chemistry notes ch 1

7/30/2019 Chemistry Notes Ch 1

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1.1 ATOMIC THEORY

1. Atoms are composed of protons, neutrons and electrons. A proton has a +1 charge, an

electron a 1 charge and a neutron is neutral.

2. According to the atomic theory, an atom is visualised in terms of two main regions.

These regions are the nucleus and the surrounding space occupied by the electrons.

3. The nucleus is the central part of the atom and is composed of protons and neutrons.

The nucleus has a positive charge equal to the number of protons. The nucleus is

extremely small compared with an atom. The diameter of an average nucleus is 10 15

mcompared with an average atomic diameter of 10 10 m. That is, the diameter of an atom

is about 100,000 times larger than the diameter of its nucleus. The nucleus contains over

99.9 % of the mass of an atom. This is due to the relatively large masses of the proton

and neutron compared with the mass of the electron. The nucleus is very dense. This is

due to its large mass and small volume.

4. The electrons move in a relatively large space, often called an electron cloud, outside

the nucleus. Because electrons are negatively charged, they are kept moving around the

nucleus by attractive electrostatic forces between them and the positively charged

nucleus.

Chemistry for WA 1: 1.1 Atomic Structure. Read Pages 3 4. Do Review Exercise 1.1.

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1.2 DIFFERENT TYPES OF ATOMS

1. In an electrically neutral atom, the number of protons equals the number of electrons.

2. The atomic number (Z) of an element is the number of protons in the nucleus of an

atom of that element.

3. The mass number (A) of an atom of an element is the sum of the numbers of protons

and neutrons in the nucleus of that atom.

4. The structure of an atom can be represented asA

ZX where X is the symbol of that

atom.

5. Isotopes are atoms with the same atomic number but different mass numbers. Anexample would be the three isotopes of Hydrogen: H-1, H-2, H-3.

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6. Ions are charged atoms. Positive ions have more protons than electrons and negative

ions have more electrons than protons.

7. Positively charged ions, or cations, are formed when one or more electrons are

removed from an atom. The resulting ion therefore has more positively charged protons

then negatively charged electrons. Negatively charged ions, or anions, are formed when

an atom gains one or more electrons. The resulting ion therefore has more negativelycharged electrons than positively charged protons.

Chemistry for WA 1: 1.2 Different Types Of Atoms. Read Pages 5 11. Do Review

Exercise 1.2.

Exploring Chemistry: Set 4 Atoms and Isotopes. Experiment 7 Fireworks

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1.3 ARRANGEMENT OF ELECTRONS IN ATOMS

1. Because they are extremely small and can move at speeds approaching that of light, it

is difficult to describe the position and motion of electrons in a nucleus. The Quantum

Mechanical Theory provides a mathematical model describing the position and

properties of electrons.

2. In the quantum mechanical model, electrons are considered to exist in energy levels or

shells. These energy levels or shells are denoted by the numbers 1, 2, 3, 4. or

alternatively by the letters K, L, M, N. The first energy level, or the K shell, is the

lowest energy level and is closest to the nucleus. The second energy level, the L shell, is

the next energy level, and so on. In successive shells the electrons, in general, have

increased energy.

3. From quantum mechanics it has been calculated that the maximum number of

electrons that a shell can accommodate is 2n2, where n is the shell number. Therefore,

the first shell can accommodate up to 2 electrons, the second can accommodate up to 8

electrons and the third can accommodate up to 18 electrons.

Shell (n) 1 2 3 4

Maximum number of electrons (2n2) 2 8 18 32

4. In atoms, electrons tend to be in shells with the lowest possible energy, that is, in the

shells that are closest to the nucleus. This is called the ground state of the atom. When

determining which shell the electrons are in for a particular atom or ion, the electrons

are generally added, beginning with the first shell, then second shell, and so on. A

magnesium atom with 12 electrons will have 2 electrons in the first shell, 8 electrons in

the second shell and 2 electrons in the third shell (2, 8, 2). A calcium atom with 20electrons will have 2 electrons in the first shell, 8 electrons in the second shell, 8 electrons

in the third shell and 2 electrons in the fourth shell (2, 8, 8, 2) as shown in the diagram

below:

This representation showing the arrangement of electrons in an atom is more like the

model proposed by Niels Bohr in 1913, rather than the quantum mechanical model. In

his theory Bohr suggested that electrons move around the nucleus in circular orbits,

which seems to suggest the diagram above. However according to the quantum

mechanical model it is not possible to specify the exact location or path of an electron

around the nucleus. All that can be done is to predict the probability of locating an

electron in a given region in space. Electrons occupy regions of space called orbitals.

These regions of space are sometimes called electron clouds.

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5. The electrons in the outer shell of an atom are often called valence or outer-shell

electrons. For instance, potassium, with an electron configuration of 2, 8, 8, 1, has 1

valence electron, and sulfur, with a configuration 2, 8, 6 has 6 valence electrons. Many of

the properties, particularly the chemical properties, of substances are determined by the

number of valence electrons in the atoms.

ELECTRON CONFIGURATION EXAMPLE:

Give the electron configuration of the following:

(a) A neutral atom with 12 protons.

(b) A sulfide ion, S 2.

SOLUTIONS:

(a) A neutral atom with 12 protons will have 12 electrons. The electron configuration is

therefore 2, 8, 2.

(b) Sulfur has an atomic number of 16, and so the atom will contain 16 protons. If the

ion has a charge of 2, it must therefore contain two more electrons than protons. As a

result, there must be 18 electrons, which means an electron configuration of 2, 8, 8.

Chemistry for WA 1: 1.3 Arrangement Of Electrons In Atoms. Read Pages 12 15. Do

Review Exercise 1.3. Page 15.

Exploring Chemistry: Set 5 Atomic Structure and the Periodic Table.

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1.4 WHERE ARE ALL THE ATOMS?

1. About 74% by mass of atoms making up the universe are estimated to be hydrogen,

about 24% by mass are helium and the remaining different types of atoms make up

about 2%.

2. Of the different types of atoms making up the Earth, iron (35%) oxygen (30%), silicon(15%) and magnesium (13%) are the most abundant followed by nickel, sulfur, calcium

and aluminium.

3. In the human body, the distribution of various types of atoms is different again.

Humans are made up of 65% oxygen, 18% carbon, 10% hydrogen, 3% nitrogen, 2%

calcium and smaller percentages of numerous other types of atoms.

4. Of the 100 or so different types of atoms known to scientists, 89 are known to occur

naturally. The others have been artificially made and exist for very short times before

they undergo radioactive decay.

5. ELEMENTS are substances composed of only one type of atom. Examples include

gold (Au), silver (Ag) and chlorine (Cl). Elements are represented by symbols consisting

of one or two letters. Each chemical atom is composed of a given type of atom. These

elements may exist as individual atoms or as groups of atoms. The noble gases, for

example helium (He), exist as separate atoms. Elements such as oxygen (O2) and

hydrogen (H2) however, exist as diatomic molecules.

6. COMPOUNDS are substances composed of two or more different types of atoms

strongly bonded together in fixed proportions by mass. Water (H2O) and sodium oxide

(Na2O) are examples of compounds. A compound can be represented by a chemical

formula that uses the symbol for each type of atoms that appears in a molecule of

formula unit of the compound.

7. PURE SUBSTANCES have constant compositions. Elements and compounds are

examples of pure substances.

8. MIXTURES have two or more substances mixed together in any proportions. Sea

water is an example of a mixture.

Chemistry for WA 1: 1.4 Where Are All The Atoms. Read Pages 16 17. Do Review

Exercise 1.4. Page 18.


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1.5 THE PERIODIC TABLE A PREVIEW

1. To systemise much of what is known about the properties of the known elements, they

are often displayed in what is called the periodic table. A periodic table is shown below:

2. The horizontal rows in the periodic table are called PERIODS. The period number

corresponds to the number of the shell in which the outermost electrons are located in

the neutral atoms. Potassium has its outermost electron located in the fourth shell (2, 8,8, 1) and is in period 4.

3. The vertical columns in the periodic table are called GROUPS. They used to be

numbered using Roman Numerals from 1 to 8. A more recent recommendation from the

International Union of Pure and Applied Chemistry (IUPAC) for identifying groups is

to number them from 1 to 18.

4. Elements within a particular group have the same number of valence electrons. These

are electrons in their outer shell. For the group 2 elements all have two electrons in their

outermost shell: beryllium (2, 2), magnesium (2, 8, 2), calcium (2, 8, 8, 2), strontium (2,

8, 18, 8, 2) and barium (2, 8, 18, 18, 8, 2).

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5. Elements with the same number of electrons in their outermost shell display similar

chemical properties. For example, lithium (Li), sodium (Na) and potassium (K), in group

1, are all soft, reactive metals, and helium (He), neon (Ne) and argon (Ar), in group 18,

are all very unreactive gases.

6. In the periodic table the metals tend to be on the left-hand side of the table and the

non-metals on the upper right-hand side of the table (Hydrogen is the exception to this).Some elements, notably boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony

(Sb) and tellurium (Te) possess a mixture of both metallic and non-metallic properties.

These elements are often referred to as semimetals or metalloids.

Chemistry for WA 1: 1.5 The Periodic Table A Preview. Read Pages 18 21. Do

Review Exercise 1.5. Page 22.


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1.6 FIREWORKS, FLUORESCENT TUBES AND FLAME TESTS

1. Many atoms emit light of a distinctive colour when they are heated in a flame or their

vapours are exposed to an electric discharge. Some of these atoms and their colours are

shown below:

Atom Colour emitted when heatedSodium Orange

Potassium Violet

Copper Green

Calcium Red

Barium Yellow

Strontium Scarlet

2. These characteristic colours arise because electrons in atoms occupy definite energy

levels (shells). When a substance is heated to a high temperature, electrons in its atoms

may absorb definite amounts of energy and jump from their ground state energy levelsto higher energy levels to form excited atoms. These excited atoms are unstable and the

electrons move back to lower energy levels. When an electron jumps from a higher

energy level to a lower energy level, light (or a photon) of a specific energy is released or

emitted. The energy of this emitted light is exactly equal to the difference in energy

between the two energy levels in the atom. This is evidence for the Bohr model of

electron energy levels.

3. The emission of light by atoms such as neon, argon or mercury is utilised in the many

neon signs used in advertising. Fireworks often contain small quantities of different

metal compounds so that when the fireworks burn, the particular metal atoms emit

photons of light with characteristic wavelengths. The flame test is often used to identifyatoms. Fluorescent lights in the home contain low pressure mercury vapour inside the

lighting tube which is coated with zinc sulfide. Electricity is used to excite the mercury

atoms causing them to emit ultraviolet light. This ultraviolet light then excites the atoms

in the zinc sulfide coating. As the excited electrons return to their ground state they

release light of many different energies in the visible range to produce white light.

Chemistry for WA 1: 1.6 Fireworks, Fluorescent Tubes and Flame Tests. Read Pages 22

24. Do Review Exercise 1.6. Page 24. Do End of Chapter 1 Questions Pages 25 28.Exploring Chemistry: Set 5 Atomic Structure and the Periodic Table. Experiment 7.

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CHAPTER 1 ATOMIC THEORY: ADDITIONAL EXERCISES & PROBLEMS

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SOLUTIONS

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SOLUTIONS

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chemistry notes ch 1

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