Atoms, Elements, and Their Properties

ATOM – Key Notes

• An atom is the basic unit of a chemical element.
  It is the smallest particle of an element that can take part in a chemical reaction.
• Atoms are the basic building blocks of all matter.
  Everything around us—solids, liquids, gases—is made of atoms.
• Atoms are extremely small.
  Their size is measured in nanometers or picometers. A single atom cannot be seen with the naked eye.

STRUCTURE OF THE ATOM

• Atoms are made up of subatomic particles.
• These particles are very tiny and include:
  1. Protons
     • Positively charged (+1)
     • Found in the nucleus (center of the atom)
     • Has a relative mass of 1 atomic mass unit (1 amu)
  2. Neutrons
     • Have no charge (neutral)
     • Also found in the nucleus
     • Have a relative mass of 1 amu
  3. Electrons
     • Negatively charged (–1)
     • Move around the nucleus in energy levels or shells
     • Very tiny mass (approximately 1/1836 of a proton)
     • Responsible for chemical reactions and bonding

IMPORTANT TERMS

• Atomic Number (Z): Number of protons in an atom.
  It determines the identity of an element.
• Mass Number (A): Total number of protons + neutrons.
• Isotopes: Atoms of the same element with the same number of protons but different number of neutrons.
• Nucleus: Dense central part of the atom containing protons and neutrons.
  It carries a positive charge.
• Electron Cloud: Region around the nucleus where electrons move.

ADDITIONAL POINTS (GOOD FOR EXAMS)

• Atoms are electrically neutral because number of protons = number of electrons.
• Electrons in the outermost shell determine chemical properties.
• The nucleus contains almost all the mass of the atom.
• Chemical reactions involve loss, gain, or sharing of electrons, not protons or neutrons.

ELEMENTS – Key Notes

• An element is a substance that consists of only one type of atom.
  This means every particle in the element is made of atoms with the same number of protons.
• Elements cannot be broken down into simpler substances by ordinary chemical means.
• There are over 118 known elements, each represented by a chemical symbol (e.g., H for Hydrogen, O for Oxygen, Na for Sodium).

EXAMPLE: HYDROGEN

• Hydrogen is the simplest and lightest element.
• A hydrogen atom contains:
  • 1 proton (in the nucleus)
  • 1 electron (orbiting around the nucleus)
• Most hydrogen atoms have no neutrons, though some isotopes do.

ADDITIONAL POINTS ABOUT ELEMENTS

• The atomic number of an element equals the number of protons in its atoms.
  For hydrogen, the atomic number is 1.
• Elements are organized in the Periodic Table based on their atomic number and properties.
• Examples of elements include:
  • Oxygen (O)
  • Carbon (C)
  • Iron (Fe)
  • Gold (Au)
• Two or more atoms of the same element can join to form molecules, e.g.:
  • O₂ (oxygen molecule)
  • N₂ (nitrogen molecule)