Atomic Structure And The Periodic Map

Atoms can be identified by the light they emit. In a discharge tube or flame test, the color is not a paint like coating. It comes from electrons in the atom releasing specific packets of energy as photons. Because each element has its own set of allowed electron energies, each element produces a distinctive set of bright lines rather than a continuous rainbow.

To connect evidence to identity, compare the positions of the bright lines for several elements and treat the pattern as a fingerprint rather than a single color.

From spectrum to structure

A line appears when an electron drops from a higher energy state to a lower one and the atom emits a photon with energy $E=h\nu$. Larger gaps between allowed energies produce higher frequency light, which is bluer. Smaller gaps produce lower frequency light, which is redder. The fact that only certain frequencies appear is evidence that electron energies in atoms are quantized rather than continuous.

Atoms are mostly empty space, so electron motion happens far from the nucleus compared with the nucleus size. The nucleus contains nearly all the mass because it contains protons and neutrons, while electrons contribute very little mass but control bonding and chemical reactivity through their charge and spatial distribution.

Notation that counts particles

Atomic number $Z$ is the number of protons in the nucleus and fixes the element’s identity. Mass number $A$ is the total number of nucleons, so $A=p+n$. An isotope is an atom of the same element with a different number of neutrons, so isotopes share $Z$ but have different $A$. An ion is an atom with a net charge because it has gained or lost electrons, so $q=p-e$ in units of the elementary charge.

Use these relationships to move between symbols and particle counts. For (,^{23}{11}!Na), $p=11$, $n=23-11=12$, and a neutral atom has $e=11$. For (,^{35}{17}!Cl^{-}), $p=17$, $n=35-17=18$, and the $-1$ charge means $e=p+1=18$. Practice converting both ways and always check that $A$ equals $p+n$ and that charge matches $p-e$.