1A)atoms,elements and molecules

• Atoms are the smallest unit that can exist independently.
• The periodic table is an arrangement of elements based on their atomic number (number of protons).
• Electrons orbit around the nucleus in defined regions of space known as electronic shells.
• Compounds are formed when two or more elements chemically combine to form a new substance with different properties than its constituent elements.
• Elements are substances made up of only one type of atom.
• The nucleus, formed by protons and neutrons, occupies only a tiny fraction of the total volume of an atom.
• Elements cannot be broken down into simpler substances by chemical means.
• Atoms are the smallest particles that can exist independently.
• Atoms consist of protons, neutrons, and electrons.
• Metals have high melting points, good conductors of heat and electricity, and form positive ions when they react with acids.
• Non-metals have low melting points, poor conductors of heat and electricity, and form negative ions when they react.
• Metals have high melting points, good conductors of heat and electricity, malleable, ductile, shiny appearance, and form positive ions when they react.
• Ionic compounds contain positively charged metal cations and negatively charged non-metal anions held together by ionic bonds.
• Metals have high melting points due to strong metallic bonding between positive metal ions and delocalized electrons.
• Protons have a positive charge (+1) and are located in the nucleus.
• Electrons have a negative charge (-1) and move around the nucleus at high speeds.
• Neutrons do not carry any electrical charge (-0) and are also found in the nucleus.
• Atoms can gain or lose electrons to become charged particles called ions.
• Ions have either a positive charge (+) if they have lost electrons or a negative charge (-) if they have gained electrons.
• Isotopes are atoms of the same element but with different numbers of neutrons.
• Molecules are groups of two or more atoms held together by covalent bonds.
• Compounds are pure substances composed of different elements joined together in fixed proportions.
• The periodic table is organized based on atomic number (number of protons), electron configuration, and properties such as metals/non-metals, reactive/less reactive, and noble gases.
• Periodic trends include increasing metallicity from left to right across periods, decreasing ionization energy from top to bottom within groups, and increasing atomic radius from left to right across periods.
• Covalent compounds contain two or more non-metal elements sharing pairs of electrons to form chemical bonds.