ATOMS, ELEMENTS, COMPOUNDS AND MOLECULES | Study Notes | Chemistry | Grade 10 | OBC

ATOMS, ELEMENTS, COMPOUNDS AND MOLECULES: ATOMIC STRUCTURE AND PERIODIC TABLE LEARNING OBJECTIVES By the end of this topic, learners should be able to: • Describe what an atom is and its basic structure. • Describe the relative charges of protons, neutrons, and electrons. • Describe the approximate relative masses of protons, neutrons, and electrons. CORE CONCEPTS AN ATOM An atom is defined as the smallest particle of an element that can take part in a chemical reaction. Atoms are the fundamental building blocks of all matter. Everything around us, from the air we breathe to the ground we walk on, is made up of atoms. STRUCTURE OF AN ATOM (BOHR MODEL) According to the Bohr model, an atom has a central, dense part called the nucleus, which is surrounded by negatively charged particles called electrons. These electrons move in specific paths or energy levels, often referred to as electron shells or orbits, around the nucleus. The nucleus contains two types of particles: 1. Protons: Positively charged particles. 2. Neutrons: Particles that have no electrical charge (they are neutral). Electrons are much smaller and lighter than protons and neutrons. They orbit the nucleus at a relatively large distance, making most of an atom empty space.

Figure: The Bohr model illustrating the structure of a Lithium atom

SUBATOMIC PARTICLES: PROTONS, NEUTRONS, AND ELECTRONS Atoms are made up of even smaller particles called subatomic particles. The three main subatomic particles are protons, neutrons, and electrons. Each has unique properties regarding its charge, mass, and location within the atom. RELATIVE CHARGES AND MASSES Scientists use relative charges and relative masses to compare the properties of these tiny particles. A relative charge expresses the charge of a particle compared to a standard unit, and a relative mass expresses its mass compared to a standard unit (usually the mass of a proton or neutron). • Proton: * Relative Charge: +1 (positive one) * Approximate Relative Mass: 1 (one atomic mass unit, a.m.u.) * Location: Found in the nucleus. • Neutron: * Relative Charge: 0 (zero, it is neutral) * Approximate Relative Mass: 1 (one atomic mass unit, a.m.u.) * Location: Found in the nucleus. • Electron: * Relative Charge: -1 (negative one) * Approximate Relative Mass: 11840 (approximately one one-thousand eight hundred fortieth of a proton's mass) * Location: Orbits the nucleus in electron shells. The table below summarises the key properties of these subatomic particles:

Properties of Subatomic Particles

Particle	Location in Atom	Relative Charge	Approximate Relative Mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Electron Shells	-1	11840

Figure: Summary of subatomic particle properties

Figure: Illustrating the vast empty space within an atom

SUMMARY • An atom is the smallest part of an element that takes part in a chemical reaction. • Atoms consist of a central nucleus and electrons orbiting in shells. • The nucleus contains protons (positive charge) and neutrons (no charge). • Electrons are negatively charged particles that orbit the nucleus. • Protons and neutrons have an approximate relative mass of 1. • Electrons have a very small approximate relative mass of 11840. • The atomic nucleus is extremely small compared to the overall size of the atom, meaning atoms are mostly empty space. PRACTICE QUESTIONS EASY 1. Define the term 'atom'. 2. Name the three main subatomic particles found in an atom. 3. State the relative charge of a proton. MEDIUM 1. Describe the structure of an atom according to the Bohr model, mentioning the location of its subatomic particles. 2. Compare the relative masses of a proton and an electron. 3. Which subatomic particle has no electrical charge and where is it located? HARD 1. An atom of Sodium (Na) has an atomic number of 11 and a mass number of 23. a) How many protons are in the Sodium atom? b) How many electrons are in the Sodium atom? c) How many neutrons are in the Sodium atom? 2. Explain why an atom, despite containing charged particles, is electrically neutral overall.

Answers

EASY
1. An atom is the smallest particle of an element that can take part in a chemical reaction.
2. Protons, neutrons, and electrons.
3. The relative charge of a proton is +1.

MEDIUM
1. According to the Bohr model, an atom has a central nucleus containing protons and neutrons. Negatively charged electrons orbit this nucleus in specific energy levels called electron shells.
2. A proton has an approximate relative mass of 1, while an electron has an approximate relative mass of 11840. This means a proton is significantly heavier than an electron.
3. A neutron has no electrical charge and is located in the nucleus of the atom.

HARD
1. a) The atomic number (11) represents the number of protons. So, there are 11 protons. b) In a neutral atom, the number of electrons is equal to the number of protons. So, there are 11 electrons. c) The number of neutrons = Mass Number - Atomic Number = 23 - 11 = 12 neutrons. 2. An atom is electrically neutral because the number of positively charged protons in its nucleus is equal to the number of negatively charged electrons orbiting the nucleus. The positive and negative charges cancel each other out, resulting in an overall neutral charge.

COMMON MISTAKES TO AVOID • Confusing Mass and Charge: Remember that protons have a +1 charge, electrons have a -1 charge, and neutrons have zero charge. Both protons and neutrons have an approximate relative mass of 1, while electrons have a negligible mass. • Incorrect Location of Particles: Do not place electrons in the nucleus or protons/neutrons outside the nucleus. Protons and neutrons are always in the nucleus; electrons are always in shells around the nucleus. • Assuming Solid Structure: Avoid thinking of atoms as solid spheres. Most of the atom is empty space, with a tiny, dense nucleus and electrons orbiting far away. EXAM TIPS • Memorise Key Definitions: Make sure you can accurately define 'atom', 'nucleus', 'proton', 'neutron', and 'electron'. • Practise Drawing Diagrams: Be ready to draw and label a simple Bohr model of an atom, showing the nucleus and electron shells with subatomic particles. • Understand Relative Properties: Know the relative charges and masses of protons, neutrons, and electrons without hesitation. • Be Precise with Language: When describing the structure, use terms like "orbits in shells" for electrons and "dense, central nucleus" for protons and neutrons. QUICK REVISION SUMMARY • Atom: Smallest particle of an element in a chemical reaction. • Structure: Nucleus (centre) and electron shells (orbits). • Nucleus contains: Protons (+1 charge, 1 mass) and Neutrons (0 charge, 1 mass). • Electron Shells contain: Electrons (-1 charge, 11840 mass). • Overall Atom Charge: Electrically neutral due to equal numbers of protons and electrons. • Atomic Space: Atoms are mostly empty space.

study-notes — Chemistry (ATOMS, ELEMENTS, COMPOUNDS AND MOLECULES)

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