Understanding the Atom
An atom with 3 protons and 4 neutrons is an isotope of lithium, specifically lithium-7. To determine its valency, we need to consider its electron configuration.

Protons and Electrons
- The number of protons in an atom defines its atomic number and, consequently, its identity. In this case, with 3 protons, the element is lithium (Li).
- In a neutral atom, the number of protons equals the number of electrons. Therefore, this lithium atom has 3 electrons.

Electron Configuration
- The electron configuration of lithium is 1s² 2s¹.
- This means there are 2 electrons in the first shell and 1 electron in the second shell.

Valency Explanation
- Valency is the ability of an atom to bond with other atoms, determined by the number of electrons in the outermost shell (valence shell).
- For lithium, only 1 electron is present in the outer shell (2s¹).

Determining Valency
- Lithium can lose this 1 electron to achieve a stable electronic configuration, similar to that of the noble gas helium.
- Consequently, lithium has a valency of +1.

Conclusion
- Therefore, the atom with 3 protons and 4 neutrons (lithium-7) has a valency of +1, indicating its tendency to lose one electron during chemical reactions. This property is fundamental in understanding how lithium interacts with other elements.