The atomic number of an element is determined by the number of protons in the nucleus of its atoms. In a neutral atom, the atomic number also corresponds to the number of electrons surrounding the nucleus.
In general, the atomic number increases across the periodic table from left to right. This pattern is observed in both metals and non-metals. However, the behavior of electrons and the trend in atomic number is different for metals and non-metals, leading to distinct properties.
In metals: Metals are found on the left side of the periodic table. As you move from left to right across a period, the atomic number increases. This means that the number of protons and electrons in the atoms also increases. Metals tend to lose electrons and form positive ions (cations) when they undergo chemical reactions. For example, sodium (Na), a metal, has an atomic number of 11. It readily loses one electron to achieve a stable electron configuration like neon (atomic number 10), resulting in a Na+ cation with 11 protons but only 10 electrons. This loss of electrons contributes to the increase in atomic number as you move from left to right in the periodic table.
In non-metals: Non-metals are found on the right side of the periodic table. As you move from left to right across a period, the atomic number still increases, but non-metals tend to gain electrons during chemical reactions to achieve a stable electron configuration. Non-metals have a higher tendency to gain electrons and form negative ions (anions). For example, oxygen (O), a non-metal, has an atomic number of 8. It readily gains two electrons to achieve a stable electron configuration like neon, resulting in an O2- anion with 8 protons but 10 electrons. This gain of electrons contributes to the increase in atomic number as you move from left to right in the periodic table for non-metals.
In summary, the behavior of electrons and the tendency to gain or lose electrons during chemical reactions differentiate the trend in atomic number between metals and non-metals. Metals generally lose electrons and increase their positive charge, while non-metals tend to gain electrons and increase their negative charge.