a symbol on the periodic table.)

1.31 Some examples of molar mass

Substance / Relative Atomic Mass () / Molar Mass (g/mol)
Copper (Cu)
Molecular oxygen (O2)
Table salt (NaCl)
Glucose (C6H12O6)

An Important formula

Using the given formula, calculate the number of moles in 300 g of glucose. Show your work.



Through experimentation, the number of atoms in 12 g of carbon was determined to be ______atoms. This is also known as Avogadro’s number or the Avogadro constant in honour of ______(1776-1856).

 AVOGADRO’S NUMBER represents the number of entities in a ______. It equals ______of those entities.