GRADE 9 Integrated Science MIXTURES,ELEMENTS AND COMPOUNDS – Structure of the atom Notes

• Recall the parts of an atom: protons, neutrons and electrons.
• Understand atomic number and mass number.
• Describe electron arrangement in shells and the idea of valence electrons.
• See how atoms link to form compounds (basic idea of ionic and covalent bonds).

1. What is an atom?

An atom is the smallest particle of an element that still shows the properties of that element. Everything around you is made of atoms: air, water, metals, living things.

2. Parts of the atom

• Nucleus — centre of the atom. Contains protons (positive, +) and neutrons (neutral, 0).
• Electrons — tiny, negatively charged particles (–) that move around the nucleus in shells or energy levels.

Charges: proton = +1, electron = −1, neutron = 0. In a neutral atom, number of electrons = number of protons.

3. Atomic number and mass number

Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons.

4. Simple visual: Carbon atom (Bohr-style)

5. Electron arrangement and valence electrons

Electrons occupy shells (energy levels) around the nucleus. A simple rule for the first shells:

• K shell (1st) — up to 2 electrons
• L shell (2nd) — up to 8 electrons
• M shell (3rd) — up to 18 electrons (but for our level often use 8 as a useful limit)

Valence electrons are electrons in the outermost shell. They determine how an atom reacts and how it forms compounds.

6. How atoms form compounds — simple idea

Atoms become more stable by obtaining a full outer shell. They do this by:

• Ionic bonding — transfer of electrons. Example: Sodium (Na) gives an electron to Chlorine (Cl) to form Na+ and Cl− → NaCl (table salt).
• Covalent bonding — sharing electrons. Example: Two hydrogen atoms share electrons with one oxygen to form water (H2O).

7. Key terms (short)

• Atom — smallest part of an element.
• Element — substance made of only one kind of atom (e.g., O, C, H).
• Compound — substance made when atoms of different elements join (e.g., H2O).
• Isotope — same element (same protons) but different number of neutrons.
• Valence electrons — electrons in the outermost shell; determine chemical behaviour.

8. Short exercises (try these)

1. What is the atomic number of oxygen? (Hint: O is element 8)
2. How many electrons does a neutral sodium atom (Na, Z=11) have?
3. Carbon-14 has 6 protons. How many neutrons does it have?
4. Explain in one sentence the difference between ionic and covalent bonding.

Atoms contain protons, neutrons and electrons. The atomic number tells us how many protons (and for neutral atoms, electrons) there are. Electrons in outer shells (valence electrons) control bonding and formation of compounds. Understanding the atom helps explain why elements react and form the substances we use every day — like water and salt.