UNIT 1:C

What is an atom?

The smallest part of an element that can exist and still have the properties of that element.

What is a molecule?

A group of two or more atoms chemically bonded together.

What are the three sub-atomic particles in an atom?

Proton, neutron, electron

Where is the proton found in an atom?

In the nucleus (centre of the atom)

Where is the neutron found in an atom?

In the nucleus (centre of the atom)

Where is the electron found in an atom?

In electron shells (orbitals) around the nucleus

What is the relative mass of a proton?

1

What is the relative mass of a neutron?

1

What is the relative mass of an electron?

1/1840 (approximately 0.0005 - negligible)

What is the relative charge of a proton?

+1

What is the relative charge of a neutron?

0 (neutral)

What is the relative charge of an electron?

-1

What is the atomic number (Z)?

The number of protons in the nucleus of an atom.

What is the mass number (A)?

The total number of protons and neutrons in the nucleus of an atom.

What is the formula to calculate the number of neutrons?

Number of neutrons = Mass number (A) - Atomic number (Z)

In a neutral atom, how do the numbers of protons and electrons compare?

Number of protons = Number of electrons (they are equal)

Calculate the number of protons, electrons and neutrons in a sodium atom with atomic number 11 and mass number 23.

Protons = 11, Electrons = 11, Neutrons = 23 - 11 = 12

Calculate the number of protons, electrons and neutrons in a carbon atom with atomic number 6 and mass number 12.

Protons = 6, Electrons = 6, Neutrons = 12 - 6 = 6

What are isotopes?

Atoms of the SAME element (same atomic number) that have DIFFERENT numbers of neutrons (different mass numbers).

Do isotopes have the same chemical properties? Why?

Yes. Chemical properties depend on electron arrangement. All isotopes of an element have the same number of electrons, so same chemical properties.

Do isotopes have the same physical properties?

No. Different mass (different number of neutrons) affects physical properties like density and rate of diffusion.

Give three examples of elements that have isotopes.

Hydrogen (protium, deuterium, tritium), Carbon (C-12, C-13, C-14), Chlorine (Cl-35, Cl-37)

What is the difference between Carbon-12 and Carbon-14?

Carbon-12 has 6 protons, 6 neutrons (mass number 12). Carbon-14 has 6 protons, 8 neutrons (mass number 14). Same element (carbon) but different isotopes.

What is relative atomic mass (Aᵣ)?

The weighted average mass of an atom of an element, taking into account all its naturally occurring isotopes, measured relative to 1/12th the mass of a carbon-12 atom.

Does Aᵣ have units?

No - it is a relative value with no units.

Why is Aᵣ usually not a whole number?

Most elements exist as a mixture of isotopes. Aᵣ is the weighted average of all isotopes, so it is usually a decimal.

Write the formula for calculating relative atomic mass from isotopic abundances.

Aᵣ = (mass₁ × % abundance₁) + (mass₂ × % abundance₂) + … divided by 100

Chlorine has two isotopes: Cl-35 (75%) and Cl-37 (25%). Calculate the Aᵣ of chlorine.

Aᵣ = (35 × 75) + (37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5

Bromine has two isotopes: Br-79 (50.7%) and Br-81 (49.3%). Calculate the Aᵣ of bromine.

Aᵣ = (79 × 50.7) + (81 × 49.3) ÷ 100 = (4005.3 + 3993.3) ÷ 100 = 7998.6 ÷ 100 = 79.99

Magnesium has three isotopes: Mg-24 (78.99%), Mg-25 (10.00%), Mg-26 (11.01%). Calculate the Aᵣ of magnesium.

Aᵣ = (24 × 78.99) + (25 × 10.00) + (26 × 11.01) ÷ 100 = (1895.76 + 250.0 + 286.26) ÷ 100 = 2432.02 ÷ 100 = 24.32

What is the relative mass and charge of the nucleus compared to the whole atom?

The nucleus contains almost all the mass (99.9%+) but very little of the volume. The nucleus is positively charged.

If an atom has 11 protons and 12 neutrons, what is its atomic number and mass number?

Atomic number = 11, Mass number = 11 + 12 = 23. This is a sodium atom (Na-23).

Gallium has Aᵣ = 69.7 and two isotopes: Ga-69 and Ga-71. If the abundance of Ga-69 is 65%, what is the abundance of Ga-71?

Abundance of Ga-71 = 100% - 65% = 35%

Write the standard notation for Carbon-14.

¹⁴₆C

Write the standard notation for Chlorine-37.

³⁷₁₇Cl

What is the electron arrangement of a neutral atom with atomic number 11?

Atomic number 11 = 11 protons = 11 electrons. Electron arrangement: 2, 8, 1 (sodium)

What is the difference between an atom of an element and a molecule of an element?

An atom is a single particle of an element. A molecule of an element contains two or more atoms of the SAME element chemically bonded (e.g., O₂ is a molecule made of two oxygen atoms).

Give an example of a diatomic molecule.

H₂ (hydrogen), O₂ (oxygen), N₂ (nitrogen), Cl₂ (chlorine), F₂ (fluorine), Br₂ (bromine), I₂ (iodine)

Which elements exist as individual atoms (not molecules)?

Noble gases (Helium, Neon, Argon, etc.) exist as individual atoms. Metals exist as giant lattices of ions, not molecules.

What is the significance of Carbon-12 in relative atomic mass?

Carbon-12 is the standard. Relative atomic mass is measured relative to 1/12th the mass of a carbon-12 atom, which is defined as exactly 12.000.

If an element has only one naturally occurring isotope, what will its Aᵣ be?

Its Aᵣ will be a whole number equal to the mass number of that isotope. Examples: Sodium (Na-23, Aᵣ = 23), Fluorine (F-19, Aᵣ = 19).

Why do isotopes of the same element have different physical properties?

Physical properties depend on mass. Different number of neutrons = different mass, affecting density, rate of diffusion, etc.

What is the approximate size of an atom?

About 0.1 nanometres (1 × 10⁻¹⁰ m)

What is the approximate size of a nucleus compared to an atom?

The nucleus is about 10,000 times smaller than the atom. If an atom were a football stadium, the nucleus would be a pea at the centre.

Copper has Aᵣ = 63.5. It has two isotopes: Cu-63 (69.2%) and another isotope with abundance 30.8%. Calculate the mass number of the second isotope.

Let mass of second isotope = m. (63 × 69.2) + (m × 30.8) ÷ 100 = 63.5. 4359.6 + 30.8m = 6350. 30.8m = 1990.4. m = 64.62 ≈ 65. Second isotope is Cu-65.

What does the term "nucleon number" mean?

Another name for mass number - the total number of protons and neutrons in the nucleus.

What does the term "proton number" mean?

Another name for atomic number - the number of protons in the nucleus.

How does increasing the number of neutrons affect the chemical properties of an atom?

It does not affect chemical properties. Chemical properties depend only on electron arrangement, which is unchanged by adding neutrons.

In the notation ²³₁₁Na, what do the numbers represent?

23 = mass number (protons + neutrons), 11 = atomic number (protons). Sodium has 11 protons, 11 electrons, 12 neutrons.

What is a radioactive isotope? Give an example.

An unstable isotope that decays spontaneously, emitting radiation. Examples: Carbon-14, Uranium-235, Iodine-131.

Gallium has Aᵣ = 69.7. Ga-69 has abundance 65% and Ga-71 has abundance 35%. Show the calculation for Aᵣ.

Aᵣ = (69 × 65) + (71 × 35) ÷ 100 = (4485 + 2485) ÷ 100 = 6970 ÷ 100 = 69.7

What is the relationship between the number of protons and the identity of an element?

The number of protons (atomic number) determines which element an atom is. Each element has a unique atomic number.

What is the main difference between an atom and an ion?

An atom is neutral (equal protons and electrons). An ion has a charge because the number of protons and electrons are not equal (atoms that have gained or lost electrons).

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