Chemistry Test on Atomic Structure : Protons, neutrons and electrons
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Introduction to Atomic Structure
What is an atom?
An atom is the smallest particle of an element that still has the properties of that element. Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons.
$$ \text{Protons and neutrons} \rightarrow \text{found in the nucleus} $$
$$ \text{Electrons} \rightarrow \text{found in shells (energy levels) around the nucleus} $$
Atomic structure helps us understand how elements are identified, how ions form, and why isotopes of the same element can behave slightly differently.
1. The Three Subatomic Particles
Rule: Each subatomic particle has a specific charge, mass, and location within the atom.
| Particle | Charge | Relative Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | ~1/1836 (negligible) | Shells around nucleus |
Worked Example 1: An atom contains 6 protons, 6 neutrons, and 6 electrons. What is the overall charge of the atom?
Step 1: Add up the positive charge from the protons.
$$ 6 \times (+1) = +6 $$
Step 2: Add up the negative charge from the electrons.
$$ 6 \times (-1) = -6 $$
Step 3: Combine the charges (neutrons contribute 0).
$$ +6 + (-6) = 0 $$
Answer: The atom is neutral (charge = 0), because the number of protons equals the number of electrons.
2. Atomic Number and Mass Number
Rule: The atomic number tells you the number of protons in an atom. The mass number tells you the total number of protons and neutrons.
$$ \text{Atomic number} = \text{number of protons} $$
$$ \text{Mass number} = \text{protons} + \text{neutrons} $$
Worked Example 2: An atom of aluminium has an atomic number of 13 and a mass number of 27. How many neutrons does it have?
Step 1: Rearrange the mass number formula to find neutrons.
$$ \text{neutrons} = \text{mass number} - \text{atomic number} $$
Step 2: Substitute the values.
$$ 27 - 13 = 14 $$
Answer: 14 neutrons
Worked Example 3: An atom of fluorine has 9 protons and 10 neutrons. What is its mass number?
Step 1: Add the protons and neutrons together.
$$ 9 + 10 = 19 $$
Answer: Mass number = 19
3. Neutral Atoms vs. Ions
Rule: In a neutral atom, the number of electrons equals the number of protons. When an atom loses or gains electrons, it becomes a charged particle called an ion.
$$ \text{Atom loses electrons} \rightarrow \text{forms a positive ion (cation)} $$
$$ \text{Atom gains electrons} \rightarrow \text{forms a negative ion (anion)} $$
Worked Example 4: A sodium atom has 11 protons and 11 electrons. It loses 1 electron to form an ion. What is the charge of the ion?
Step 1: Work out the new number of electrons.
$$ 11 - 1 = 10 \text{ electrons} $$
Step 2: Compare protons (still 11) to electrons (now 10).
$$ \text{charge} = (+11) + (-10) = +1 $$
Answer: The ion has a charge of +1 (written as Na⁺)
Worked Example 5: An oxygen atom has 8 protons. It gains 2 electrons to form an ion. What is the charge, and how many electrons does the ion now have?
Step 1: A neutral oxygen atom has 8 electrons (equal to protons).
$$ 8 + 2 = 10 \text{ electrons} $$
Step 2: Compare protons (8) to electrons (10).
$$ \text{charge} = (+8) + (-10) = -2 $$
Answer: The ion has a charge of -2 (written as O²⁻) and has 10 electrons
4. Isotopes
Rule: Isotopes are atoms of the same element (same number of protons) but with a different number of neutrons, giving them different mass numbers.
Worked Example 6: Chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons. How many neutrons does each isotope have?
Step 1: For chlorine-35, subtract the atomic number from the mass number.
$$ 35 - 17 = 18 \text{ neutrons} $$
Step 2: For chlorine-37, do the same calculation.
$$ 37 - 17 = 20 \text{ neutrons} $$
Answer: Chlorine-35 has 18 neutrons; chlorine-37 has 20 neutrons. Both isotopes have the same number of protons (17), so they are still chlorine — but they have different masses.
5. Real-World Application
Question: A carbon atom used in an archaeological dating test is identified as carbon-14. A standard carbon atom (carbon-12) has 6 protons and 6 neutrons. How many neutrons does carbon-14 have, and is it the same element as carbon-12?
Step 1: Both isotopes of carbon have the same number of protons.
$$ \text{protons} = 6 $$
Step 2: Find the neutrons in carbon-14 using the mass number.
$$ 14 - 6 = 8 \text{ neutrons} $$
Step 3: Compare to carbon-12, which has 6 neutrons.
$$ 8 \text{ neutrons (C-14)} \neq 6 \text{ neutrons (C-12)} $$
Answer: Carbon-14 has 8 neutrons. It is still the element carbon (because it has 6 protons, same as carbon-12), but it is a different isotope because the number of neutrons differs.
Key Takeaways
1. Protons and neutrons are found in the nucleus; electrons orbit the nucleus in shells.
2. The atomic number equals the number of protons, and the mass number equals protons plus neutrons.
3. In a neutral atom, protons and electrons are equal in number; losing or gaining electrons creates a charged ion.
4. Isotopes have the same number of protons but a different number of neutrons, giving different mass numbers.
5. Practising with both particle-counting problems and real-world examples (like isotopes used in dating) builds a stronger understanding of atomic structure.
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